13. Mendelian Genetics
Monohybrid Crosses
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In this video we're going to distinguish between the genotypic and phenotypic ratios. And so the genotypic ratio as its name implies, is going to be the ratio of different genotypes observed amongst the offspring. And so the characteristic genotypic ratio from a monohybrid cross is 1 to 2 to 1, where there is 1 homozygous dominant individual for every 2 heterozygous individuals for every one homozygous recessive individual. Now the phenotypic ratio, as its name implies, is going to be the ratio of different phenotypes of observed amongst the offspring. And so the characteristic phenotypic ratio from a monohybrid cross is 3 to 1, where there are 3 individuals that display the dominant phenotype and one individual that displays the recessive phenotype. And so, of course, previously we had done a monohybrid cross here, and, of course, the results of the monohybrid cross are in this Punnett square. And what you can see is that, once again, the genotypic ratio shows the ratios of the genotypes. So there is again 1 homozygous dominant individual, 2 heterozygous individuals or possibilities, and 1 homozygous recessive possibility. And so that 1 to 2 to 1 is coming straight from that. 1 homozygous dominant, 2 heterozygous, and 1 homozygous recessive. Now again, the genotypic ratio is different than ratio. The phenotypic ratio looks at the phenotypes, and the phenotypes are the colors of the peas. And so amongst the offspring, there are 3 possibilities for the offspring to be yellow and one possibility for the offspring to be green. And so, the ratio is 3 to 1. And those are the characteristic genotypic and phenotypic ratios of monohybrid crosses, and that's important to to keep in mind. So this here concludes our introduction to genotypic and phenotypic ratios, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.
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