A geneticist crossed two green corn plants (with unknown genotype) and obtained

ID: 47739 • Letter: A

Question

A geneticist crossed two green corn plants (with unknown genotype) and obtained 4,696 seedlings with the following phenotypes:

3583 green seedlings

853 white seedlings

260 yellow seedlings

Part 1. Develop a gene interaction model to explain these results. (Hint: to get started examine the data and see if the actual numbers are closed to any modified Mendelian ratios.) Define the genes and symbols for alleles in your model. Clearly explain how the genes interact to give these results. Give the genotypes of the two original green plants that were crossed. Show the cross worked out to explain how ratios close to these numbers would be obtained.

Part 2. Given the model you developed,calculate exactly how many of each phenotype would you expect out of 4,696 seedlings if they came out in the exact ratios predicted by your genetic model. Use the Chi-squared test to see how well the observed data fit what you expect from your genetic model.

Explanation / Answer

a. In the given example, the ratio of offsprings is nearly 12:3:1. This ratio is obtained in dominant epistasis.

To obtain given ratio of phenotypes the condition follows dominant epistasis where color is recessive at one allelic pair which has to be expressed before a specific color allele. Green is dominant which is represented as G located as first gene followed by second gene. The second gene is dominant to white compared to yellow hence they are represented as W and w.

The green colored seedlings will be totally 12 offsprings. The dominant allele negates effect of W allele.

The white colored seedlings will be 3 offsprings. The recessive allele of green allows expression of white phenotype which is dominant over yellow.

The yellow color seedlings will be 3 offsprings. The recessive allele of green allows expression of yellow phenotype.

The genotype of the parents is GgWw (heterozygous).

b. Chi square test is useful for comparing observed results against the predicted ones.

Phenotype

Observed

Expected

(O-E)2

(O-E)2/E

Green

3583

3522

0.017

White

853

880.5

27.5

0.031

Yellow

260

293.5

33.5

0.114

Total

4696

0.162

Thus,