Q31 — Linkage Disequilibrium Between M* and Flanking Loci R/Q

Theoretical B Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license

Linkage, Recombination & Genetic Mapping

Linkage Disequilibrium (LD) is a non-random co-occurrence of alleles at different loci. Loci can be on the same or separate chromosomes. Let’s consider biallelic loci R and Q, located on the same chromosome. Around 1000 years ago, a neutral mutation (M*) arose in a single human individual (ID-1) at a locus M, which is located between R and Q. ID-1 was homozygous at R and Q. Table 1 shows the frequency of association between M and R/Q in the chromosomes of today’s 200 heterozygous carriers of M*, who inherited M* from ID-1. Assume that there is no selection against any allele at loci R, Q, and M, and that the population size is infinite.

Allele pair on the same chromosome	Frequency
R1-M*	12
R2-M*	188
Q1-M*	164
Q2-M*	36

Table 1. Occurrence of R/Q alleles paired with the M allele, from 200 heterozygous carriers descended from ID-1.*

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. If, in a population, the frequency of gametes having dominant alleles of genes A and B together is equal to the product of frequencies of these alleles, then there is no LD between dominant alleles of genes A and B.

B. The genotype of ID-1 is most likely R2R2/Q1Q1.

C. M is closer to locus Q than to locus R.

D. In the next 1000 years, you can expect LD between M* and R/Q alleles to decrease.

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