Solutions For An Introduction to Genetic Analysis Chapter 2 Problem 34P

The results regarding the inheritance of the small-colony phenotype are deduced as follows: -

a. Consider a diploid meiocyte that is heterozygous for one gene. It is S⁺/S and S is the allele which confers the small colony phenotype. This diploid meiocyte after replication and segregation gives rise to two products of meiosis of genotype S⁺ and two of genotypes S. If we analyze the spores of many meiocytes randomly, we can notice that 50% are normally sized colonies and 50% are small colonies if the small colonies are due to the mutation in a single gene. There were 188 normal-size colonies and 180 small-sized colonies. This supports the theory that the phenotype is the result of the mutation in a single gene.

b. The diploid meiocyte S+/S after replication and segregation of S+ and S would give a tetrad of two meiotic products or spores of genotype S+ and two of S, all contained in a membrane sac called ascus.

So, S⁺/S----S⁺

S ⁺ tetrad in ascus

S

S

An ascus from this cross looks like this.