Q78 of 167 Page 195

The person A has only B chromosomes in all its gametes. On the other hand, another person C has chromosome D in half of gametes and chromosome E in the other half of gametes. When chromosomes B and D combine during fertilization, a female zygote results. On the other hand, combination of B and E chromosomes produces a male zygote.

(a) What are chromosomes?


(i) B


(ii) D, and


(iii) E ?


(b) Out of B, D and E, which two chromosomes are of the same type?


(c) Which chromosome is smaller in size?


(d) What is the general name of chromosomes such as B and E?


(e) Out of the two persons A and C, which one is


(i) male, and


(ii) female ?

(a) (i) The chromosome B are X chromosomes because if person A is entirely madeup of B chromosomes it means the person is a female an has all X chromosomes.

(ii) The chromosome D is X chromosome again because when B and D combine a female offspring is produced. A female zygote can only be produced when both of the chromosomes in the gamete are ‘X’. Chromosome B is X as already said above; now chromosome D is also X. Thus only then when B and D chromosome i.e. X and X chromosome combine, a female zygote is produced.


(iii) The chromosome E is a Y chromosome because when B and E combine a male zygote is produced. B is a X chromosome as discussed above and when E i.e. Y chromosome combine , a male zygote is produced(XY)


(b) Chromosome B and D are of the same type, i.e. X chromosome.


(c) Chromosome E is smaller in size since it is a Y chromosome.


(d) Chromosome B (X) and chromosome E (Y) are known as Sex Chromosomes


(e) (i) C is male since it has half genes from D and half genes from E.


D is a X chromosome and E is a Y chromosome so when they combine a male zygote is formed.


(ii) A is female since it has all B chromosomes in it i.e. X chromosome which is only possible in female.


More from this chapter

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76

A Mendelian experiment consisted of breeding tall TT pea plants bearing violet flowers with short t pea plants bearing white w flowers. The progeny all bore violet W flowers, but almost half of them were short. This suggested that the genetic make-up (or genotype) of the tall parent can be depicted as:

(a) TTWW


(b) TTww


(c) TtWW


(d) TtWw


Give reason for your choice

 77

A person first crossed pure-bred pea plants having round-yellow seeds with pure-bred pea plants having wrinkled-green seeds and found that only A-B type of seeds were produced in the F1generation. When F1 generation pea plants having A-B type of seeds were cross-bred by self-pollination, then in addition to the original round-yellow and wrinkled-green seeds, two new varieties A-D and C-B type of seeds were also obtained.

(a) What are A-B type of seeds?


(b) State whether A and B are dominant traits or recessive traits.


(c) What are A-D type of seeds?


(d) What are C-B type of seeds?


(e) Out of A-B and A-D types of seeds, which one will be produced in


(i) minimum numbers, and


(ii) maximum numbers, in the F2 generation ?

 79

Mendel first crossed pure-bred pea plants having round-yellow seeds with pure-bred pea plants having wrinkled-green seeds and found that only round-yellow seeds were produced in the F1 generation. When F1 generation pea plants having round-yellow seeds were cross-bred by self-pollination, then peas having round � yellow seeds, round green seeds, wrinkled-yellow seeds and wrinkled-green seeds were produced. Mendel collected a total of 2160 seeds.

(a)What will be the number of?


(i) Round green seeds


(ii) Wrinkled green seeds


(iii) Round yellow seeds, and


(iv) Wrinkled-yellow seeds


(b) Which 'ratio' as established by Mendel have you made use of in answering the part (a) above?

 80

Pure-bred round-yellow pea seeds have genotype RRYY and the pure-bred wrinkled-green pea seeds have genotype rryy. Keeping this in mind, write the phenotypes of the following genotypes of hybrid pea seeds:

(a) Rryy


(b) rrYy


(c) rrYY


(d) RrYy


(e) RRyy