7. A genetic cross was set between two heterozygous mice with agouti coat colour.
Agouti is dominant over normal coat colour. The F1 phenotypic ratio with regards to
coat colour (agouti: wild type) was 2:1. A possible explanation for this could be:
a. The agouti gene converted to wild type counterpart.
b. Homozygous lethality associated with the agouti locus.
c. Incomplete penetrance.
d. Variable expressivity.
A genetic cross between two heterozygous mice for agouti coat color (Aa x Aa), where agouti (A) dominates normal/wild type (a), normally expects a 3:1 phenotypic ratio. The observed 2:1 ratio (agouti:wild type) deviates due to homozygous lethality at the agouti locus, a classic CSIR NET genetics example. This occurs because AA homozygotes die embryonically, leaving only Aa (agouti) and aa (wild type) survivors in a 2:1 proportion.
Option Analysis
Each option explains the unusual 2:1 ratio in this agouti coat color mice genetic cross.
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a. The agouti gene converted to wild type counterpart: Gene conversion implies a mutation changing A to a, but this would not produce a consistent 2:1 ratio across F1; it suggests a permanent allelic shift, unsupported by Mendelian inheritance or lethality evidence.
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b. Homozygous lethality associated with the agouti locus: Correct explanation. In Aa x Aa crosses, Punnett square yields 1 AA:2 Aa:1 aa, but lethal yellow (A^Y) or similar AA causes embryonic death, reducing survivors to 2 agouti (Aa):1 wild type (aa), matching the 2:1 ratio precisely.
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c. Incomplete penetrance: This means some Aa genotypes fail to show agouti despite having the allele, potentially altering ratios, but it would not predict exactly 2:1; lethality provides a cleaner genotypic elimination fit.
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d. Variable expressivity: Refers to varying agouti intensity in Aa individuals, affecting phenotype degree but not eliminating classes to yield discrete 2:1; it explains variation, not deviation from 3:1.
Punnett Square Breakdown
For agouti coat color mice genetic cross (Aa x Aa):
| A | a | |
|---|---|---|
| A | AA (lethal, dies) | Aa (agouti) |
| a | Aa (agouti) | aa (wild type) |
Surviving F1: 2 agouti:1 wild type = 2:1 ratio, hallmark of homozygous dominant lethality in CSIR NET problems.
CSIR NET Relevance
This question tests lethal alleles and modified Mendelian ratios, common in CSIR NET Life Sciences genetics (Part B/C). Recognize 2:1 as lethality signature versus 3:1 standard dominance.
