today's video is sort of a continuation of our last video on dna and we're going to take a look at a whole bunch of different terms to see what an allele is and what the differences are between dominant and recessive homozygous and heterozygous and also genotype and phenotype now all of these terms are related to genes and remember a gene is just a segment of dna that codes for a particular type of protein together these genes we inherit determine the characteristics that we go on to develop sometimes these traits are determined by a single gene like the ones that code for fur color and mites and red green color blindness in humans more often though our characteristics are determined by several different genes that interact with each other for example there are loads of genes that code for height which is one of the reasons why we're all different sizes if we look back at the definition of a gene you'll notice that it codes for a particular type of protein the reason i say type of protein is that there are often multiple forms of the same protein and so there'll be a different genetic code for each of those forms and we call these different codes alleles so you can think of alleles as different versions of the same gene because we have two copies of every gene one from each of our parents it means that we'll have two alleles of each gene and these could both be the same allele or they could be two different alleles if they were the same we'd say they were homozygous for that allele whereas if they were different we would be heterozygous this brings a bit of a problem though say we had a mouse that was heterozygous for fur color with one allele coding for purple fur and the other for green fur what color would our mouse be well it's not going to be a mix of the two instead one of the alleles will be dominant and the other one will be recessive and it's always a dominant one that gets expressed so if the purple allele was dominant to the green allele which would make that green allele recessive then our heterozygous mouse would be purple this means that the only way to have a green mouse is if both of its alleles were for green fur which we would call homozygous recessive on the other hand the purple allele will always be expressed when it's present regardless of whether the mouse is heterozygous or homozygous for the purple allele now the last two terms that we need to cover are genotype and phenotype the genotype is basically the entire collection of alleles that we have so if we take the three mice that we used before the heterozygous mounts the homozygous dominant and the homozygous recessive one we would say that they all have different genotypes because they have different alleles and so different genetic codes however your phenotype is the characteristics that you get from your genotype so as both the heterozygous mouse and the homozygous dominant mouse are purple we would say that they have the same phenotype even though they had different genotypes meanwhile because our homozygous recessive mouse is green rather than purple it has a different phenotype as well as a different genotype anyway that's it for this video in our next one we're going to take a look at genetic diagrams so cheers for watching and we'll see you soon you