Two different antigen molecules that occur on the surface of blood cells of wild

Question

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.

What are the complete genotypes of the two parental strains? Select two.

aabbcc

PartB

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

PartC

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?

PartD

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?

PartE

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?

PartF

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?

aabbcc

PartB

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

PartC

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?

PartD

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?

PartE

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?

PartF

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?

Explanation / Answer

Part A:

The genotype of the two parental strains would be aaBBcc (codes only antigen 1) and aabbCC (codes only antigen 2). The offspring would be aaBbCc that code for both antigens 1 and antigen 2.

Part B:

When two F1 mice are crossed the cross would be as follows:

The offspring obtained by crossing the F1 individuals would be:

aaBBCC, aaBbCC, aaBBCC, aaBbCc, aaBbCc, aabbcc, aaBbcc, aabbCc, aaBBCc, aaBbcc, aaBBcc, aaBbCc, aaBbCC, aabbCc, aaBbCc, aabbCC.

Therefore, the offspring with both antigen 1 and 2 are: aaBBCC, aaBbCC, aaBBCC, aaBbCc, aaBbCc, aaBBCc, aaBbCc, aaBbCC.

Hence, a total of 8 individual offsprings out of 16 express both antigen 1 and antigen 2.

Part C:

Therefore, from the above cross the offspring with only antigen 1 are: aaBbcc, aaBbcc, aaBBcc.

Hence, 3 out of 16 individuals are expected to contain only antigen 1 without antigen 2.

Part D:

From the above cross between F1 individuals the proportion of individuals which contain only antigen 2 are: aabbCc, aabbCc, aabbCC.

Hence, 3 out of 16 individuals are expected to contain only antigen 2 without antigen 1.

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