In: Biology
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
+/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 |
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+ |
vg |
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+/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/+ |
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Cy |
+ |
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+/+ |
+ |
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 |
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+ |
se |
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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 |
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+ |
ab |
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+/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.