The cotton species Gossypium hirsutum has a diploid chromosome number of 52. Two

Question

The cotton species Gossypium hirsutum has a diploid chromosome number of 52. Two other cotton species G. thurberi and G. herbaceum each have a diploid number of 26. Hybrids between these species show the following pairing at meisois:

Hybrid Pairing arrangement

G hirsutum x G thurberi 13 small bivalents and 13 large univalents

G hirstum x G herbaceum 13 large bivalents and 13 small univalents

G thurberi x G herbaceum 13 large univalents and 13 small univalents

Explain these results.

13.7 Yellow body color in Drosophila is caused by a mutant allele y of a gene located on the tip of the X-chromosome (the wild type allele causes a gray body). In a radiation experiment, a wild type male Was irradiated with X rays and then crossed with a yellow-bodied female. Most of the male progeny Were yellow, as expected, but the scanning of thousands of flies revealed two gray-bodied (wild type) Males. These gray-bodied males were crossed with yellow bodied females with the following results: Parents Gray male 1 x yellow female Gray male 2 x yellow female Explain these results. Offspring yellow females; gray males ½ yellow females. ½ gray females ½ yellow males; ½ gray males

Explanation / Answer

G. hirsutum, 2n = 52; n = 26

G. thurberi, 2n = 26; n = 13

G. herbaceum, 2n = 26; n = 13

Only homologous chromosomes pair with each other to form bivalents.

From the given data, it appears that G. hirsutum has acquired half the number of chromosomes from G.thurberi and the other half from G. herbaceum.

1.

G. hirsutum X G. thurberi

13 chromosomes from G. thurberi form 13 bivalents with their respective chromosome pairs from G. hirsutum.

The remaining 13 chromosomes from G. hirsutum remain as monovalents.

2.

G. hirsutum X G. herbaceum

13 chromosomes from G. herbaceum form 13 bivalents with their respective chromosome pairs from G. hirsutum.

The remaining 13 chromosomes from G. hirsutum remain as monovalents.

3.

G. thurberi X G. herbaceum

Since none of these chromosomes are homologous to each other, all of them remain as monovalents.

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