Question

Linkage

a) how do you determine the types of gametes expected from a double heterozygous individual when there is linkage and when the genes assort independently.

b) How do you identify the parental and the recombinant type gametes from data on the frequency of different types of offspring in a testcross and from information about the genotypes of the parents of the double heterozygote.

c) How do you calculate the map distance when given testcross results.

Answer 1

a)

Considering a test cross: with A and B as dominant, a and b as recessive.

• Cross: AaBb x aabb
• For AaBb- gametes= AB, ab, Ab, aB
• For aabb-gametes = ab

	ab	ab
AB	AaBb	AaBb
Ab	Aabb	Aabb
aB	aaBb	aaBb
ab	aabb	aabb

For indebendent assortment: ratio= 1:1:1:1

1. Coupling conformation:More recombinants:
2. Repulsion conformation: More parental:

b)

• Genes located in the same chromosome may be close together or linked.
• For linked gene: considering that the heterozygotes are F1-hybrids from true breeding parents, F2 will have 3:1 ratio.
• If the linkage is incomplete, recombination frequency or crossover will be less than 50 %, If complete linkage: no crossover or R.F will be 0%
• RF is half the frequency of crossing over. If crossover occurs half of the time RF is ½ of 50% or 25%.
• Cis configuration (coupled)-
• If alleles of both genes are located in the same chromosome.
• Or dominant alleles are present in one chromosome and recessive in another.
• More number of cross-overs (recombinants) than non-cross-overs (parents).
• Trans configuration (repulsed)-
• If alleles of both genes are located in different chromosome.
• Or homologous chromosomes have dominant and recessive alleles from different chromosomes.
• More number of cross-overs (recombinants) than non-cross-overs (parents).

c)

• Recombination frequency (RF or θ) indicates the frequency of chromosomal crossover between two genes.
• RF measures genetic linkage and the loci of gene on a chromosome. Recombination frequency (measured in %) determines the distance of two gene loci (measured in centi-Morgan (cM) or as map unit (MU).
• During recombination process (like meiotic recombination, where recombination occurs between non-sister chromatids), the further the genes are the more is their tendency to cross over or higher is the recombination frequency (as indicated by Morgan).
• While genes that are close together are linked and the recombination frequency is less.
• Usually if recombination frequency is less than 50%, it indicates that the genes are linked.

Thus, recombination frequencies can be used to determine the map distances of two gene loci.

If RF = 1%, map distance = 1 cM.

In this case, the gene loci of two genes, gene 1 and gene 2 are indicated, with their respective RF.

The RF given for a pair of genes is equal to their map distance.

This can be indicated as:

R.F.= (Number of recombinants/ Number of recombinants + Number of non-recombinants) x 100