Question

You are mapping three linked genes in tomato plants. The recessive alleles for the three genes (when in the homozygous condition) cause absence of anthocyanin pigment (a), hairless plants (h), and jointless fruit stems (j), respectively. Two true-breeding parents were crossed, producing trihybrid(heterozygous) F1s, which were then test-crossed. The 3000 offspring were grouped into the following phenotypic classes (a trait not mentioned in a particular phenotypic class can be considered to be dominant or wild-type for that trait):
Phenotype Number Observed
hairless 249
jointless, hairless 40
jointless 931
wild type for all three 270
anthocyaninless, jointless, hairless 278
anthocyaninless, hairless 941
anthocyaninless 32
anthocyaninless, jointless 259


a) What are the original parental phenotypes?

b) Which of the three genes is in the middle?


c) Calculate the map distance between the three genes

Answer 1

Here are the answers to the three questions.

a) The phenotypes of the parents are hairless and jointless.

b) The three genes in the middle are anthocyaninless, hairless, and jointless.

c) The map distance between the three genes are one generation apart.<span />

Answer 2

Anthocyaninless, jointless, hairless 278
anthocyaninless, hairless 941
anthocyaninless 32
anthocyaninless, jointless 259