Question

In: Biology

In a cross of two flies +/vg Cy/+ +/se +/ab X +/vg +/+ se/se +/ab what...

In a cross of two flies +/vg Cy/+ +/se +/ab X +/vg +/+ se/se +/ab what proportion of the offspring will be mutant in phenotype for all four markers?

ANSWER: 1/64

Please explain the process

Solutions

Expert Solution

+/vg Cy/+ +/se +/ab

+/vg +/+   se/se +/ab

vg, Cy, se and ab are the mutant alleles for the four genes. Upper case of the first letter in the gene indicates a dominant phenotype. Lower case of the first letter in the gene indicates a recessive phenotype.

Consider the Punnett squares for crosses for each gene separately:

a. +/vg X +/vg

Vestigial gene is inherited in recessive fashion. Hence, two copies of vg should be present to see mutant phenotype.

+/vg

+

vg

+/vg

+

+/+

Homozygous dominant

Wild type phenotype

+/vg

Heterozygous

Wild type phenotype

vg

+/vg

Heterozygous

Wild type phenotype

vg/vg

Homozygous recessive

Mutant phenotype

Vg/Vg is the mutant phenotype.

Probability of Vg/Vg is 1/4.

b. Cy/+ X +/+

Cy gene (curly wings) is dominant inherited gene and a mutant phenotype will be seen with only one mutant allele (Cy/+ or Cy/Cy).

Cy/+

Cy

+

+/+

+

Cy/+

Heterozygous

Curly wing mutant phenotype

+/+ recessive

Homozygous

Wild type phenotype (no curly wings)

+

Cy/+

Heterozygous

Curly wing mutant phenotype

Homozygous recessive

Wild type phenotype (no curly wings)

Probability of mutant phenotype (Cy/+) = 2/4= 1/2

c. +/se   X se/se

Se is sepia brown phenotype gene and is inherited in recessive fashion.

+/se

+

se

Se/se

se

+/se

Wild type phenotype

Heterozygous

se/se

sepia brown

Homozygous recessive

se

+/se

Wild type phenotype

Heterozygous

se/se

sepia brown

Homozygous recessive

Probability of mutant phenotype= se/se= 2/4=1/2.

d. +/ab X +/ab

The ab (abrupt) gene is recessive. Hence, two copies of mutant ab alleles are required to inherit the phenotype.

+/ab

+

ab

+/ab

+

+/+

Homozygous dominant

Wild type phenotype

+/ab

Heterozygous

Wild type phenotype

ab

+/ab

Heterozygous

Wild type phenotype

ab/ab

Homozygous recessive

Mutant phenotype

Probability of mutant phenotype= ab/ab= 1/4

Hence,

Probability of obtaining progeny flies that are mutants for all genes in the +/vg Cy/+ +/se +/ab X +/vg +/+   se/se +/ab cross= Multiplication of individual probabilities for each gene

Probability of obtaining progeny flies that are mutants for all genes in the +/vg Cy/+ +/se +/ab X +/vg +/+   se/se +/ab cross= 1/4 X 1/2 X 1/2 X 1/4= 1/64

Thus, probability of offspring’s that are mutant for all genes= 1/64.


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