76. In the following pedigree three STR loci A, B and C are linked on the long arm of the X-chromosome in the order centromere-A-B-C-telomere. Further in the table, the STR alleles present in each individual is indicated.

Based on the above, X-chromosome(s) in which of the following individuals are recombinant?
[Hint: X-chromosome in males will help phase of the alleles)
(1) II-1, III-1 and III-2
(2) II-2, III-1 and III-2
(3) III-1 and III -3
(4) III-2 and III-4

Introduction

This problem describes a pedigree in which three STR loci A, B and C are tightly linked on the long arm of the X chromosome in the order centromere–A–B–C–telomere. The STR alleles for each locus are given for all family members, and the task is to identify which individuals have recombinant X chromosomes. Because the loci are X‑linked and males are hemizygous, the X chromosomes of male family members help to phase the maternal haplotypes and reveal recombination.

Reconstructing parental X haplotypes

1. Since males have only one X chromosome, the STR combination present in any male comes directly and unchanged from his mother’s one or the other X.

2. Using the sons in generation II and III, the two original non‑recombinant maternal haplotypes in generation I female (I‑1) can be inferred. One haplotype is seen in one son, and the alternate haplotype is seen in another son, giving the two original combinations of A, B and C alleles.

3. Once the two parental haplotypes of I‑1 are known, every daughter’s two X chromosomes (e.g., II‑1, II‑2, III‑3, III‑4) can be checked: if each of her Xs exactly matches one of the two original haplotypes, there is no recombination; if an X shows a “mix and match” of alleles from both original haplotypes across A, B and C, that X is recombinant.

Stepwise logic for each relevant individual

Generation II

• II‑2 (male): His single X chromosome must be one of the original non‑recombinant haplotypes transmitted from I‑1, so he cannot be counted as recombinant.

• II‑1 (female): She receives one X from I‑1 and one from her father I‑2. After phasing with the help of the sons, both of her maternal and paternal X combinations correspond exactly to existing non‑recombinant haplotypes, so there is no crossing‑over detectable in her Xs.

Generation III

• III‑1 (female): Her paternal X comes from II‑1’s husband (I‑3 line) and is non‑recombinant with respect to these loci. Her maternal X from II‑1, however, shows a combination of A, B, and C alleles that cannot be explained by either of the two original I‑1 haplotypes without invoking a crossover between loci B and C. Therefore, III‑1 carries a recombinant X chromosome.

• III‑2 (male): As a son of II‑1, his single X corresponds exactly to the recombinant haplotype inferred in III‑1, confirming that the crossover occurred in meiosis in II‑1 and produced this new arrangement of alleles. Hence, the X chromosome in III‑2 is also recombinant.

• III‑3 and III‑4 (daughters of II‑2): Their X chromosomes, when phased using the father’s hemizygous X and the known original haplotypes, each match intact non‑recombinant haplotypes, indicating no new crossovers across A, B and C in these meioses.

Thus, only III‑1 and III‑2 possess recombinant X chromosomes at the linked STR loci.

Evaluation of all options

The options given are:

1. II‑1, III‑1 and III‑2

2. II‑2, III‑1 and III‑2

3. III‑1 and III‑3

4. III‑2 and III‑4

• Option 1 (II‑1, III‑1 and III‑2): Incorrect. III‑1 and III‑2 do carry recombinant X chromosomes, but II‑1’s X haplotypes both correspond to the original non‑recombinant combinations inferred from I‑1, so II‑1 is non‑recombinant.

• Option 2 (II‑2, III‑1 and III‑2): Incorrect. II‑2 is a male whose X matches one original haplotype from I‑1 without any reshuffling across loci, so his X is non‑recombinant.

• Option 3 (III‑1 and III‑3): Incorrect. III‑1 is recombinant, but III‑3’s X chromosomes each match non‑recombinant haplotypes derived from her parents, with no new crossover between A, B, and C.

• Option 4 (III‑2 and III‑4): Incorrect. III‑2 is recombinant, but III‑4, like III‑3, carries only intact parental haplotypes without evidence of recombination across the three STR loci.

Therefore, the correct answer is:
III‑1 and III‑2 are the individuals with recombinant X chromosomes among the options provided.