Q.13 In mice, a trait is determined by a dominant allele Y and recessive allele y. What
proportion of the offspring from a YY × yy cross is expected to be homozygous
recessive in F1 generation?
(A) 0
(B) 0.25
(C) 0.5
(D) 1
The correct answer is (A) 0. In a YY × yy cross, all F1 offspring are heterozygous Yy, so none are homozygous recessive (yy).
Punnett Square Analysis
The YY parent produces only Y gametes, while the yy parent produces only y gametes. Every fertilization results in Yy offspring, confirming 100% heterozygous and 0% homozygous recessive in F1.
Option Explanations
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(A) 0: Correct, as no yy combination forms in F1 from pure dominant × pure recessive cross.
-
(B) 0.25: Incorrect; this 1:4 ratio occurs in F2 from Yy × Yy self-cross, not F1.
-
(C) 0.5: Incorrect; a 1:2 ratio (50%) appears in testcross (Yy × yy), not this parental cross.
-
(D) 1: Incorrect; full recessive requires yy × yy parents, producing only yy offspring.
In mice genetics, the YY × yy cross represents a classic monohybrid cross where Y (dominant allele) determines the trait and y (recessive allele) does not express unless homozygous. This cross produces uniform F1 offspring, making the YY yy cross F1 homozygous recessive proportion key for understanding Mendelian inheritance. CSIR NET aspirants often encounter such questions to test Punnett square application and genotypic ratios.
Genetic Cross Mechanics
Homozygous dominant YY yields only Y gametes; homozygous recessive yy yields only y gametes. The Punnett square shows:
| Y | Y | |
|---|---|---|
| y | Yy | Yy |
| y | Yy | Yy |
All F1 are Yy (heterozygous), so YY yy cross F1 homozygous recessive proportion equals 0. No yy forms without recessive gametes from both parents.
CSIR NET Relevance
For Genetics CSIR NET preparation, recognize F1 uniformity in pure-line crosses versus F2 segregation (3:1 phenotypic). Options like 0.25 confuse with self-pollination; master these distinctions.
Common Pitfalls
Tricky cases involve lethal alleles (e.g., yellow mice YY lethal), but here no lethality mentioned—pure Mendelian. Practice distinguishes F1 from testcross ratios.
