Question

Fauna hypotheticus crosses with a closely related animal that has 2n = 40 small chromosomes. They produce a viable hybrid offspring. Studies of the chromosomes of the hybrid animal during meiosis show both paired and unpaired chromosomes.

2.2 a. How many chromosomes are found in somatic cells of the hybrid offspring? (1)

b. Explain why the hybrid animal is sterile. (1)

c. Explain the presence of the paired chromosomes. (1)

Answer 1

ANSWER 2.2.A :-

1. The first important point is to identify the number of chromosomes which are present in the animal Fauna hypotheticus. This animals is closely related to another animal which has a diploid chromosome number of 40. This indicates that at the time of segregation, the gamete will contain 20 chromosomes.
2. There is no mention about the number of chromosomes which are present in the Fauna hypotheticus. Lets assume that the animal has 38 as the diploid chromosome content. This means that the gamete will have 19 chromosomes.
3. At the time of fertilization, the gametes will fuse which will lead to the total of (20+19 = 39) chromosomes in the hybrid offspring. Out of these, two will be the sex chromosomes and the remaining chromosomes will be autosomal. So, there will be a total 37 autosomes and 2 sex chromosomes indicating a total of 39 chromosomes in the somatic cells of the hybrid organism.

ANSWER 2.2.B :-

1. First of all, the two animals being closely related does not indicates that their chromosomes will be homologous to each other and thus there will be errors which will occur during the process of gamete formation in the animal which is also called as Meiosis (during which crossing over takes place).
2. These are some reasons which are responsible for the hybrid animal being sterile.

ANSWER 2.2.C :-

1. The last question is quite uninformative. It would be better if more details are provided. The point to be included is that there wont be formation of pairs between the chromosomes of the two related animals in the hybrid offspring.
2. These are in short not compatible to form a pair as can be seen in case of homologous chromosomes.