47. A pedigree shown below depicts that the individual l-I is heterozygous for a dominant disease allele D and for molecular markers MI/M2.
The paternal molecular markers present in the progeny individuals are indicated in the pedigree.
The following statements may be drawn from the above pedigree:
A. The two loci D/d and MI/M2 appears to be linked
B. The recombination frequency between the two loci is 20%
C. If LOD score comes out to be 3, then it ensures that the two loci are independently assorting
D. A LOD score <1 would have ensured that the two Loci are linked
Which combination of the above statements can correctly interpret the depicted pedigree?
(1) C and D (2) only C
(3) A and B (4) only D
Question Recap in Simple Words
The pedigree shows an individual I‑1 heterozygous for a dominant disease allele D and for marker alleles M1/M2; paternal marker alleles present in each child are indicated under the symbols.
Using this information, four statements (A–D) are given about linkage, recombination frequency and LOD score, and the task is to choose which combination correctly interprets the pedigree.
Step 1: Work out coupling phase and recombinants
I‑1 is D/d, M1/M2; from the segregation pattern, most diseased children receive the same paternal marker allele, indicating that D is coupled with one marker allele (say D–M1), while d is with the other (d–M2).
Counting progeny types from the key in this CSIR question gives 8 parental‑type and 2 recombinant‑type offspring, so recombinants are 2/10 = 0.2, i.e. 20% recombination.
- Parental gametes: D–M1 and d–M2 (most progeny).
- Recombinant gametes: D–M2 or d–M1 (rare progeny).
This clearly suggests linkage with recombination frequency r ≈ 0.2.
Step 2: Evaluate each statement
Statement A
A. The two loci D/d and M1/M2 appear to be linked.
Because most affected offspring carry the same paternal marker and only a minority show recombinant combinations, D and M1/M2 segregate together more often than expected for independent assortment (which would give 50% recombination).
Therefore, the loci appear linked, so statement A is true.
Statement B
B. The recombination frequency between the two loci is 20%.
From the pedigree, 2 of 10 progeny are recombinants, giving recombination fraction r = 2/10 = 0.2 = 20%.
Thus statement B is also true.
Statement C
C. If LOD score comes out to be 3, then it ensures that the two loci are independently assorting.
LOD score is the log10 of the odds that loci are linked at a given recombination fraction versus unlinked (independent assortment).
A LOD score of 3 means the data are 1000 times more likely under linkage than under independent assortment, so it supports linkage, not independence; therefore statement C is false.
Statement D
D. A LOD score < 1 would have ensured that the two loci are linked.
Conventionally, a LOD score ≥ 3 is strong evidence for linkage, scores ≤ −2 support no linkage, and values between about −2 and +3 are inconclusive.
A LOD score less than 1 (especially if near 0 or negative) does not “ensure linkage”; rather, it suggests weak or no evidence for linkage, so statement D is false.
Final choice among options
- A: true (loci appear linked).
- B: true (recombination ≈ 20%).
- C: false (LOD 3 supports linkage, not independent assortment).
- D: false (LOD < 1 does not ensure linkage; evidence is weak or against linkage).
Therefore, the correct combination is option (3) A and B.
