A species of cereal rye (Secale cerale) has a chromosomal number of 14, while a

Question

A species of cereal rye (Secale cerale) has a chromosomal number of 14, while a species of Canadian wild rye (Elymus canadensis) has a chromosomal number of 28. Sterile hybrids can be produced by crossing Secale with Elymus. (Suggested homework 2:32) a. What would be the expected chromosome number in the somatic cells of the hybrid? b. Given that none of the chromosomes pair at meiosis I in the sterile hybrid, speculate on the anaphase I separation pattern of these chromosomes. Assume that you were examining a first polar body and noted that it had one copy (dyad) of each chromosome except chromosome 21, but two copies (dyads) of chromosome 21. What would you expect to be the chromosome 21 complement in the second oocyte?

Explanation / Answer

Ans. 2A. Part A: Gametes are haploid (n) with respect to the chromosomal number of the parent or somatic cells (2n, diploid).

So,

Chromosome number in gamete of Secale = 7 (= 14 / 2)

Chromosome number in gamete of Elymus = 14 (= 28 / 2)

A hybrid is the cells resulted from fusion of gametes of two unrelated species.

So,

Expected chromosome number in hybrid =

chromosomes no. in gamete of Secale + Chromosome number in gamete of Elymus

                                                                        = 7 + 14

                                                                        = 21

Part B: Anaphase I is the phase of segregation of homologous chromosomes, whereas Anaphase II results segregation of sister chromatids.

A homologous chromosome consists of a pair of chromosomes sharing “homologous region” derives from male and female parent. For example, chromosome number 7 from father and chromosome number 7 from mother pair during prophase 1 to form a homologous pair (actually, a tetrad as chromosomes are duplicated during S-phase).

Lacking homologous chromosomes (two half sets are derived from different species, so such chromosomes do not share a homologous region), the hybrid does not show homologous pairing.

Therefore, the hybrid has 21 chromosomes in G-phase. The following events take place during meiosis I-

I. Chromosomes duplicated to forms 21 sister-chromatids (two duplicated chromosomes, say 2 copies of chromosome 7 from female parent, attached at centromere).

II. Since homologous chromosomes are absent, there is no homologous pairing during prophase 1.    

III. During anaphase I, there would be 21 independent (NOT in pair) sister-chromatids. The 21 sister chromatids would segregate randomly to two opposite poles during this phase.     

#3. First polar body forms after first meiotic division (meiosis I).

There is 2 dyads (2 chromosomes attached at centromere) of chromosome 21 in polar body- however, it was supposed to have one dyad while the other dyad was supposed to be transferred to the second oocyte.

So, second oocyte lacks chromosome 21 dyad.

The genotype of the second oocyte would be – 22 dyads, lacking chromosome 21 dyad.   

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