Biology hmwk. Genetics....?

Question

Cabbage is crossed with raddish of chromosome number 18 for both. The chromosome number of gametes is 9 for both and the chromosome of the sterile hybrid is 18.

Why is this hybrid plant unable to produce gametes?
How may fertile polyploids arise from sterile hybrids?
Give one adventage of polyploid plants to humans.

Answer 1

1. In each chromosome pair the two chromosomes are from different species. They aren't homologues and can't form bivalents during meiosis, so meiosis fails to occur and therefore there are no gametes.

2. Unlike in animals plant germ cells ultimately arise from ordinary (somatic) tissues rather than specialized germ cells. There are at least 2 ways of producing 'unreduced' gametes which are diploid rather than haploid. The first is through mitosis, the second is that there is a doubling of all chromosomes in somatic cells so in the cabbage x radish cross this cell would have 36 rather than 18 chromosomes. If these cells goes onto form gametes then they are able to undergo meiosis as there are now 2 sets of homologous chromosomes. If these unite then fertile tetraploid offspring are the result (i.e. each cell contains 4 sets of chromosomes). A polyploid formed from 2 different species is called an allopolyploid.

3. New species of plants - the cereal triticale is a wheat x rye allopolyploid.

Answer 2

reason for the plant not producing gametes is that : cabbage and radish are two different plants or species so hybrid is infertile and doesnt produce gametes. hope this helps for th 1st question :)