Question

Assume that in watermelon plants the genes for flower color (Y=yellow: y = orange flowers) and fruit shape (R = round; r = oval) are on the same chromosome. You cross two true-breeding plants, one with yellow flowers and round fruit and the other with orange flowers and oval fruit. You take one of the Findividuals and cross it with a plant that is homozygous recessive for both traits. You get the following in the F₂: 300 yellow flowers, round fruit; 305 orange flowers, oval fruit: 295 yellow flowers, oval fruit, 320 orange flowers, round fruit. What is the map distance between the genes for flower color and fruit shape?

O 4.0
O 33.0
O over 50 or the genes are on two different chromosomes
O 25.0
O 40.0

Answer 1

The map distance between the genes for flower color and fruit shape in watermelon plants is approximately 50.4 centimorgans, suggesting they may be on the same chromosome but are quite far apart.

Step-by-step explanation:

The question asks us to calculate the map distance between two genes, for flower color and fruit shape, using the F2 generation outcome from a given cross in watermelon plants. To determine the map distance, we first identify recombinant phenotypes, which are the phenotypes that do not match the parental phenotypes. These are yellow flowers, oval fruit (295) and orange flowers, round fruit (320).


The total number of offspring is 300 + 305 + 295 + 320 = 1220. The map distance is calculated by adding the number of recombinant offspring (295 + 320) and dividing by the total number of offspring, then multiplying by 100 to get a percentage.

Map distance = (Recombinants / Total offspring) × 100
= (295 + 320) / 1220 × 100
= (615 / 1220) × 100
≈ 50.4%

The map distance is approximately 50cM, which suggests that the genes for flower color and fruit shape are on the same chromosome but are relatively far apart, providing evidence against independent assortment.


However, because the distance exceeds 50 centimorgans, there is the possibility that these genes behave as if they're on different chromosomes due to the high rate of recombination.