Question

a. Your dream is to find durum seeds that are white in color (wild type is amber or yellow) and so you mutagenize thousands of amber seeds and identify 6 plants segregating for white seeds

I. How would you identify true breeding white lines?

II. Determine whether the 6 individual white lines contained mutations in the same gene or different genes?

b. The following two genotypes are crossed: Aa Bb Cc dd Ee × Aa bb Cc Dd Ee. What will the proportion of the following genotypes be among the progeny of this cross? (5 points)

I. Aa Bb Cc Dd Ee

II. aa bb cc dd ee

IV. If there were 1000 offspring, how many will have the true breeding recessive genotype?

Answer 1

I. True breeding = Homozygous
To determine if a plant is true breeding, allow it it self-pollinate.
If all the progeny exhibit white seeds, the plant is homozygous.
If there is more than one phenotype, the plant is not homozygous.

II. If two recessive mutants are crossed and the resulting F1 exhibits the mutant phenotype, both mutants are said to be allelic i.e. located in the same gene.
If two recessive mutants are crossed and the resulting F1 exhibits the WT phenotype, both mutants are said to be non-allelic i.e. located in different genes.

This is applicable only to recessive mutants.

b. Parental cross: AaBbCcddEe X AabbCcDdEe
I. Required progeny = AaBbCcDdEe
Probability = 1/2 X 1/2 X 1/2 X 1/2 X 1/2
= 1/32

II. Required progeny = aabbccddee
Probability = 1/4 X 1/2 X 1/4 X 1/2 X 1/4
= 1/256

III. The probability for true breeding recessive homozygote = 1/256
i.e. one in 256
~4 in 1000 (or 3 in 1000)