Question

Maize breeders is trying to determine the gene order on a metacentric chromosome. They testcross a maize plant heterozygous for three linked genes with the following recessive alleles (a,b, and d), the following phenotypes and numbers of progeny were obtained (see to right). Assume a dominant recessive relationship between the alleles for each gene (+ is wildtype phenotype).

++d - 460

ab+ - 476

+bd - 19

a++ - 20

a+d - 12

+b+ - 12

A) (2 points) Which pairs of genes show repulsion in the progeny of the test cross?

B) (3 points) Draw a genetic map of these three genes. Include the correct gene order and map distances based on the three point cross.

C) (3 points) At another locus on the chromosome the genetic map has already been determined;

Using a cross of +++/abg backcrossed to abg/abg, gene A is found to be in the middle and shows linkage to G (40 cM). From a two point cross the recombination frequency between A-E is close to 50% (effectively unlinked).

A comparison of the genetic map with the physical map indicates that there is a 'cold spot' between the E and F loci. There is a 'hot spot' between the A and G loci. Identify the location of the centromere and predict which gene is closest to the telomere? Explain your reasoning and include a drawing of the map with the loci, the centromere and telomere labelled

Answer 1

A. Repulsion is when a pair of homologous chromosomes have one dominant and one recessive on each chromosome linked to each other. All offspring of all homologous chromosomes have a dominant and recessive alleles in one chromosome.

The gene order is a-d-b. There are no double cross overs. The cross overs are single cross overs (SCO).

Distance between genes can be calculated by recombination frequency as follows: