Question

Kari has identified three loci, Q, R, S, found in order on the q arm of a pair of homologous chromosomes. In recent experiments, she has generated a triple heterozygote from a triple dominant parent (QQRRSS) and a triple recessive parent (qqrrss). a) Sketch the arrangement of the alleles on each homologue after the chromosomes have condensed, but before any crossing over has occurred. b) Name the possible genotypes of a gamete derived from a chromatid for which no homologous recombination has occurred. c) Name the possible genotypes of a gamete for which a single crossover has occurred. d) Sketch one double crossover scenario that will produce new recombinants that were not possible in (b) or (c).

Answer 1

a). QRS/QRS x qrs/qrs----Parents

QRS qrs--------Gametes

QRS/qrs ---------F1

b). QRS/qrs

The above genotype produces only two types of gametes are produced when recombination is absent. Those gametes' genotypes are QRS & qrs.

c). QRS/qrs

If single crossover occurs between Q & R, then the gametes are as follows.

QRS, Qrs, qRS & qrs

If single crossover occurs between R & S, then the gametes are as follows.

QRS, QRs. qrS & qrs

d).

Double crossover recombinants are QrS & qRs