Question

Question 3 (5 points, plus 5 extra credit points). Imagine you want to breed cats with short legs called munchkins. The munchkin phenotype is dominant. However, you have never been able to get a pure breeding population of munchkins! Whenever you cross two munchin cats, you find that 2/3 of the kittens are munchkins and 1/3 are normal sized. A. (2.5 points) What must be the genotypes of the parents and the F1 Offspring? B. (2.5 points) Provide an explanation for the non-Mendelian ratios observed in the F1. C. (2.5 points). How could you test your explanation in part B. D. (2.5 points) If you crossed two F1 munchkins, what ratio of their progeny do you think would be munchins? Show your work for credit.

Answer 1

Since munchkin is dominant but we can never get a pure breeding line of this. Meams we can have only heterozugous line not homozygous.

A.

The genotype of parental generation will be Mm × Mm

Punnet square:

Gametes	M	m
M	MM	Mm
m	Mm	mm

Since we can't get homozygous for munchkin means MM is lethal.

And Mm will be munchkin that can survive. And mm will be normal sized cat.

So, the genotype of F1 progeny will be Mm and mm because MM can't survive as its lethal when both M and M comes togather.

B.

The expected genotypic ratio should be 1:2:1

But the observed ratio is 2:1

The deviation from Mendalian ratio is because of the lethality when both M comes togather. MM genotupe dies in very early development. Allele M is lethal when homozygous but not when heterozygous. Thus it is known as recessive lethal allele.

C.

The explaination can be tested because if we will cross two munchkin with homozugous character we would not get normal sized cats.

Gamete	M	M
M	MM	MM
m	Mm	Mm

All the cats would be munchkin with genotypic ratio 1:1 but we are getting normal sized cat also. Means no progeny is homozygous for M.

D.

If we will cross two F1 munchkin cats: Mm × Mm

Punnet square:

Gamete	M	m
M	Mm	Mm
m	Mm	mm

Again we will get 2/3rd of the pregeny as munchkin. And 1/3 will be normal sized cats. MM will not survive due to lethality.

The ratio will come out to be 2:1 for Munchkin : normal size