hello and welcome back to a-level biology help today i'm going to be taking you through the inheritance chapter for hoa a-level biology also near the end of the video i'll be going through a few exam style questions and explain their mark schemes and as always there will be timestamps in the comments section so that you can skip to the different sections in the video if you do not wish to watch the whole thing right okay so this chapter covers a lot of content so this video may be quite long but remember there are timestamps in the comments section but this is the content that we are going to cover but not necessarily in this order so the concept of genotype and phenotypes multiple alleles dominant recessive and codominant alleles homozygous and heterozygous alleles monohybrid and dihybrid crosses crosses involving sex linkage autosomal linkage multiple alleles and epistasis and finally the chi-squared test so let's get started so what is a genotype and what is a phenotype well a genotype is the genetic constitution of an organism so all of its genes but the phenotype is the expression of the genotype and this interaction with the environment so a phenotype is like the feature so for example blue eyes but the genotype is the actual um gene or the allele that it carries remembering that alleles are variants of a particular gene so now i'm i'm going to introduce to you this concept of multiple alleles an example of multiple alleles are eye colour so you have a blue green brown hazel etch your alleles so there are multiple alleles for one gene so the gene would be eye colour so there are multiple alleles for this so what causes multiple alleles well multiple alleles are caused by mutations in a gene that occur at different positions or different loci in the gene so at different positions in their base sequence so these mutations at different positions produce multiple alleles of a single gene this leads me on to the concept of homozygotes and heterozygotes which you will have hopefully covered at gcse so in diploid cells chromosomes form pairs called homologous chromosomes which you will be familiar with if you studied the meiosis chapter so alleles at a specific locus so specific position in a gene can be homozygous if they're the same type of allele so two dominant or two recessive values for example or heterozygous if both alleles are different so the main point here to take away is that you have two copies of um a particular allele for the genotype as chromosomes for homologous chromosomes so one allele comes from e for your parents so you have two copies of each allele and they're homozygous if they're the same type or heterozygous if they're both different if an ariel is dominant it is expressed in the phenotype of a heterozygote so only one copy is needed for expression so one copy is needed for expression even if there is a presence of a recessive allele you'll still um have the phenotype of the dominant allele and an allele is recessive if it's not expressed in the phenotype of hepatozygote so this means that two copies have to be present for expression so one from each parent so here is an example that is often used so the cystic fibrosis allele is often used as it cystic fibrosis is one of the most common autosomal recessive disorders so we can um denote alleles as letters capital letters are normally mean the dominant allele so here i've written that capital c is the normal allele which is dominant and lowercase c is the cystic fibrosis allele so that is the recessive value so if your genotype is two capital cs this means that you are homozygous dominant as the two alleles are the same and they are both dominant this means that you are not affected by cystic fibrosis if you have one capital c and one lowercase c this means that you are heterozygous so you have two different alleles this means that you are not affected by cystic fibrosis but you are a carrier as you have one recessive value if you carry two lower cases this means that you are homozygous recessive so this means that you are affected by cystic fibrosis so these i just get my pen tool out all of these are what we call the genotype so now i'm going to introduce to you the concept of codominant alleles which will probably be new to you so codominant alleles are when both alleles are equally dominant at and are equally expressed in the phenotype so an example would be blood type a b as you have the alleles for blood type a and blood type b but they are equally dominant and equally expressed so your blood type is a b so to put that into context here are the um different alleles for blood type so we write this as i o which is blood group o i a which is a and then iv which is b the io allele so build type o is recessive and i a and ib are co-dominant so if your genotype was ioio you you would have blood group o if it was iaio you would have blood group a if it was ibio you would have blood group b and then if it was iaib you would then have blood group a b as they are codominant alleles so um monohybrid crosses can be used to predict the outcome of crossing um two different genotypes so here i have for example two um parents with two different genotypes one is homozygous dominant and one is homozygous recessive homozygous dominant individuals unaffected with cystic fibrosis and the homozygous recessive individual is affected so these are the parental genotypes so we can predict the outcome of the probability that their offspring will have particular genotypes of crosses so here are the gametes the gametes are just single letters in this case as meiosis has occurred to produce gametes and um gametes are haploid so we have four different gametes in total a capital c another capital c lowercase c and a lower case c and if we cross c's in what we call a punnett square we get these genotypes so these are all the possible genotypes that you could get enough spring now notice that they are all the same so if you cross a homozygous dominant individual with a homozygous recessive individual 100 of the offspring will be heterozygous carriers so they carry one recessive allele for cystic fibrosis but what happens if we cross two of these heterozygotes so we call this the f1 generation so what happens if we cross two of these heterozygous individuals so here are the parental genotypes which are both heterozygous they are both unaffected by cystic fibrosis but they are carriers so the gametes for this will be a big c and then little c and the b c and little c and if we cross c's using a punnett square we get this so we have three different genotypes in total we have a homozygous dominant so big c big c then two lots of heterozygotes so big c little c and one um homozygous recessive so we can say that 25 of individuals or the offspring would be homozygous dominant so they are unaffected by cystic fibrosis so there is a 25 chance that the offspring will be homozygous dominant and then we have two heterozygotes in our grid so this means that there's a fifty percent chance that the ocean will be heterozygous and therefore unaffected but carriers of the cystic fibrosis allele then we have one um heterozygous not heterozygous sorry homozygous recessive individual so there's a 25 percent chance that the um individual will be or the offspring will be homozygous recessive so will be affected by cystic fibrosis so when two heterozygotes are caused and you have a recessive disorder then there is a 25 chance that the offspring will be affected now notice this is a crucial thing to remember with monohybrid crosses there is a three to one ratio of unaffected to affected phenotypes this ratio is absolutely crucial to remember as it often asks you about this in exam questions so crosses can be used to predict an unknown genotype as well so used to determine an unknown genotype the unknown genotype is crossed with a homozygous recessive individual and you see the outcomes of the offspring to predict what the unknown genotype is so if all the offspring has a dominant phenotype this means that the unknown genotype is homozygous dominant so um the offspring will be heterozygous so this would show that um the unknown genotype is homozygous dominant as they must um inherit the homozygous dominant allele from the unknown genotype as the as it has been crossed with a homozygous recessive individual however if half of the offspring have the recessive phenotype then the unknown genotype is heterozygous as some individuals have inherited the recessive annual from the individual with the unknown genotype you can also use dihybrid crosses to cross in or predict the outcome of two genes together so this describes the inheritance of two different genes so here we have two parental genotypes within two different genes so one gene um affects the shape of the p and one gene affects the color of the p so capital r allele means that the um p is round and lowercase r which is your recessive allele means that it is wrinkled capital y means that the p is green and then the recessive alleles so lowercase y means that the p is yellow so here we have the parental genotypes so we have homozygous dominant individuals so this has round green peas and a homozygous recessive individual which has wrinkled yellow peas this means that all the other will be heterozygous for both genes so they they have one copy of each of the alleles so capital r lowercase r capital y lowercase y so what happens if we cross the heterozygotes in the dihybrid cross but we get slightly different ratios and with monohybrid crosses so here's the parental genotypes and here we have all the possible gametes so for each um individual we have the gametes capital r capital y capital r lowercase y lowercase r capital y and then lowercase r lowercase y it's crucial to remember that um the annuals of the same gene are not inherited in gametes together so if we make a cross out of these gametes we get this which is quite complicated table so let's interpret the results so as you can see around one two three four five six seven eight nine of the individuals have a round green phenotype as they have the capital r and the capital y allows present which are dominant three of the individuals out of 16 have a round yellow phenotype as they have the dominant allele for the shape however they have they are homozygous recessive for the collar gene so they are round and yellow three out of 16 of the individuals in the offspring have a wrinkled green um phenotype so that they are homozygous recessive for their shape so they are wrinkled but they have at least one copy of the dominant allele for collar so they are wrinkled and green then one individual is homozygous recessive for both genes so one out of 16 has a wrinkled yellow um phenotype so this means we can say that we have a nine to three to three to one ratio of phenotypes this is the key ratio in dihybrid crosses so it is crucial that you remember this nine to three to three to one ratio of phenotypes and genotypes for a dihybrid cross so now i'm going to talk about something called autosomal linkage so autosomes are basically chromosomes that are not your sex chromosomes so also same with linkage is when two or more genes are located on the same autosome when two or more genes are located on the same autosome this means that they are less likely to be separated during crossing over so this means that they are often inherited together and this can have impacts on um the ratio of phenotypes in a cross so offspring are more likely to resemble the parental genotype if the alleles of the parental genotype are linked as they are less likely to be separated during crossing over of meiosis so here we have an example so the alleles capital a and capital b are linked and the alleles cap lowercase a and lowercase b are linked as well so this means that um it is more likely that only two types of gametes would be formed so capital a and b and lowercase a and b so it's highly likely that you'll get a genotype of lowercase a and capital b and or capital a and lowercase b so this is crucial as it changes the ratio of phenotypes so these are the genotypes that you are likely to get so they are resemble parental genotypes the next type of linkage is called sex linkage which is linkage that occurs on the sex chromosomes so sex linkage is when gene the gene is on the x chromosome so the or sometimes they are carried on the y chromosome but at a level you need to be more familiar with x linkage so the gene is situated on the x chromosome and if the allele carried on the x chromosome is a recessive allele this means that males are more likely to have them for example hemophilia because they only require one copy of the sex-linked allele recessive allele for expression instead of two this is because males have one x and one y chromosome to make up their six chromosomes and that's the recessive allele is carried on the x chromosome and males only have one x chromosome this means that they only require one copy for it to be expressed obviously obviously for females they need to carry two and when males have a an x-linked recessive condition this means that they have inherited it from their mother as males inherit their only inherit their y chromosome from their father so here is a cross-involving um linkage so here we have crossed a um dominant male so the degener x capital h y and we have a heterozygous female so x capital h x lowercase h the lowercase h indicates the disease allele which is recessive so we have 25 is a normal female which is homozygous dominant 25 is a normal um female but is it is a carrier as it is heterozygous 25 is a normal male as this male has inherited the dominant allele and 25 percent hemophiliac male as this male has inherited the sex-linked recessive allele from their mother obviously if he had a y-linked disorder then this means that the male would inherit it from the father and females could not have the disease at all as females have two x chromosomes and they don't have a y chromosome at all so now i'm going to talk about this concept called epistasis epistasis is an interaction between two genes where one gene masks the expression of the other gene in the phenotype the suppressing gene so the gene that causes the epistasis is called the epistatic gene and the gene that is being masked or suppressed by the epistatic gene is called the hypostatic gene and these epistatic interactions can be either dominant or recessive in dominant epistatis expression of a dominant allele so if one or two of the dominant alleles are present of the episodic gene this means that the expression of the hypostatic gene is always repressed and in recessive epistasis two copies of the episodic allele are required to amass the expression of the hypostatic gene so let's look at an example so here we have a cross between two mice and the um genotype capsule a capital a or capital a lowercase a produces an aguti color which is like a brown color homozygous recessive individuals are albino and then the capital b allele also produces a negoti um phenotype but a capital b produces a black phenotype but as you can see by this dihybrid cross here the ratios aren't normal so they are in a ratio of nine to three to four instead of nine to three to three to one this is because well this is an example of recessive epistasis as when two copies of the lowercase a so that albino alleles are present these form epistasis over the lowercase b allele so as you can see by these four individuals here they are all albino despite the presence of the well if you look at this individual this is albino because the two copies of the lowercase a allele mask the expression of both lowercase b alleles so this is why this individual is albino instead of black if epistasis didn't occur then this individual would be black as two coppers of the black allele are present however episodes occur so it is albino so the main ratio that you need to know for epistasis while recessive epistasis is nine to three to four so the last bit of content that i am going to talk about is the chi-squared test which is a statistical test that we use a lot in biology the chi-squared test determines the probability of an unexpected result being due to chance or being significant and we base the chi-squared test on a null hypothesis the null hypothesis is n is explains that any difference that occurs between observed and expected result is due to chance so it's not significant and this is the formula formula that we use to calculate a chi-squared value so to calculate chi squared we calculate the sum of observed results from a minus minus expected result squared divided by expected results so o equals observed results and e equals expected results so now i'm going to go through a couple of examples so here i have done an experiment in which you flip a coin which would either land on heads or tails and i have done this 50 times and my results so my observed results were 36 heads and 14 tables so now you need to figure out your expected results so as there are two categories and they have an equal chance of landing on each other then your expected results will be 25 each as we have 50 flips in total so now you need to figure out the observed minus expected so 36 minus 25 is 11 and 14 minus 25 is minus 11. square those results and they will both be 121 as if you multiply a negative number by a negative number you get a positive number and if we divide this value by the expected results we both get 4.84 and if we add this together we get our chi-squared value of 9.68 now another example would be the round and ring called and phenotype that we mentioned earlier when we were talking about dihybrid crosses so if we had 100 of these peas and we observe that 80 of them were round and 20 were wrinkled so to calculate your expected results you need to have knowledge of monohybrid crosses the monohybrid crosses produce a three to one ratio of dominant two recessive phenotypes as the round phenotype is dominant to wrinkled then we would expect that the values if we had 100 ps would be 75 round and 25 wrinkled and if we minus 75 and 25 from the observed results we would get 5 and minus 5 respectively square then we will get 25 and then if we divide them by the expected results for round p's we'll get 0.33 and for wrinkles we would get one add these together and we would get our chi-squared value as 1.33 but how can we tell if these results are significant so i'm going to use our example of the coin flipping experiment first we need to look at this table which looks very complicated but it's not really that complicated once you explain it so first you need to look at what we call the degrees of freedom which is this section here this row the degrees of freedom are the number of categories minus one so we have two categories in that our experiment heads and tails so if we take away one from that we would have a degree of freedom of 1. so here we have our chi-squared value of 9.68 now the second thing that we need to do is to find the critical value for the degree freedom of 1. the critical value is the value at a five percent chance that the results are due to chance so the 0.05 value here represents five percent so we need to look along degree of freedom of one to where it says not 0.05 sometimes the 0.05 or 5 and value here is known as the p value so here we have our critical value here which is 3.84 but our chi-squared value is above above the critical value and it actually goes off the scale so our chi squared value is above the critical value so we can conclude that our results are significant this is because our results are less than five percent likely to be due to chance this is because the um on the top row here the numbers go down so two point five percent then one percent so less than five percent likely to be to be due to chance so when something is less than five percent likely then the results are significant if they are more than four more than five percent likely then they are not significant this means that we can reject the null hypothesis as a null hut null hypothesis suggests that the difference between observed and expected results is not significant so next if we take our example from a round and recalled piece experiment our chi-squared value is 1.33 and obviously that was two categories so our degree of freedom is one again but this time the um chi-squared value is below the critical value as 1.33 is below 3.84 when a chi-squared value is less than the critical value we can say that our results are not not significant and they are more than five percent likely to be due to chance because if you look along the top row here that's 10 and 90 95 etc etc this means that we can accept the null hypothesis this time i hope that makes some kind of sense to you so that is all the content for this video and now we are going to get onto some exam style questions so the first question says in birds males are xx and females are xy so this is the opposite to humans use this information to explain why recessive sex-linked characteristics are more common in female birds than male melbers so this is basically asking you to explain the idea of sex linkage so as we said when a sex characteristic is recessive this means that the individual carrying only one x chromosome only has to have one copy of the recessive allele to display the sex-linked characteristic this so this is what i've written females only need one recessive allele for the characteristic to be expressed in the phenotype so let's look at the march scheme so you could have put the recessive alleles always expressed in females or females have one recessive allele all males need two assessor values or males need to be homozygous recessive or males could have dominant and recessive alleles or be heterozygous or carriers so i've written that females need one recessive allele so we would get the mark in this question also it accepts y chromosome does not carry a dominant allele so you could have written that to get the mark also it says other answers must be in the context of an allele not chromosome or gene so it is important that you refer to alleles not chromosomes or genes to get the milk so let's move on to the next question in chickens a gene on the x chromosome controls the rate of feather production the allele for slow feather production f is dominant to the allele for rapid feather production lowercase f the following figure shows the results produced from crosses carried out by a farmer so male chickens are xx and female chickens are xy so this diagram here is what we call a pedigree diagram so we have the main parents here and their offspring and then the offspring of the offspring so here we have the key so a filled in square means a male with rapid feather production an empty square means a male with slow feather production a circle filled circle means a female with rapid feather production and the unfilled circle means a female with slow feather production so let's look at the question then the question says explain one piece of evidence from the figure which shows that the allele for rapid feather production is recessive so if we look at the pedigree diagram here we see that one and two so the parents have both have slow further production so this means that they carry at least one dominant allele as a dominant allele in kosovo so feather production however you can see that number five here has the is homozygous um recessive so has rapid feather production this means that um both one and two must be heterozygotes so this is what i've written in my answer one and two have slow feather production whilst five has rapid feather production which shows that one must be a carrier of the recessive angle so actually one and two are not heterozygous as this is an x-linked condition so as the five is a female this means that the um um recessive excellent characteristic must have been inherited from the father as females inherit their x chromosome from the father in the case of chickens so let's look at the mark scheme here the first mining point says one two and five so you need to have mentioned these in your answer we mentioned them all so we'll get that mark also it accepts for one mark that one and two have slow feather production but produce one offspring with rapid feather production we actually put this so we would get the mark also it says neutral any reference of three being offspring of one so the examiner doesn't really want you to put this however it is still technically right and the second marking point says one must possess or pass on the recessive allele so the allele term here is underlined so you need to refer to that to get the mark or one must be a carrier heterozygous or if the slow feather production is recessive all offspring of one and two would be slow or if rapid further production was dominant one would have rapid feather production so you put any of these points to get the mark we wrote that one must be a carrier of the recessive allele so we will get this all two marks for this question also it says reject both parents also be carriers such possessor recessive allele as this is asking you about sex-linked conditions also it says reject one of the parents must be a carrier or heterozygous so if it's not specified you can't get any marks so as these two points here say reject then if you put any of these points you don't get any marks at all for the question even if one of the marking points was correct so let's move on so the next question is give all the possible genotypes for the following chickens from the figures so chicken five and chicken seven so if i just get my pencil out so here is chicken five which is a female rapid feather production and then chicken seven which is a male with slow feather production as the female has rapid feather production this means that it carries a recessive allele and female chickens have an xy genotype so this means that the female carries one recessive allele so the genotype for chicken five must be x lowercase fy however chicken 7 could have a number of possibilities so obviously it can't be homozygous recessive as it has slow feather production and not a rapid further production but these are the potential genotypes that could be x capital f x lower case f so it could be a heterozygote because um one of its um siblings has the um rapid feather production condition or it can be homozygous dominant so x capital f x capital f please note that these capital s are supposed to be in superscript however i couldn't figure out how to do it so let's look at the mark scheme so 5 is x lowercase fy or you can just write lowercase f or locus fy we wrote this so we would get the that mark so for seven you could write x capital f x lowercase f and x um capital f x capital f either way around we write both of these so we'll get all too much for this question here it says note allow five equals x in lowercase f y x lowercase f fy so you can put them twice to get a mark also here it accepts for both 5 and 7 a different letter than f however lowercase and capital letter must be corresponding to that shown in the answer so you don't have to use the letter f you can use any letter in the alphabet but we use f normally because f stands for feather but it doesn't really matter so the next question says a cross between two chickens produce four offspring two of these were males with rapid feather production and two were females of slow further production give the genotypes of the parents so the males of rapid feather production are obviously homozygous and recessive as the male chickens carry two x chromosomes and females with so feather production must be um just carry one dominant alley also capital f as they only carry one x chromosome this means that the genotypes of the parents must be um well the father must be in heterozygote so the father is x capital f x lower case f and the mother is x lowercase f y because um obviously the recessive arrow needs to be carried by both parents for a male to have rapid feather production as the male needs two copies of the recessive allele to express the um rapid feather production in its phenotype so if we look at the mark scheme so we got it right so we it says x capital f x lowercase f and x lower case f y so we've got the mark for this question so you can put any one of these combinations to get the mark down here it says accept a different letter than f so again you can use any other letter the alphabet but it prefers if you use the letter f to denote feather right that is all i want to say for this video thank you very very much for watching if you have any questions big or small please leave them in the comments i'll be more than happy to answer them and also i'll see you in the next video