Q. Why do all the gametes formed in human females have an X-chromosome?

Ans. The sex chromosome in human female is homomorphic i.e., they contain same chromosome XX. During meiosis process at the time of gamete formation all egg cell will get one copy of X chromosome, hence all the gametes formed in human females have an X-chromosome.

Q. How does the creation of variations in species promote survival?

Ans. Due to errors in DNA copying some variations in a species occur. Species with useful variations get adapt to the changing environment and they have better chances of survival and produce their offsprings.

Q. Only variations that confer an advantage to an individual organism will survive in a population. Do you agree with this statement? Why or why not?

Ans. Yes, we agree with this statement that only variations that confer an advantage to an individual organism

Q. If a trait A exists in 10% of a population of an asexually reproducing species and trait B exists in 60% of the same population, which trait is likely to have arisen earlier?

Ans. Trait B would have arisen earlier than trait A because as species are asexually reproducing, there would be very minor differences generated due to small inaccuracies in DNA copying which results in variations in trait A. But as trait B occurs in more number in the population as compared to trait A so Trait B would have arisen earlier than trait A.

Q. How do Mendel’s experiments show that traits may be dominant or recessive?

Ans. When Mendel crossed pure tall (TT) pea plants with pure dwarf (tt) pea plants, in F1 generation he found that all pea plants were tall (Tt). There were no dwarf plants produced in F1 generation. When he self-pollinated these F1 plants, in F2 generation he obtained tall and dwarf plants in the ratio 3: 1. Thus as three-fourths of the plant in F2 generation are tall and one-fourth is dwarf so tall is a dominant trait whereas dwarf is a recessive trait [which expressed itself only in homozygous condition]. So he concluded that for a particular trait [here in this example height of the plant] it may be dominant or recessive.

Q. Outline a project which aims to find the dominant coat colour in dogs.

  • Ans. Consider a homozygous black coat colour male dog (BB) breed with homozygous white coat colour female dog (bb). In F1 generation all dogs with black coat colour are produced. When these are again breed we get black and white coat colour dogs in F2 generation in the ratio 3 : 1. Thus, we can say black colour is dominant over white.
277

Q. A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits blood group A or O is dominant? Why or why not?

  • Ans. No, this information is not enough to find out which blood group is dominant. Blood group A can be represented as IAIA or IAIO which is the father’s blood group. Blood group O is represented as IOIO which is the mother’s blood group. The blood group of daughter is also O which might be inherited from her mother or father so it is difficult to assume which blood group is dominant.

Q. How is the sex of the child determined in human beings?

  • Ans. In human beings, there are two types of sex chromosome X and Y; female have XX chromosome whereas male have XY chromosome. Females produce eggs which carry only X chromosomes but males contain half of the sperms with X chromosomes and other half with Y chromosomes.

During fertilisation when X carrying sperms fuse with an egg which contains X chromosome the offsprings will be a female (XX). But when Y bearing sperms fuses with an egg (X) the offspring will be male (XY). Thus the sex of a child is determined by the type of sex chromosome X or Y received by the male gamete.

Q. How is the equal genetic contribution of male and female parents ensured in the progeny?

  • Ans. Human beings contain 23 pairs of chromosomes-22 pairs are autosomes and one pair sex chromosomes. During meiosis process gametes are formed in sex cells where the chromosome number is halved(n). At the time of fertilisation when male gamete fuses with female gamete the diploid number (2n) is restored back in zygote. Thus half of the chromosomes come from father and other half from mother. In this way meiosis process ensured equal genetic contribution of male and female parents in the progeny.