42. In a family, father is homozygous dominant (AA) for a gene A and his wife is homozygous for its recessive allele (aa) showing albino phenotype. It was surprising that their child showed the albino phenotype. Which of the following phenomenon can explain the phenotype?
(1) Nondisjunction
(2) Uniparental Disomy
(3) Gene conversion
(4) All of theabove

Genotype logic

• Father: AA (both alleles dominant, normal pigmentation).

• Mother: aa (both alleles recessive, albino).

• Normal Mendelian expectation: all children Aa (pigmented carriers).

• Observed child: albino, so functionally aa at this locus.

For that to happen, the allele contributed by the father must effectively behave as a, even though his germline was genotypically AA.

Why option (3) gene conversion fits

Gene conversion is a non‑reciprocal recombination event where the sequence on one chromatid is “copied over” to its homolog, converting one allele to the other during meiosis.

If, during spermatogenesis in the father:

• One A allele is converted to a in a meiotic cell, that cell will produce gametes carrying a instead of A.

• Fertilization of such a paternal a gamete with maternal a gives an aa zygote → albino child.

Thus, gene conversion provides a direct molecular route for an apparent AA parent to transmit an a allele without the parent being genotypically Aa in routine testing.

Why the other options are less appropriate

1. Nondisjunction

   • Nondisjunction changes chromosome number (aneuploidy, e.g., trisomy, monosomy), not the identity of an allele on a given locus.

   • It might create gametes with extra or missing chromosomes, but it will not convert A to a or explain an AA parent transmitting a recessive allele at normal chromosome copy number.

2. Uniparental disomy

   • Child receives both copies of a chromosome from one parent, none from the other.

   • If this occurred here and the child got both maternal chromosomes, the genotype would be aa without needing gene conversion.

   • However, the question highlights a normal AA father and aa mother and asks which phenomenon explains the child’s phenotype; classic exam framing for this scenario points to sequence-level change in one parental allele, i.e., gene conversion, rather than a whole‑chromosome UPD event.

3. All of the above

   • Incorrect because nondisjunction alone cannot yield aa from AA×aa at this locus, and uniparental disomy is not generally grouped together with nondisjunction and gene conversion as a single explanatory category in such single‑gene questions.

Summary

A surprising albino (aa) child from AA × aa parents is best explained, at the single‑gene level, by gene conversion converting one paternal A allele to a during meiosis, so the correct option is (3) Gene conversion.