Questions: Gene Conversion and Nonreciprocal Recombination

Question 1

In tetrad analysis of a heterozygous yeast strain (Aa), a researcher observes 3 A spores and 1 a spore from a single meiosis rather than the expected 2:2 ratio. This 3:1 ratio is most directly explained by:

• A. Nondisjunction during meiosis II, causing one A chromosome to be mis-segregated
• B. A crossover occurring between the A and a loci that swapped the entire chromosomal arm
• C. Gene conversion: mismatch repair of heteroduplex DNA replaced the a allele sequence with a copy of the A allele sequence on one chromatid
• D. The a allele having a growth disadvantage that caused the a spore to divide fewer times

Answer: (C)

A 3:1 spore ratio (instead of the Mendelian 2:2) is the hallmark of gene conversion. During recombination, strand invasion creates heteroduplex DNA where one strand comes from each homolog. If the two alleles differ at a nucleotide in this heteroduplex region, a mismatch forms. Mismatch repair can resolve this by overwriting the a sequence with the A sequence (or vice versa), converting one chromatid's allele. Nondisjunction would produce aneuploid spores, not a 3:1 ratio. A simple crossover would still give 2:2 ratios at the point of exchange.

Question 2

Biased gene conversion at a locus in a large population favors GC alleles over AT alleles during mismatch repair. What population-level consequence would you expect over many generations?

• A. GC alleles would decrease in frequency because they are more likely to be repaired away
• B. No change in allele frequencies, because gene conversion is random by definition
• C. GC alleles would increase in frequency regardless of their fitness effects, because the repair bias acts as a weak directional force independent of natural selection
• D. GC alleles would increase only if they also confer a selective advantage

Answer: (C)

Biased gene conversion acts like directional selection: in every heterozygote, mismatch repair slightly favors copying the GC allele onto the AT chromosome rather than the reverse. This directional repair bias systematically shifts allele frequencies toward GC at recombination hotspots over generations, independently of whether GC alleles are beneficial, neutral, or even slightly deleterious. This is why recombination hotspots tend to be GC-rich in the genome — not because GC content has a functional advantage there, but because of the physical chemistry of mismatch repair.

Question 3

In standard reciprocal crossing over between homologs, a heterozygote (Aa) produces gametes in a 2:2 ratio (2 A and 2 a), while gene conversion at the same locus can produce a 3:1 or even 4:0 ratio.

• T. True
• F. False

Answer: True

Reciprocal crossing over physically exchanges chromosomal segments between homologs symmetrically — what one chromosome gives, the other receives. A heterozygote therefore still has one A chromatid and one a chromatid after crossing over, giving 2A:2a in the final products. Gene conversion is nonreciprocal: one allele overwrites the other without an equal exchange in the opposite direction. Mismatch repair converts one copy of (say) the a allele to A, yielding 3A:1a. In principle, if both heteroduplex mismatches are repaired the same way, a 4:0 ratio is possible, though rare.

Question 4

Gene conversion involves an unequal physical exchange of chromosomal segments, where one chromosome receives more sequence than it gives to its homolog.

• T. True
• F. False

Answer: False

Gene conversion does not involve a physical exchange of chromosomal material at all. It is a nonreciprocal information transfer: during mismatch repair of heteroduplex DNA, one strand is rewritten to match the other, but no chromosomal segment physically moves from one chromosome to the other. Both chromosomes remain intact in terms of segment composition — what changes is the sequence of one allele, overwritten to match the donor sequence. Unequal crossing over (a different event) does involve asymmetric physical exchange, but that is distinct from gene conversion.

Question 5

Explain how heteroduplex DNA and mismatch repair together produce gene conversion, and why the information transfer is nonreciprocal.

Model answer: During meiotic recombination, Spo11 creates a double-strand break and the ends are resected to expose single-stranded tails. One tail invades the homologous chromosome and base-pairs with the complementary strand, displacing the other strand. The region where one strand comes from one chromosome and the other strand from the homolog is called heteroduplex DNA. If the two chromosomes differ at a nucleotide within this region, the result is a mismatched base pair. Mismatch repair enzymes detect and correct this mismatch, but they can only rewrite one strand to match the other — not exchange information symmetrically. If the invading strand is rewritten, the original allele is restored; if the resident strand is rewritten, it now carries the invader's sequence. Either way, one allele has been overwritten by the other — gene conversion — and information flowed in one direction only.

The nonreciprocity follows directly from the repair mechanism: repair converts the mismatch to a Watson-Crick base pair by editing one strand, not by swapping segments. The chromosome that 'donated' the sequence is unchanged; only the 'recipient' chromosome is altered. This contrasts with reciprocal crossing over, where both chromosomes exchange physically equivalent segments and both are modified symmetrically.