Question

Assume that three loci, each with two alleles (A and a, B and b, C and c), determine the differences in height between two homozygous strains of a plant. These genes are additive and equal in their effects on plant height. One strain (aa bb cc) is 10 cm in height. The other strain (AA BB CC) is 22 cm in height. The two strains are crossed, and the resulting F1 are interbred to produce F2 progeny. Give the phenotypes and the expected proportions of the F2 progeny.

Answer 1

Given,

aabbcc = 10 cm

AABBCC = 22 cm

The genes are additive which means that each dominant allele will contribute to the overall height. The base minimum height is 10 cm (no dominant allele) and the maximum height is 22 cm (all six dominant alleles). Thus, six dominant alleles contribute 12 cm of height to the base height, which means that each dominant allele adds 2 cm of height.

When, aabbcc X AABBCC :

AaBbCc ( F1 )

When, AaBbCc X AaBbCc :

7 types of phenotype is possible. They are:

6 dominant alleles = 22 cm

5 dominant alleles = 20 cm

4 dominant alleles = 18 cm

3 dominant alleles = 16 cm

2 dominant alleles = 14 cm

1 dominant allele = 12 cm

0 dominant allele = 10 cm

They will be in the ratio = 1 : 6 : 15 : 20 : 15 : 6 : 1 as given by the ratio of additive effect governing three loci. The intermediate phenotype ( 16 cm ) will be maximum in number and the numbers will decrease as we move to the extremes.

Answer 2

Step-by-step explanation:

In the given question, the trihybrid cross has been provided for the height trait and it is provided that recessive alleles (aabbcc) contribute 10 cm whereas dominant allele (AABBCC) contribute 22cm.

The height contributed by each dominant allele (A+A+B+B+C+C)= 22cm

The height contributed by each recessive allele (a+a+b+b+c+c)= 10 cm

The dominant genotype is 12 cm taller than recessive genotype which indicates that dominant allele contributes 2 cm.

In trihybrid cross-64 genotype will be formed with these dominant allele based on which the height of the plant will be determined.

The genotype with dominant allele (Height) and F2 progeny are

0 (10cm) - 1/64

1 (12cm)- 6/64

2 (14cm)- 15/64

3 (16 cm)- 20/64

4 (18cm)- 15/64

5 (20 cm)- 6/64

6 (22 cm)- 1/64