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An Introduction to Mendelian Genetics

Video transcript

an introduction to Mendelian genetics now before we start let's review the idea that human cells contain 46 chromosomes which contain the DNA that makes each cell unique and 23 of these chromosomes were inherited from a person's father and 23 were inherited from the mother so we can say that each person is made up of a combination of genetic code from both of their parents now sometimes we like to say that we have 23 pairs of chromosomes instead of saying that we have 46 total because that way we remind ourselves that for each chromosome you have a maternal and paternal copy now the first thing I want to introduce is the term allele now if we have a chromosome here then an allele is one small section on that chromosome that codes for a specific gene that makes you you now since humans have at least two copies of each chromosome we can say that humans usually have at least two alleles for every specific gene one allele from their mother and one from their father so let's look at an example and we'll start by talking about blood type now I'm sure that you've heard that blood types are usually named with letters like a B and O but what does that actually mean well there's a specific allele that codes for blood type and let's say that we have this guy here and his alleles both code for blood type a and I'll use the letter A for that and let's say we have this girl here who has one allele coding for a and another allele coding for blood type O now for the guy if he has both alleles coding for blood type a then it's pretty clear that when we check his actual blood type it will be a but for the girl we're not so sure since she has one of each so now I'm going to introduce a couple new terms to you and the first is that since the guy has two alleles that code with the same thing both code for blood type a then we say that this guy is homozygous and Homo means the two alleles are the same Homo the same and zygous refers to the mixture of DNA that he got from his parents so someone who is homozygous got the same allele from both parents now in the case of the girl is she going to have blood type A or blood type O well it turns out that she's going to have blood type A and that's because the aleo is the dominant allele while the o le o is the excessive allele when an allele is dominant that means that if someone has two different alleles it'll be the dominant one that wins so in this case since a is dominant over O which is recessive a will win and she'll have blood type-a now since this girl has two different alleles we call her heterozygous since hetero means different and zygous reversed the same thing the mixture of DNA that she got from her parents now I want to introduce two more terms we can describe a person's genes in two different ways we can look at the person's individual alleles and we call this the genotype so for this guy his genotype is a a referring to his two alleles which both code for blood type a but we can also look at a person's physical traits which we call the phenotype so for this guy and girl the phenotype would be blood type a so you can see that genotype and phenotype are different but it is possible for two different genotypes to make this same phenotype since some alleles are dominant over others now let's talk about gene inheritance for a bit so let's say that our guy and girl from before have offspring together we can use something called a Punnett square to determine what different genotypes their kids could have now each of the parents two alleles are on separate chromosomes so each parent will contribute one of their two alleles to the child and the punnett square allows you to determine all possible combinations so if we take the father's alleles and line them up vertically and then take the mother's alleles and line them up horizontally we can fill in the chart to find the possible genotypes for our offspring in this case two of our boxes will have the a a in them and two will have a oh in them so that means that half of the children will have the genotype a a and half of the children will have genotype a oh but since both of these genotypes code for the same phenotype all of the children will have the blood type a phenotype now let's see what happens if we change our father's genotype to match our mothers genotype now only one quarter of the children will have the AAA genotype half will have the AO genotype since the order of the two alleles doesn't matter Oh a and a are the same and 1/4 will have the OO genotype so this means that 75% of the children will have blood type A in their phenotype since a a and a O make blood type a but 25% of the children will have the blood type O phenotype since oo makes blood type O so what did we learn well first we learned what an allele is and the difference between homozygous and heterozygous as well as the difference between dominant and recessive traits in relation to alleles second we learned about the difference between genotype and phenotype and how the genotype refers to a person's DNA while a phenotype reverse the physical traits that the DNA codes for and finally we learned about how we can use a Punnett square to determine how different alleles will be inherited from two parents