an Austrian monk called Gregor Mendel used pea plants to discover the laws of inheritance this was long before anything was known about the structure of DNA or genes so in the 19th century his work was largely misunderstood but today he's known as the father of genetics so let's learn about his [Music] laws in the last video we learned a lot of the key words needed to understand the inheritance now we know how alol work we can use them to predict the genotypes and phenotypes of children or offspring let's look at eye color in these parents the mom's phenotype or physical characteristic is blue eyes and the dad's phenotype is brown eyes for the genotype we need to know which Ali is dominant and which is recessive remember you'll be told this or we'll be able to work it out from the information given in this situation the blue ey Al is recessive so we're going to label it with a lowercase letter a little B and so the mom's genotype is Little B little B the dad has a dominant Al which gets a capital B so his genotype is Big B little B but if they have a child how do we know what genotype and phenotype it can have well to do this we're going to use punet squares this this is just a genetic diagram and to start off it looks a little bit like a knot and cross's GD we'll start by splitting up the mum's Ali because a child only gets half of its genes from its mother so we split up the little be and the next little B we'll do the same for the dad for the same reason that the child gets half its DNA from his dad as well so we split up the Big B little B now we work out the combinations by reading down and across and we get Big B little B in the next box we get another Big B little B why not pause and see if you can work out the remaining two boxes in this one we get little B little B and it's the same in the last box great so now we have all the genotypes we'll work out the probability of each you could be asked to give this as a fraction or a percentage so we'll do both okay so you can see we've got two Big B little be's and two little B little be's that means we've got 50% of each or a half so we write the Letters Out Big B little B and we do the same for the little B little B now will write the phenotypes for each genotype you need to write them Below in the same order or beside the genotype so it's very obvious exam questions will normally structure this for you so the Big B little B will give you brown eyes as the brown eye Al is dominant the little B little B will give you blue eyes because you have two recessive alos lastly you can be asked to give the expected ratio this is one of the MTH skills you're required to know for your exam since it's both 50% as a ratio this is 1: one again you write it in the same order as you've given the phenotypes if you're studying higher tier you need to be able to make the punet squares yourself if you're studying Foundation you need to be able to complete one and the questions will be more structured I recommend you watch this either way as it's good practice okay let's read the question two smooth pee plants are crossed they are both heteros zagas work out the proportions and ratios of the genotypes and phenotypes for this cross cross just means they're reproducing together pause and grab some pap and try and work it out yourself then press play when you're ready to go through it okay so let's start with the parent genotypes they are both heterozygous which means they have different Al the two Al we've got are smooth is dominant and wrinkled is recessive we'll use the letter s the dominant one gets a capital and the wrinkled one gets a lowercase it doesn't matter what letter you use CU we haven't been given it so both parents genotype because their heterosis will be big S little s now we're going to do our cross so we start by splitting up each parent genotype into big S little s then we work out the different combinations if you didn't get to this point pause anyway and try and fill out the combinations okay so this box will be big S big S this is big ass little s another big ass little s and two little s es notice how I'm really emphasizing the size of the letters that's important okay now let's write out what Offspring genotypes we have we can see we've got one big S big S which is 25% we've got two big ass little asses which gives us 50% and we've got one little s little s which is 25% the question has asked us for the ratios of the genotype so if we work it out we've got 1 to two to one if you wrote your genotypes in a different order you may have your ratio in a different order just check carefully next we'll work out The Offspring phenotypes every offspring that has a dominant alel or a big S is going to be smooth but every offspring that has two little s's is going to be wrinkled we can see that we've got three offspring that have got the big dominant s so 75% are going to be smooth and that leaves 25% will be wrinkled finally we have to give this as a ratio and we compare 75 to 25 that's 3: 1 the last thing we're going to look at is sex inheritance this this person's phenotype is female and this person's is male sex is controlled by an entire chromosome as opposed to one or multiple genes in females the sex chromosomes are XX and in males they are X Y we can construct a punet square to show The Offspring probabilities doing this very quickly we can see that we end up with two XX Offspring and two XY Offspring so so the probability is 50% female and 50% male you have to memorize the sex chromosomes for females and Meals one tip I use to help me remember this is that my female friends tend to send me a little kiss kiss like an XX at the end of messages more than my meale friends do pause and try the quick questions then press play when you're ready to go through them one what does homo sagus mean this is having two of of theum alos 2 a complete this punet Square We've Got Big D Big D Big D Little D Big D Big D and Big D Little D B give each possible genotype as a fraction well you can see that we've got half Big D Big D and a half Big D Little D it adds up to one C give the possible phenotypes as a percentage so big D is no dimples and we can see that every possible Offspring has a big D so it is 100% no dimples and therefore 0% dimples four write the sex chromosomes of human females this is XX make sure those xes are really obvious how did you do next up we're learning about inherited disorders click here to watch and here to subscribe thanks and bye