Chapter 10, Problem 10A

Summary Introduction

To analyze:

Why a cross between a horse and a donkey produces a mule which is usually sterile.

Introduction:

Reproductive cells which pass on genetic traits from the parent to child are produced by the process of meiosis. In order to maintain the same number of chromosomes in each generation, an organism produces gametes. Gametes are sex cells and have half the number of chromosomes.

Answer to Problem 10A

A horse with 64 chromosomes will produce gametes with 32 chromosomes.

A donkey with 62 chromosomes will produce gametes with 31 chromosomes.

Now when there is fertilization between a gamete of horse and a gamete of donkey, a mule is produced having (32+ 31) 63 chromosomes. The mule will produce gametes which will not have equal number of chromosomes. Hence, it is sterile.

Explanation of Solution

When one haploid gamete fuses with another haploid gamete, the process is called fertilization. The result of fertilization is a diploid cell with 2n number of chromosomes; n chromosomes from male parent plus n chromosomes from female parent. Fertilization allows genetic variations in organisms.

When a donkey and a horse mate, the result of fertilization is a mule. A mule has a set of 32 chromosomes from horse and 31 chromosomes from donkey. These chromosomes are not exact match of each other. There is also one extra chromosome that does not have a pair. So during meiosis in a mule, the gametes do not get a copy of each chromosome. The chromosomes do not form homologous pairs. Hence, the gametes do not form sperms and eggs and this makes the mule sterile.