if you're studying for the AP Bio exam or for a unit 5 AP bio test and you're feeling nervous and scared about topics like meiosis and Di hybrid crosses or linkage and recombination that makes so much sense those are difficult topics but don't worry in this video I'm going to teach you everything that you need to know so that you can Crush that next test or the AP Bio exam my name is Glenn woken Feld also known as Mr I'm a retired AP biology teacher I love b i o l o g y here's what we're going to cover in this video we're going to start with meiosis that'll lead us to sex determination and that'll lead us to an investigation of what can go wrong in meiosis then mandelian genetics we'll start with key Concepts we'll look at monohybrid crosses and dihybrid Crosses then non-mendelian genetics which includes linkage and recombination sexlink alals non-nuclear inheritance and genotype environment interaction to help you study I've put together a checklist that you can download at AP bios. checklist meiosis the big picture what is meiosis why is it important meiosis is how sexually reproducing UK carots that includes animals plants fungi protests transmit genes from one generation to the next it creates variation between parents and their offspring and creates variation Among The Offspring Describe the life cycle of sexually reproducing ukar in ukots adults have specialized tissues testes and ovaries for creating gametes gametes are sperm and egg cells and they do that through this process that we'll be exploring called meiosis the sperm fertilizes the egg and that produces a zygote or a fertilized egg that zygote then divides and develops the tissues differentiate to produce an adult organism in relation to meiosis compare haid and diploid cells these are super important terms for understanding meiosis parents have two sets of chromosomes in all of their body cells with the exception of their gtes those chromosomes are paired so like for example here's chromosome one there are two of them here's chromosome 2 there are two of them one was inherited from one parent one was inherited from the other those pairs are said to be homologous and that's a term that we'll explain in the next slide when parents pass on their chromosomes to the Next Generation they can't pass all of them on if they did then the chromosome number would double in every generation so what happens in meiosis is is a having of the number of chromosomes and the half number of chromosomes kind of rhymes with the word half or it begins with the same prefix it's called haid so notice that there are four chromosomes down here there are only two over here what's the difference the difference is what happened during meiosis which is division of cells that involves reduction reduction division define homologous chromosomes homologous romes are the matching chromosomes that you inherited from your parents so for example here's chromosome 3 one of them came from your mom one of them came from your dad here's chromosome four it's the same thing here's chromosome five all the way on down the line they are not identical how could they be your Mom and Dad aren't identical so the chromosomes that they passed on to you wouldn't be identical they do have the same genes in the same order but the AL the specific code that is in the location where those genes are found that might be different let's use the analogy of a gene as a recipe well the one that you inherited from your mom if that were a recipe for tomato sauce maybe that one has a lot more garlic and the one that you inherited from your dad that might have a lot more basil in it well let's now think more biologically if C refers to a specific protein then the DNA that's coding for a specific sequence of amino acids might be slightly different in what you inherit from your mom and your dad and that might even be to the extent where the amino acid sequence of that protein differs so the genes are the same but the AL might be different that's what homologous means more essential meiosis genetics vocabulary compare and contrast germ cells gametes and somatic cells germ cells are are the diploid cells that are found in the testes and the ovaries that undergo meiosis they produce gametes and after meiosis we have sperm and egg cells that are haid in a human being the diploid number is 46 that's 23 pairs of chromosomes in the hpid gtes the chromosomes aren't paired anymore so there were just 23 chromosomes in the sperm 23 in the egg now the sperm goes ahead and fertilizes the Egg and that egg or zygo will divide develop the cells will differentiate and what you'll wind up with are somatic cells those are the cells of the body the diploid cells that make up the body tissues somatic cells are diploid germ cells are diploid gametes are hloy what happens during meiosis meiosis is reduction division why reduction division it's cell division that reduces chromosome number cells go from diploid with two sets of chromosomes to haid with one set of chromosomes the first step is DNA replication which we see over here at number one it creates doubled chromosomes consisting of two sister chromatids what's going on even though meiosis is reduction division it starts in exactly the same way that mitosis starts with a round of DNA or chromosome replication so if you look at this cell over here there are four chromosomes if you look over here there are still four chromosomes but each one is doubled consisting of two sister chromatids in step two we have meiosis 1 and what meiosis one does is it separates the homologous pairs so this chromosome is homologous to this one this one this one and they're going to be separated so now each of the result ing gametes only has one member of each homologous pair that means it's now a haid cell but each chromosome is still doubled and that's why we have meiosis 2 what meiosis 2 does is it separates the sister chromatids the result is four unique haid gametes compare mitosis and meiosis mitosis consists of one round of cell division that separates the sister chromatids the cells begin as diploid and they end as diploid the daughter cells over here are clones of the parent cell it's used for growth and repair meiosis consists of two cell divisions meiosis one over here separates homologous pairs so here we have the doubled chromosomes homologus pairs they get separated in meiosis 1 meiosis 2 separates sister chromatids we go from diploid to hloy it's used to create gtes for reproduction it introduces variation the daughter cells are unique how meiosis creates variation I'm Mr W from learn biology.com where we believe that interaction and feedback is what leads to deep substantial learning we're so sure of that that we provide a money back guarantee that comes with your subscription what are the two ways that meosis generates diversity are shown here the first is independent assortment the second is crossing over and genetic recombination we'll explain both of those now explain what independent assortment is and how it generates genetic diversity note that the phases of mitosis and meiosis have the same names the same designations but because there are two cell divisions in meiosis we have to give them a kind of suffix so in is there's interphase one followed by prophase 1 metaphase 1 Etc then there's a cytokinesis interphase 2 followed by a prophase 2 metaphase 2 anaphase 2 Etc the reason why that's important is because the events that we're going to talk about happen in between prophase 1 and metaphase 1 that's where independent assortment really takes place what happens is that during prophase one homologous pairs hair up and if you think about that it's quite poignant and extraordinary in the adult organism who's undergoing meiosis the chromosomal parents wind up finding each other and what I mean by that is that in the germ cells of an organism do meiosis the mother and the father's chromosome number one will find one another and I'm not kidding they actually embrace and chromosome 2 does that chromosome 3 does that all those chromosomes find one another and embrace one another during metaphase 1 they're pulled by spindle fibers just as happens in mitosis to the cell equator right so here we have that but the way that each pair gets dragged to the middle is independent of every other pair so In This Very simplified system it's possible that the paternal chromosomes might be on the left side and the maternal ones might be on the right side it's equally possible that you could have this Arrangement versus this Arrangement whereas you have paternal chromosome one on the left maternal chromosome one on the right whereas maternal chromosome 2 is on the left and paternal chromosome 2 is on the right it's as random as flipping a coin and that Randomness is essential what happens is that with two homologous pairs four different chromosome arrangements are possible that 2^ squared in the gamet in other words what we're going to do now during anaphase is we're going to pull these homologous pairs apart so one possible Arrangement is like this and then in the gam we have paternal chromosome 1 paternal chromosome 2 and in this gam we have maternal chromosome 1 maternal chromosome 2 and if the chromosomes are organized like this then this gam can have paternal chromosome one paternal chromosome 2 and this gamet can have maternal chromosome 1 and paternal chromosome 2 now you can play around with this you can make a cutout little pieces or you can label coins M1 P1 M2 P2 and you can try different combinations but you won't get more than four in a system that has four chromosomes as its diploid number with three homologous pairs then the math takes you to two cubed that's eight possible arrangements and with 23 pairs like we have in Homo sapiens you have 2 to the 23rd possible Arrangements that's 8, 388,000 68 possible combinations that's the chance that any two sperm cells or any two egg cells would have exactly the same array of maternal and paternal chromosomes so what's the chance that you and a sibling would have the same chromosomal inheritance in other words you'd inherit the same array of chromosomes in your dad's sperm and the same array in your mom's egg well those are independent events so the same egg it's 1 over 2 to the 23rd the same sperm it's 1 over 2 to the 23rd you multiply those two together and that's 1 in 70 trillion you ever wonder why you're different from your siblings this is is only one of the reasons why that's why myosis is so phenomenal so this is independent assortment what every chromosome does is independent of every other chromosome pair and that creates tremendous diversity in The Offspring independent assortment is a phenomenal engine for creating diversity but there's yet another one in meiosis that's crossing over what is crossing over and how does it create variation when those homologous PIR s pair up during prophase one they don't only Embrace they Embrace in such an intense way that they actually exchange parts so this Embrace is called synapsis and at a point called a kazma segments of DNA will move from one homologue to the other the result is that you start like this this array of four sister chromatids is called a tetr four tetrad and so this is what it's like before crossing over and then after crossing over it's like this well you'll notice this isn't really a maternal chromosome anymore it's a maternal chromosome with a paternal piece and the same thing for this one so crossing over creates what's called recombinant chromosomes and these have unique and novel sequences of DNA how does sexual reproduction create diversity one engine for diversity is independent assortment and how it randomly arrays different combinations of chromosomes in the gametes the other engine is crossing over and genetic recombination which creates uniquely recombinant chromosomes and then finally there's fertilization where the sperm and egg from different individuals combin in a fertilized egg egg and that is the third generator and that's why sexual reproduction creates diversity it explains you it explains me biology is it amazing or what meiosis the whole shebang let's walk through the entire process we begin with interphase and the thing to remember about interphase of meiosis is that it does exactly what interphase of mitosis does it replicates the chromosomes duplicates the DNA that's why when we get to prophase one each chromosome consists of two sister chromatids what else happens during prophase 1 the homologous pairs pair up the maternal and paternal chromosomes find one another and they Embrace they do this thing called synapsis and crossing over where they actually exchange pieces of DNA so that's happening during prophase during metaphase the spindle fibers pull these homologous pairs to the center of the cell and remember that each pair is pulled independently from every other that's the source of one of the main sources of variation in meiosis independent assortment the way that these maternal and paternal chromosomes get array is completely independent it's two to the number of pairs if you want to mathematically calculate the number of chromosomal Arrangements in the gametes during anaphase 1 the homologous pairs are pulled apart during Tila Phase 1 a new nucleus forms there's a cyto canis one and an interphase 2 that's not shown in this diagram that moves us to prophase 2 where the chromosomes once again condense note that whereas there were four chromosomes in prophase one there's only two in this cell in prophase 2 we've gone from diploid over here to hloy over here that's the main transition or one of the main transitions that occurs in meiosis 1 during meiosis 2 during metaphase the doubled chromosomes get pulled to the cell equator and then they get pulled apart during anaphase 2 then there's a tease where a new nuclear membrane forms and then there's another cyto canis the result four haid gametes each consisting of single chromosomes we've gone from diploid to haid from doubled chromosomes to single chromosomes and each of these haid gametes is unique it's genetically unique and that's what happens during meiosis at learn biology.com we understand why students struggle with AP Bio it's a hard hard course the material is complex the vocabulary is ridiculous and the pace is withering it's natural to feel overwhelmed and inadequate to get an A or a four or a five you need an easier way to study and that's why we created learn biology.com it has quizzes it has flashcards it has interactive tutorials about every topic in the AP Bio curriculum it has a comprehensive AP Bio exam review system use learn Das bio ology.com and you'll gain the skills and confidence that you'll need to Ace your biology course and to crush it on the AP Bio exam so here's your plan go to learn biology.com we've got free trials from June through March for both teachers and students you won't believe how much you'll learn topic 5.6 part one chromosomal inheritance sex determination how is sex determined in mammals we've seen this image before it's it's a cotype it shows all of the homologous pairs paired up chromosomes 1 through 22 are called autosomes they're paired homologous pairs they're the same in chromosomal males and chromosomal females but the last pair are called the sex chromosomes females have two X chromosomes males have an X chromosome and a y chromosome unlike the a omes chromosome pairs 1 through 22 in humans the X and the Y are quite different from one another for one thing unlike those homologous pairs they don't cross over and swap pieces of DNA the x is a normal chromosome it has a variety of alals related to various nonsex relating functions that includes immune function Vision blood clotting and so on the Y has a region that's called Sr y it's indicated by this yellow bar over here and that initiates the development of the testes during early embryonic development and later on production of testosterone which winds up differentiating the body into its male form during fertilization it's the sperm that determines the chromosomal sex of the zygote which becomes the embryo which becomes the baby which becomes the person the males which have their 22 autosomes and then in and a y chromosome during meiosis will pass on either an X chromosome or a y chromosome because even though they're not truly homologous they will get separated just like how the other homologous pairs get separated the egg has two X chromosomes so every single egg cell is going to have the X chromosome if the egg is fertilized by a x carrying sperm then the zygote will have two x chromosomes and it'll develop into a female if the sperm that's carrying the Y chromosome does the fertilization then the resulting zygote will be XY and it'll develop into a male that's chromosomal sex termination in all of the mammals the fact that half of the eggs will be fertilized by an EXC carrying sperm and half will be fertilized by a y carrying sperm explains the fact that among the births in any m population the initial ratio of males to females will be 5050 Birds also have a chromosomal system of sex determination but it's different it's kind of a flipped version of the one in mammals in Birds it's the egg that determines the chromosomal sex of The Offspring that's because the females have what we call a z chromosome and a w chromosome and during meiosis half of the eggs will wind up being Z and half will be W the females will pass on either that Z or W chromosome the males have two Z chromosomes so when they form their sperm all of the sperm carry Z fertilization of a z carrying egg will lead to a male whose z z fertilization of a w carrying egg will lead to a female who is ZW that's chromosomal sex determination in birds as in mammals because half of the eggs will be Z eggs and half the eggs will be W eggs the result is that among the initial births that happen in any bird population half of those birds will be male and half of those birds will be female among certain reptiles sex is determined by the temperature at which the embryos develop some reptiles lay their eggs in a nest that's dug in the sand and as you might imagine it's warmer on the top closer to the Sun and cooler down below these animals develop based on a pivot point that's represented by tpiv over here so in sea turtles the eggs that develop above the Pivot Point become female here's the proportion of males over here so if you're above the pivot point if you're in the warmer area you'll develop into a female if you're in the cooler area over here below the Pivot Point you'll develop into a male and then at the pivot point it's pretty much random 50/50 in another kind of reptile called a two Atara it's the opposite so basically if you develop above the pivot point then you have a higher chance of being a male if you're below the pivot point then the proportion of males goes down you have a higher chance of being a female and in crocodiles there are two pivot Points a low temperature one and a high temperature one in the coolest and the warmest temperatures the eggs develop into females in other words the proportion male way down but at intermediate temperatures over here the eggs develop into males how is sex determined in ants bees and wasps this is completely mind-blowing it's a system that's called Hao diploid sex determination or Hao diploidy the males are hloy they develop from unfertilized eggs so here's a male here haid the females which includes the queen and all of the workers they're all diploid and they develop from fertilized eggs so the mother in a bee Colony just to use that example is the queen she undergoes normal meiosis when she creates her eggs but the father is a haid male also called a drone and he can't really do meiosis because he's haid he's not diploid so essentially he passes on 100% of his chromosomes in the sperm that he creates the consequence is that all the bees in a hive with the exception of the drones all of the worker bees are sisters but they're more closely related to one another than any two mammal sisters are just think about it mammal sisters inherit half their mom's genes half of their dad's genes so they're 50% related to one another whereas these sisters inherit half of their mom's genes and 100% of their dad's genes so they're 75% related to one another they're more closely related to one another than they would be to their own offspring so that is thought to be an explanation of why the workers cooperate with one another to help the queen create more offspring to keep the hive going to find food all of these incredible foraging behaviors that you find in ants wasps and other social insects but it's not a complete explanation because like for example the termites are also social but they don't have this h hapl loyed system so biology amazing this is as much as you need to know about Hao diploid for the AP Bio exam topic 5.6 part two chromosomal inheritance non-disjunction and chromosomal variation what is non-disjunction what are its consequences nondisjunction is kind of a cool word so a junction is where things come together a disjunction would be things coming apart and non-disjunction means things failing to separate it's when the homologous pairs or the sister chromatids don't separate during meiosis there's a couple of variations in meiosis one the homologues don't separate and as a result so you see that over here this blue uh chromosome over here these homologs didn't separate they stayed together so the result is that in meiosis 2 uh we're going to have three chromatids on this side three over here one over here one over here so 50% of the gametes are n haid plus one extra and 50% are n minus one so n the haid number missing a chromosome if non-disjunction occurs during meosis 2 it involves the sister chromatids not separating in meiosis 2 the sister chromatids don't pull apart so the result is that 20 5% of the gametes are n + 1 25% are n minus1 in other words the haid number but missing one and then 50% of the gametes will be normal so that's what nondisjunction is and how it can happen in meiosis 1 or meiosis 2 we saw in the last slide how non-disjunction during meiosis 1 or meiosis 2 can result in gamt that have an abnormal number of chromosome omes if the eggs are n + one they have the haid number plus one more then the zygo will have an extra chromosome then what we have is a triom and what that means Tri three instead of a homologous pair with two we have three the most famous example of that is Down syndrome which is a triom of the 21st chromosome which has various developmental consequences and developmental delays if the are nus1 like over here or over here and the sperm again are carrying a normal number of chromosomes so that's not necessarily the case you can have non-disjunction that occurs during the formation of sperm as well but then the zygotes will have a missing chromosome so they'll have all of the homologous pairs but one will be short and the result of that is a monosomy and most of those aren't really survival able except in the case of the sex chromosomes so uh one to know is called turna syndrome where instead of females having two X chromosomes they have one and there's a significant amount of variation that could happen in the sex chromosomes there can be men who are born with an extra X chromosome there can be men who are born with an X and two Y chromosomes so those are all chromosomal variations that come about through non-disjunction followed by fertilization I want to acknowledge how difficult and complex some of these Concepts can be and I want to encourage you to go to learn-y. comom and with a free trial you can do the tutorials and you can use our unit reviews and it's going to really help you to get on top of this material setting you up for Success on your unit test or the AP Bio exam topic 5.3 melan genetics genes are the basic unit of heredity they are what gets passed from inance to offspring they determine traits or characteristics you can also think of genes from a molecular genetics perspective and you can think of them as a sequence of DNA nucleotides that code for RNA and ultimately code for protein and those proteins ultimately determine the characteristics of the organism describe mle's principle of segregation in your answer cover the difference between the terms homozygous and heterozygous just historical note Gregor mendal is considered the father or grandfather of genetics in the 1800s he figured out many of these basic principles that we're talking about right now every individual has two copies of each gene those copies are located on the chromosomes that are organized into homologous pairs the alals are alternative versions of those genes that might have different DNA sequences that will produce proteins that have different amino acid sequences homozygous means that the two alals are identical for example in this parent over here both of the alals are designated with a capital letter A they are the same this person is a homozygote for this particular Gene in a heterozygote the alals can be different in this individual over here one of the alals is capital A one of the alals is small a the principle of segregation shows what's happening here when individuals create gametes their sex cells they pass on only one of their two alals so the alal are together in the parent but they become segregated or separated during gamet formation here that's happening in the formation of sperm here that's happening in the formation of eggs if you feel that that corresponds to the events of meiosis you're on to something and we'll talk about that later Define the terms dominant and recessive dominant are always observed in the phenotype of The Offspring they're represented by a capital letter so for example the capital letter A so here's an individual whose homozygous dominant and they express the dominant characteristic dark black fur in these heterozygotes there's also a recessive alil but because the dominant alil is present then the characteristics of the organ ism is determined by the dominant Al a recessive alil only shows up in a homozygote so this mouse over here is homozygous recessive and therefore it has the recessive appearance that homozygous alal recedes into the background when it's paired with a dominant Al and these are represented in the melan system by a lower case letter in this case lowercase a what's the difference between genotype and phenotype type phenotype is your appearance it's the observable characteristics in an organism an example is brown eyes genotype is the genes that you've inherited the type of genes that you have it's your underlying DNA in my case my eye color is brown that's a dominant phenotype though eye colors actually complex there are many alals involved in determining eye color actually I think it's about three but I'm a heterozygote so my genotype would be like capital B little B how do I know that because my wife has blue eyes and both of my kids have blue eyes and that would only be possible if I were a heterozygote because if I was a homozygote then I could only pass on the Big B alil and then both of my children would have been heterozygotes explain what a monohybrid cross is and what the results of such a cross will be in your response create a punet square to demonstrate your understanding this which we've seen before is a punet square it uses the rules of probability to quickly predict The Offspring of genetic crosses a monohybrid cross is a cross between to heterozygotes so these are P's which was one of mle's steady organisms and this P has purple flowers the genotype is capital P Little P capital P Little P why because it's a heterozygote and this is a monohybrid cross so they're hybrid for one characteristic now the result will be Offspring in a 3:1 ratio so how do you do these pent squares well you start by identifying the genotype of the parents well in the problem it's given to you it's a monohybrid cross big p Little P crossed with big p Little P so you can think of this as a germ cell it's going to go through myos the homologous pairs are going to separate principle of segregation so some of the gametes will have big p some will have little p and that's true for each parent here big p Little P flowers are hermaphroditic so it doesn't really make sense talking about father and mother in this case what you do is then the um gametes will fertilize one another you know if this were a pollen the equivalent of sperm it would fertilize the egg assuming this is the female and this is the male over here but in other words this combines with this and we have this organism is capital P capital P it's got the dominant phenotype it's homozygous dominant this organism gets a big p and a little p it's a heterozygote and it still has the dominant phenotype same with this one over here this one gets two Little P's one from each parent and as a result it shows the recessive phenotype and that recessive phenotype is white flowers we have three individuals with the dominant phenotype one with the recessive phenotype a quarter of The Offspring are homozygous dominant half of The Offspring two quarters are heterozygous they still have the dominant phenotype and one quarter will be homozygous recessive and they'll show the recessive phenotype that's how you do a punet square for a monohybrid cross in genetics what are the P F1 and F2 Generations this is a common notation that's used in genetics and you as an AP biology student need to understand it the P generation is the parental generation and in general they're true breeding homozygotes now notice that we've jumped in complexity in this example we're not dealing with just one gene we're dealing with two so there's a gene for tallness and there's the gene for flower color and the um alternatives are tall versus short and purple flowers versus white flowers when you breed the P generation together those offspring are called f1s it's the first filial generation and in this case they're heterozygotes they're double heterozygotes because they're Big T Little T that's a heterozygote and big p Little P so double heterozygotes now let's go from the f1s to the f2s you breed together the F1 generation these double heterozygotes and you get the f2s so the F1 that's the first filial generation and the F2 it's the second generation it's the grandchildren of the P generation and crossing the dihybrid f1s why are they dihybrid because they're double heterozygotes they're hybrid they're mixed for two characteristics that's called a DI hybrid cross we're going to take that apart in just a moment M Define mle's principle of Independent Assortment independent assortment it's what we talked about in the context of meiosis this is essentially the same process mendal was able to int this just looking at the transmission of traits from one generation to the next genes carrying different traits are segregated and passed on independently from one another so in an organism that's di hybrid Big T Little T big p Little P the way that the T Gene pair gets passed on is independent from the way that the P Gene pair gets passed on and therefore in a double heterozygote a DI hybrid Big T Little T big p Little P four unique gametes can be created here they are Big T big p Big T Little P Little T big p Little T Little P how can you figure this out for yourself it's very very simple if you use the foil algorithm that's for factoring binomials you should know it from algebra if you don't just remember it first Outside Inside last so you think about this and you think about the gene pairs so you take the first one big T big p you take the outside one big T Little P that's over here then you take the inside one little T big p that's over here and then the last one little T Little P so four G can come from a DI hybrid organism this is limited to genes that are found on different chromosomes the game completely changes when those genes are on the same chromosome and we'll deal with that later explain what a DI hybrid cross is and what the results of such a cross will be it's a cross between two double heterozygotes a couple of slides ago we were looking at the production of the f2s when there was a DI hybrid cross we're cross ing Big T Little T big p Little P with the same you're going to use the foil algorithm to figure out the gametes and we just described that in the last slide you just bring these alals down and over and combine them you do that for all 16 squares of this die hybrid cross and what do you wind up with you wind up with a 9 to 3 to 3 to one ratio in The Offspring and that is actually something that's worth memorizing what's the connection between mle's laws of inheritance and what happens during meiosis we've just articulated mle's two laws the principle of segregation and the principle of Independent Assortment the principle of segregation according to mendle says that parents have two alals for each trait but pass on only one to their offspring which inherit their two alals from two separate parents in meiosis there's a diploid parent that produces hloy gametes the diploid phase corresponds to the two alals in each parent the haid phase corresponds to the one Al that the parent passes on to their offspring in zygotes that diploid condition is restored and that's not shown in this diagram over here independent assortment according to mendal is what happens to one gene pair is independent of every other Gene pair at least in the ones that mendal studied in meiosis chromos omes assort independently during metaphase 1 of meiosis What is the rule of multiplication demonstrate your understanding by explaining how to predict the probability of genotype little a little a little B little B little C little C resulting from a trihybrid cross between big a little a big b little B Big C little c times the same don't make a huge Punit Square to try and solve problems like this what you want to do instead is use one of the laws of probability the rule of multiplication here's what it is the probability of independent events occurring together is the product of their individual probabilities here's how to think about this it's like three independent punet squares because what each gene does is independent of every other Gene what a is going to do is independent of B is independent of C so three independent pent squares one for a one for B and one for C but you don't have to create three pent squares you just have to use the rule of multiplication in a cross between big a little a and big a little a the probability of little a little a is 1 out of four it's the same for B one out of four and it's the same for c one out of four these are independent events so you use the rule of multiplication it's 1/4 * 1/4 * 1/4 or 1 out of 64 topics 5.4 and 5.5 non-mendelian genetics and environment phenotype interaction non-mendelian genetics is about genetic principles that were discovered after mle's original contributions one of the most important of these involves linked genes what are link genes describe their inheritance pattern and explain which melian rule link genes don't follow link genes are genes that are on the same chromosome so fruit flies as opposed to peas were the widely used experimental organism to discover the principles of non-mendelian genetics and here you can see one chromosome from a fruit fly and there's a whole variety of genes sequences of DNA that code for specific traits that are located on the same chromosome one has to do with this kind of bristled appendages on the head One controls body color One controls one type of eye color One controls Wing length these genes are mostly inherited together which is different from the Independent Assortment that we saw with melan genetics because they're on the same chromosome these genes don't independently assort so like for example above genes T and A in this cell over here they're Linked In This cell over here they're not linked these would independently assort and these wouldn't so what happens in crosses involving link Gene note first of all that we have a different symbol system over here you can see that right over here we have B+ B+ vg+ vg+ in this system in non-mendelian genetics a plus sign indicates the wild type or the dominant Al if you have a symbol that can be more than one letter without a plus side that indicates the recessive in this P cross what we're doing is we're crossing normal body normal winged fly B+ B+ vg+ vg+ with a black body vestigial winged fly those are both recessive traits and note that all the F1 offspring are dihybrid they're B+ B vg+ VG and they have both dominant phenotypes this is what you'd expect in a melan trait for the f1s we have a gray body and normal winged fly which has all of the dominant characteristics now over here what we're doing is we're representing this chromosomally B+ vg+ and B VG notice that B+ and vg+ they're on the same chromosome and B VG are on the same chromosome too these genes are linked what if they were perfectly linked and they never separated what would you expect to happen the method here is a little bit different than the method in a melan cross we're not doing a DI hybrid cross we're doing what's called a test cross and a test cross a DI hybrid so b+b VG plus VG is being crossed with a double recessive and the way that it's done is the double recessive is the male the female is the double hybrid the double recessive I've represented this over here here's a representation of the female on one of her chromosomes she'll have B+ and vg+ on the other chromosome she'll have B and VG the male is a double mutant and he has B and VG on both of his chromosomes when the female produces gtes then half of her gtes will have B+ and vg+ and half of her gametes will have B and VG now that's assuming perfect linkage in the male all of the sperm have to have B and VG here's a Punit Square these are the eggs that's going here the other eggs are going here the other half of the eggs and all the sperm are going over here you put them together and what you'd expect is that half of the ospring would be B+ B vg+ VG the other half of the offspring would be b b VG VG this organism these offspring would have normal body and normal wings that's 50% of The Offspring and the other 50% of The Offspring would have black body and vestigial wings but that is not what happens and what I'm doing here is I'm letting you know what you would expect if there were perfect linkage what actually happens in a cross involving linkage note that the numbers won't always be the same but the general concepts apply what we see is that the majority of The Offspring have parental phenotypes what is does that mean well the mom's phenotype was gray body with normal wings and many of these flies have that phenotype gray body normal Wings the father's phenotype is black body with vestigial Wings many of The Offspring have that phenotype but a significant number of Offspring Have recombinant phenotypes what are recombinant phenotypes they take one of the phenotypes of the mom and they combine it with one of the phenotypes of the dad that's what's happening over here in these flies that have a gray body like the mother and vestigial wings like the father or you can have a black body like the father with normal wings like the mother those are recombinant phenotypes why do we have most of The Offspring having parental phenotypes but a significant minority having recombinant phenotypes it's because linkage is not perfect genes that are link don't always stay together why not because during meosis there's recombination and crossing over so linked genes because of that process can separate we'll see the details of this in the next slide what happens in crosses involving link genes why are there recombinant in diagrams A and B we have a DI hybrid female and her germ cells in C we have meiosis 1 with crossing over we have homologous pairs they're going to get together they're going to swap genes and the production of recombinate chromosomes but notice that some of the sister chromatids are recombinant and some aren't so now what we're going to do is we're going to complete meiosis we're going to go through gamet formation and when we do that note that of the eggs that the female produces two of them at letter H they're recombinant what do I mean by that well they have B+ and they have VG in other words this chromosome is B+ vg+ this chromosome was B VG well the recombinant ones are the ones where those alal swapped places so this is a recombinant B plus VG this is a recombinant b vg+ but these two at letter I those are parental right they're just like the parents B plus VG plus bvg now over here we we have the male parent who's a homozygote and his gamet he can only produce one kind of gamt he's homozygous he can only produce a gamet that has B and VG here's what happens with The Offspring Offspring n and p have recombinant phenotypes in other words they have in this case graybody vestigial wings that's recombinant neither of the parent look like that and this is black body normal Wings again recombinant neither are the parent look like that but o and Q are parental phenotypes o is graybody normal wings and Q is black body vestigial Wings why are there so many more parentals than there are recombinant phenotypes it's because crossing over happens but it happens at a rate that's dependent on the distance of these alals on the chromosome and so the closer to are the less they'll tend to cross over and this frequency which is I believe 177% that represents the distance between the B and the VG on the fruit flies chromosomes what's the relationship between the percentage of recombination and the distance between any two linked genes the further apart any genes are on the chromosome the higher the percentage of recombinant gametes in the chromosome map above genes A and E will recombine the most because they're the furthest apart in other words there can be crossing over over here over here over here and all of those will get e to jump over here and Big E to jump over here that's going to look like recombination that's going to actually almost look like independent assortment because they're so far away but genes B and C they'll recombine the least because the only way that they can cross over is if there's a kayma right over here at this specific spot that would enable little CA to jump over here Big C to jump over here or B could do the same the percentage of recombination can be used to calculate the map distance between two alals so over here this is a chromosome map and it's saying here that the distance between this long bristled appendages called long Arista and gray body that's 48.5 well those are units that reflect the frequency of recombination over here between gray body and red eyes that's a little bit under that's nine recombination units and really what this amounts to is the frequency of recombinant so doing all of these mapping experiments or doing all of these breeding experiments with fruit flies the researchers at Columbia University in the 1900s Thomas Hunt Morgan and his crew were able to create maps of chromosomes and this was establishing that genes are on chromosomes and was part of the pathway that ultimately led to the double helix and all that other great stuff stuff note that these are not physical maps so that the numbers don't add up but this should get you far enough to have a basic understanding again on learnes biology.com I have a whole tutorial about linkage and recombination with practice problems that'll get you all the way there your success in AP biology starts here are you struggling with AP Bio with learnbiology dcom students get the skills and confidence to be a top student and earn fours and fives on the AP Bio exam guaranteed go to learn biology.com to find out how you can master your biology course and crush the AP Bio [Music] exam what are sexlink genes sexlink genes are located on the X chromosome males because the genes are located on the X chromosome can't be heterozygous they either have the alal or they don't so the whole heterozygous homozygous thing doesn't apply to males but females can be heterozygous as shown in this case it's X big h x little H A heterozygote or they could be homozygous dominant that would be X big h x Big H or they could be homozygous recessive X little h x little H using hemophilia as an example how can males inherit a recessive sexlink trait hemophilia is a blood clotting disorder and ESS ially hemophiliacs can't clot their blood it's much more common in males than it is in females because the alil for hemophilia the mutated Al that leads to ineffective blood clotting is on the X chromosome Suns inherit their xlink alals from their mothers so we'll look at the punet square first this punet square is from learn biology.com these boxes are things that you would drag the alals into here we have a hemophiliac X little Hy it's recessive xlink condition how did this uh young man inherit that EX-L alil from his mom mom is a heterozygote x big h x little H she's not a hemophiliac but she carries the alil it's commonly known as a carrier the dad in this case is unaffected X big Hy but it doesn't matter because dads don't pass on their X chromosomes to their sons they pass on their Y chromosomes that's why they're Sons to begin with so the mom has to be a heterozygote as shown here or she could be homozygous recessive though that's quite rare we'll look at that in the next slide here's a pedigree that shows uh the same thing basically a cross between a heterozygous female and a normal male and note that in this case the mom passed on her her normal chromosome and the dad passed on his normal chromosome it's the only EX chromosome that he has to pass on in the case of these two sons the mom passed on her defective ex chromosome with the hemophilia alal so we have two sons who are X little h y they're both hemophiliacs this daughter over here is a carrier she received the normal ex chromosome from her dad and an ex chromosome with the mutant hemophilia alil from her mom and here we have a son who inherited the normal X chromosome from the mom she had one of two to give he got lucky and the Y chromosome from the doubt so he's a normal young male examples of excellent processive conditions in humans hemophilia which we just talked about also a very common one is red green colorblindness in fruit flies which have a similar sex determination system to humans it's just the X chromosome and the Y chromosome just like mammals the AL for white eyes is a mutation that's on the X chromosome and fruit flies and in fact this was the first a that was ever located on a specific chromosome can a female inherit a recessive sexlink trait absolutely but it's uncommon here's what has to happen the male parent must have the sexlink recessive trait so here we have a wide-eyed fruit fly X little Wy the female must be a heterozygote in this case or have the trait so it could be a wide-eyed female here the female is X Big W X Little W and when we create the pent Square we can see that among the female Offspring 50% of The Offspring are carriers they have to be carriers they get the normal Al for red eyes in this case from their mom they get the recessive alal for white eyes from their dad and in this case they get the recessive alal from both parents so 50% are carriers and 50% of the females have the trait among the male offspring 50% of the males are normal why because the mom's a heterozygote she passes on her her normal ex chromosome with the red eyed alil to half of her sons but there's also the 50% chance that she'll pass on the recessive mutated Al and that's why this Offspring over here will be white-eyed but the key thing that we were looking for was how can a female wind up inheriting a recessive Sex Link condition she can if the dad has the condition and the mom is a heterozygote what is non-nuclear inheritance I'm Mr W from learn biology.com where we believe that interaction and feedback is what leads to deep substantial learning we're so sure of that that we provide a money back guarantee that comes with your subscription it's inheritance of genes that are not on a nuclear chromosome but on the chromosome of a mitochondrian or a chloroplast a nuclear chromosome is one of the autosomes or a sex chromosome that are found inside the nucleus of a eukaryotic cell these genes that are on mitochondria or chloroplast are only passed on to The Offspring through the female gamate why let's look at the example of of sperm sperm have mitochondria the mitochondria power the flagellum that enables the sperm to swim towards the ovam but when there's fertilization only the head of the sperm penetrates through the egg membrane or is allowed in through the egg membrane and all of the mitochondria are left outside so all of the mitochondria that get passed on to The Offspring are mitochondria that were passed on through the female germ cell The Inheritance pattern of these mitochondrial or chloroplast genes doesn't follow mandelian patterns you can see that in this pedigree over here where the mom passes on the mitochondrial Gene to all of her Offspring male and female but the male who inherits the mitochondrial Gene doesn't pass it on to any of his offspring the male is is essentially a mitochondrial dead end if we're talking about mitochondria where the female passes on her mitochondria to all of her Offspring in the case of mitochondrial inheritance in ukar all of one's mitochondria are inherited from one's mother and in Plants they also have mitochondria so the same rule applies but they also have chloroplasts and only the avule not the pollen passes on chloroplast and mitochondria to The Offspring so it's the same thing it's female line inheritance of alals when it relates to genes that are on an organel such as the mitochondria or a chloroplast what is incomplete dominance this is when the phenotype of the heterozygote is different from and usually intermediate between either of the homozygous phenotypes neither alil is dominant so in this case we have to use a different notation system this is carnation and C Big R with a superscript represents the red flower color alil C Big W with a superscript represents the white flower color o so here's our P generation cbig r cbig r cross with c bigw c bigw the F1 generation they're all hybrids cbig r c Big W but they're pink it's because two doses of gene expression in other words this is DNA that's coding for protein and you need that much protein in order to produce that vibrant red color if you just have one then it's not enough to push the phenotype all the way to red and you wind up with this intermediate phenotype one dose can produce sufficient pigment to create the red color let's see what happens when you do your F1 cross to produce the F2 Generation so we have a pink carnation cbig r cbig w crossed with another pink carnation cbig r cbig w what do you get in The Offspring you get one Offspring that's red because it's cbig r cbig r one offspring that is white because it's c bigw cbig w and two that have the intermediate phenotype pink because they're going to be cbig r cbig w cbig r c bigw explain how the same genotype can result in different phenotypes under different environment Al conditions this is also known as an environment genotype interaction and what happens is that factors in the environment influence gene expression and that leads to variation in the subsequent phenotype and basically what it's saying is something that's fairly well known that genes don't determine everything genes interact with the environment and note that this is the norm not the exception here are two examples to know about if you have uh hydranges in your yard you can determine the color of the hydranges because the flower color is determined not by genes but by the acidity of the soil in a more acidic soil shown over here you'll get this beautiful purple color in more alkaline soil you'll get reddish color and there's actually mixes that you can add to the water that you give to your hydranges that will adjust the alkalinity or acidity of the soil example number two is dark fur regions in mammals and they develop in cooler body regions away from the core this is a Himalayan rabbit but if you can picture a Siamese cat it's pretty much the same thing so this is a cool region this is a cool region this is a cool region and all the cells have the same genes but the cells that are being exposed to cooler temperatures Express different pigmental than those that are in the warmer regions of the body there are many other examples height and weight in humans is caused by an interaction between the genes that someone inherits in the environment that they're in the way that your skin color can change in relationship to the amount of sun that you're exposed to that is genes being expressed in relationship to environmental cues we talked earlier about how sex is determined by temperature in certain reptiles another example of environment genotype interaction here are your next moves for AP bios sucess please subscribe to learn biology.com and please watch this next video