Solutions For An Introduction to Genetic Analysis Chapter 3 Problem 25P

b) The genotypic ratio which is underlying the 9:3:3:1 ratio is: 1:2:4:2:1:2:1:2:1

1 - R/R;Y/Y

2 - R/R;Y/y

4 - R/r;Y/y

2 -R/r;Y/Y

1 -R/R;y/y

2 -R/r;y/y

1 - r/r;y/y

2 - r/r;Y/y

1 - r/r;Y/Y

c) To devise a simple formula for the calculation of the number of phenotypes and the number of genotypes in monohybrid, dihybrid and trihybrid crosses, we can tabulate the data.

We can write 3, 9 and 27 as 3¹, 3² and 3³ and 2, 4 and 8 can be written as 2¹, 2² and 2³.

Hence the simple formula would be:

For genotypes: 3ⁿ where n is the number of genes

For phenotypes: 2ⁿ where n is the number of genes

d) To determine the genotype of a specific plant with round yellow peas we can use two methods:

• Testcross: We cross the plant with a homozygous recessive plant with green wrinkled peas. The F2 progeny will show the alleles present in the experimental plant.

o If the experimental plant is heterozygous for both alleles you will get a 1:1:1:1 phenotypic ratio.

o If the plant is heterozygous only for one allele then a phenotypic ratio of 1:1 is obtained.

o If the plant is homozygous for both alleles then there will be only one phenotypic category.

• Selfing: we cross the plant with itself to get the F2 progeny.

o If the plant is heterozygous for both alleles then we get a 9:3:3:1 ratio.

o If the plant is homozygous for one allele then the phenotypic ratio is 3:1:

o If the plant is homozygous for both alleles then there will be only one phenotypic category.