The Physical Basis of Heredity Practice Problems

Review the following concepts if needed:

• Genetics, Cells, and Mendel's Law for Genetics
• Chromosomes for Genetics
• Cell Division and Reproduction for Genetics
• Gametogenesis for Genetics

The Physical Basis of Heredity Practice Problems

Practice 1

Consider three pairs of homologous chromosomes with centromeres labeled A/a, B/b, and C/c, where the slash line separates one chromosome from its homologue. How many different kinds of meiotic products can this individual produce?

Solution 1

For ease in determining all possible combinations, we can use a dichotomous branching system.

Eight different chromosomal combinations are expected in the gametes. The solution can also be found by using the formula 2ⁿ, where n = the number of loci. In this case, n = 3 so the number of possible combinations is equal to 2³ = 8.

Practice 2

The horse (Equus caballus) has a diploid complement of 64 chromosomes including 36 acrocentric autosomes; the ass (Equus asinus) has 62 chromosomes including 22 acrocentric autosomes. (a) Predict the number of chromosomes to be found in the hybrid offspring (mule) produced by mating a male ass (jack) to a female horse (mare). (b) Why are mules usually sterile (incapable of producing viable gametes)?

Solution 2

1. The sperm of the jack carries the haploid number of chromosomes for its species (62/2 = 31); the egg of the mare carries the haploid number for its species (64/2 ¼ 32); the hybrid mule formed by the union of these gametes would have a diploid number of 31 + 32 = 63.
2. The haploid set of chromosomes of the horse, which includes 18 acrocentric autosomes, is so dissimilar to that of the ass, which includes only 11 acrocentric autosomes, that meiosis in the mule germ line cannot proceed beyond first prophase where synapsis of homologues occurs.

Practice 3

When a plant of chromosomal type aa pollinates a plant of type AA, what chromosomal type of embryo and endosperm is expected in the resulting seeds?

Solution 3

The pollen parent produces two sperm nuclei in each pollen grain of type a, one combining with the A egg nucleus to produce a diploid zygote (embryo) of type Aa, and the other combining with the maternal fusion nucleus AA to produce a triploid endosperm of type AAa.

Practice 4

Given the first meiotic metaphase orientation shown on the right in the diagram below, and keeping all products in sequential order as they would be formed from left to right, diagram the embryo sac that develops from the meiotic product at the left and label the chromosomal constitution of all its nuclei.

Solution 4