A cross between a male domesticated horse and a female zebra can produce an offspring called a hebra. Hebras are typically sterile. (4 points)
A. If the horse has 64 chromosomes in diploid cells and the zebra has 46 chromosomes in diploid cells, how many chromosomes does the hebra have in somatic cells?
B. Can you predict how many chromosomes the hebra would have in its gametes? Briefly explain.
C. Now just consider genetic variation in the horse. How many different chromosome combinations are possible in gametes due to the law of independent assortment? Only write as an expression, not the actual number.
The correct answer and explanation is:
A. Number of chromosomes in somatic cells of the hebra:
When a horse (64 chromosomes) is crossed with a zebra (46 chromosomes), the resulting hybrid offspring (the hebra) will inherit half of its chromosomes from each parent. Since the horse has 64 chromosomes, it will contribute 32 chromosomes, and since the zebra has 46 chromosomes, it will contribute 23 chromosomes. Therefore, the hebra will have 32 + 23 = 55 chromosomes in its somatic cells.
B. Number of chromosomes in the hebra’s gametes:
Gametes are reproductive cells (sperm or eggs), and they are haploid, meaning they only contain half of the total number of chromosomes present in somatic cells. In the case of the hebra, which has 55 chromosomes in its somatic cells, the gametes will each have half of 55, or 27.5 chromosomes. However, since the number of chromosomes must be an integer, it’s not biologically possible to have a fraction of a chromosome. As a result, the hebra’s gametes would likely be sterile because an uneven number of chromosomes would prevent proper chromosome segregation during gamete formation. This is why hybrids like the hebra are typically sterile.
C. Chromosome combinations in horse gametes due to independent assortment:
The law of independent assortment states that alleles for different traits segregate, or assort, independently of one another during the formation of gametes. For the horse, which has 64 chromosomes in diploid cells (32 pairs), the number of possible combinations of chromosomes in gametes can be expressed as 2^n, where n is the number of chromosome pairs. For the horse, since it has 32 chromosome pairs, the number of different chromosome combinations possible in gametes would be expressed as 2^32. This expression represents the number of ways chromosomes can be randomly distributed into gametes due to independent assortment.