Two different antigen molecules that occur on the surface of

Two different antigen molecules that occur on the surface of blood cells of wild-type mice are produced through this biosynthetic pathway.

Mice that are homozygous for null mutations in the gene that encodes enzyme A (genotype aa) don\'t make either antigen 1 or antigen 2. Mice that are homozygous for null mutations in the gene that encodes enzyme B (genotype bb) don\'t make antigen 1. Mice that are homozygous for null mutations in the gene that encodes enzyme C (genotype cc) don\'t make antigen 2. All three genes are unlinked.

Two different true-breeding strains of mice have been isolated that don’t make either antigen 1 or antigen 2. When an individual from one strain is crossed with an individual from the other strain, all of the F₁ mice produce both antigens.

Part A

What are the complete genotypes of the two parental strains?

Select two.

Select two.

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Part B

Two of the F₁ mice are crossed to one another. What proportion of the F₂ progeny will produce both antigen 1 and antigen 2?

Enter your answer as a fraction.

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Part C

If F₁ mice are crossed to one another, what proportion of the F₂ progeny would be expected to produce antigen 1, but not antigen 2?

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Part D

If F₁ mice are crossed to one another, what proportion of the F₂ progeny would be expected to produce antigen 2, but not antigen 1?

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Part E

If F₁ mice are crossed to one another, what proportion of the F₂ progeny would be expected to produce neither antigen 1 nor antigen 2?

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Part F

Among the F₂ progeny, there will be mice of several different genotypes that are phenotypically antigen 1^– and antigen 2^–. If you wanted to use a testcross in order to test whether any individual F₂ mouse that doesn’t produce either antigen is defective in the production of enzyme A, what would be the best genotype for the tester mouse?

List the gene in your answer in alphabetical order with no spaces between them. For example: DDEEFF, DdEdFf, ddeeff, etc.

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AABBCC

AAbbCC

AABBcc

AAbbcc

aaBBCC

aaBBcc

aabbCC

aabbcc

enzyme A enzyme B antigen 1 intermediate enzyme antigen 2

Solution

(1) As the offspring are able to produce both the antigens A and B, then each parent must be able to complement each other

Parent one genotype   AAbbcc

Parent two genotype   aaBBCC

This combination only will be able to complement each other. The offspring will be AaBbCc and can produce both the antigens.

(b) Now you want to know the fraction of offspring which can produce both antigen A and B. Only offspring with genotype A-B-C- can produce both the antigen. Now from the trihybrid cross, using the fork method, we can easily calculate

Now you have crossed F1 offspring   AaBbCc X AaBbCc

(3/4)*(3/4)*(3/4)= 27/64 can produce both antigen A and antigen B

(C) Now you want to know the fraction of offspring which can produce only antigen 1, but not antigen 2

Then the genotype should be A-B-cc

(3/4)*(3/4)*(1/4) = 9/24

(D) Now the fraction of offspring which can produce only antigen 2, but not antigen 1

(3/4)*(1/4)*(3/4) = 9/64

(E) Now to know the fraction of offspring which can neither produce antigen 1 and antigen 2

1-(27/64+9/64+9/64)=19/64

(F) The best genotype for a testcross mouse is aaBBCC, as this is deficient for enzyme A, so when it is crossed with any mice with a good copy of enzyme A, offspring will produce antigen 1 and antigen 2.

Now the possible genotypes that would give antigen 1 and antigen 2 are aabbcc, aaB-cc, aaB-C-, aabbC- and A-bbcc.