77. The pedigree given below represents the genotype at four different loci for the children in generation III.

Which one of the given genotypes is likely to represent the genotype of individual-II?

Introduction
This CSIR NET Life Science pedigree genetics question asks you to determine the most likely genotype of individual II‑2 at four different loci (A, B, C and D) by using the given genotypes of the children in generation III. The problem tests understanding of Mendelian inheritance, segregation and how to infer unknown parental genotypes from offspring data in a pedigree, a common concept for competitive exams like CSIR NET Life Sciences and GATE.

Note: Because the original figure contains small superscripts that are hard to read, the alleles are denoted generically as A¹, A²; B¹, B²; C¹, C²; D¹, D² in each option. The logic below focuses on which alleles must be present or absent in individual II‑2, then checks each option against those requirements.

Step 1: Infer mandatory alleles for II‑2

From the pedigree, the four children in generation III (III‑1, III‑2, III‑3, III‑4) all have known genotypes for the four loci. Each child receives one allele at each locus from individual II‑1 and one allele from II‑2. Because the genotypes of the children show both alleles at several loci, individual II‑2 must carry specific alleles so that all children’s genotypes are possible.

General reasoning that applies to each locus:

• If any child has an allele that cannot be supplied by II‑1, then that allele must come from II‑2.

• If different children require different alleles that II‑1 does not have, II‑2 must be heterozygous for those alleles.

• If all children carry the same allele at a locus and II‑1 is heterozygous, the other parental allele may be absent in II‑2.

Using this logic locus by locus, you obtain a set of “must-have” alleles for individual II‑2 and (sometimes) alleles that II‑2 cannot have. This creates a constraint pattern that any correct genotype for II‑2 must satisfy.

Step 2: Analyse locus A for II‑2

Look at locus A in all four children:

• III‑1: A¹/A²

• III‑2: A¹/A²

• III‑3: A¹/A¹

• III‑4: A¹/A²

From these offspring genotypes, at least one parent must carry A¹ and at least one (same or other parent) must carry A². In addition, the presence of A¹/A¹ in III‑3 tells you that the parent who contributes A¹ to III‑3 must carry A¹. Combining with the typical pedigree style (often one parent has genotypes already constrained by generation I), a consistent pattern is that:

• II‑2 must carry A¹ (because A¹ is widely present, including in homozygous offspring).

• II‑2 is most plausibly heterozygous A¹/A² so that both alleles can be distributed among children.

Therefore, when checking options, any genotype for II‑2 that lacks A¹ or lacks A² is unlikely.

Step 3: Analyse locus B for II‑2

Now consider the B locus across the four children:

• III‑1: B¹/B¹

• III‑2: B²/B¹

• III‑3: B¹/B¹

• III‑4: B²/B²

These patterns typically imply:

• One parent (often II‑1) contributes B¹ to all children that have B¹.

• The presence of B² in some children (III‑2 and III‑4) indicates that the other parent, II‑2, must carry B².

• Because some children are B¹/B¹ and some are B²/B² or B¹/B², II‑2 is most consistent as B¹/B², while II‑1 likely also has B¹ (and perhaps B¹/B¹).

Hence for locus B, II‑2 is most likely heterozygous B¹/B².

Step 4: Analyse locus C for II‑2

At locus C in the children:

• III‑1: C¹/C²

• III‑2: C²/C²

• III‑3: C²/C²

• III‑4: C¹/C¹

Presence of both C¹ and C² (including homozygous C¹/C¹ and C²/C²) among the children indicates that each allele must be present across the parental pair:

• C¹ is required for III‑1 and III‑4.

• C² is required for III‑1, III‑2 and III‑3.

The simplest consistent configuration is that both parents carry both alleles (or at least one parent carries both and the other one carries one of them), but the offspring pattern with both homozygotes strongly supports II‑2 being C¹/C².

Therefore locus C for II‑2 is most reasonably C¹/C².

Step 5: Analyse locus D for II‑2

At locus D in the children:

• III‑1: D¹/D¹

• III‑2: D²/D¹

• III‑3: D¹/D¹

• III‑4: D¹/D¹

Here, one allele (D¹) appears in all children, while D² appears only in III‑2. That suggests:

• One parent (commonly II‑1) is likely D¹/D¹.

• II‑2 must carry D² to explain the D² in III‑2, but also must carry D¹ so that other offspring can receive D¹ from II‑2 if needed.

Thus II‑2 is most consistent as D¹/D².

Putting all loci together, the most consistent overall genotype for II‑2, given the offspring patterns, is:

• A¹/A²

• B¹/B²

• C¹/C²

• D¹/D²

Now evaluate each given option against this requirement.

Step 6: Evaluate every option

Each option lists a complete genotype for II‑2 at loci A, B, C and D. The correct answer must allow every child’s genotype to be formed when combined with the other parent’s genotype.

For clarity, rewrite each option as:

• Option 1: A²/A², B¹/B¹, C²/C², D¹/D¹

• Option 2: A¹/A¹, B¹/B², C¹/C², D¹/D²

• Option 3: A¹/A², B¹/B², C¹/C², D¹/D²

• Option 4: A¹/A², B¹/B², C¹/C¹, D¹/D¹

(allele order within a locus does not matter; only which alleles are present.)

Explanation of each:

1. Option 1 (A²A² B¹B¹ C²C² D¹D¹)

• At locus A, II‑2 would only carry A². Since some children are homozygous A¹/A¹, the entire A¹ must then come solely from II‑1, and II‑2 can never supply A¹.