This problems illustrates why classical geneticists in the...

This problems illustrates why classical geneticists in the days before DNA analysis usually needed to work with traits showing complete penetrance. where the A-B genotypic class normally produces purple flowers and all other genotypic classes have white flowers. A- If the parental generation is AA bb * aa BB what phenotypic ration do you expect in the F2 generation assuming complete penetrance ? B- suppose now that only 75% of A-B individuals have purple flowers (that is, the penetrance of this trait is 75%). what phenotypic ratio do you now expect among the F2 plants? C- in doing these types of crosses, what kinds of results (other than an unexpected F2 ratio) might suggest that penetrance of the purple phenotype is incomplete?

Transcript

00:01 Is regarding a p, sweet p, shown in figure, where a dominant a and b genotype shows a purple flower and all the other genotypic flowers have white flower.

00:15 You'll have a parental cross.

00:19 You have homozygous capital a lower b and cross with homozygous lower a and capital b.

00:30 And what phenotypic ratio do you expect in the f2 generation, assuming complete penetrance? so all f1 is gonna show the same phenotype because these parents are true breed.

00:42 The parents on the left will have only one gametes, capital a and lower b.

00:47 And the parents on the right is gonna have lower a and a capital b.

00:50 So the all f1 is gonna have heterozygous a, heterozygous b.

00:56 And then of course you have a cross right here.

01:02 So from there you can see that this is a very classic, the hybrid cross was two genes, both are heterozygous.

01:11 So from our knowledge, we know that for f2, we have a planet square, which show that you have a nine to three to three ratio.

01:25 Nine is at least one capital a and capital b.

01:30 And this is gonna be nine.

01:32 And then you'll have homozygous a, sorry, heterozygous capital a and homozygous lower b.

01:41 This is gonna be three.

01:42 Or the other way around, you have homozygous lower a and at least one capital b.

01:47 This is gonna be three.

01:48 And then you'll have homozygous lower a and homozygous lower b, which is gonna be one.

01:53 So you can see according to our question, only this category is gonna show purple and the rest of them will all show white.

02:03 So we know that if we have a 100 % penetrance, which means that 100 % of individual that has a one capital a and one capital b will show purple.

02:14 So the ratio is gonna be purple versus white equals nine versus seven.

02:26 Three plus three plus one equals seven.

02:29 So purple and white will be nine to three ratio when you have a 100 % penetrance.

02:39 The penetrance means that 100 % individual has certain genotype will show a phenotype.

02:45 Now for question b, it says that, what phenotypic ratio do you expect among f2 if you have a 75 % penetrance? so again, we have the same ratio right here.

02:59 And now this time it's only 75 % of penetrance...

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