NCERT Solutions for Class 10 Science Chapter 8 Exercise

Answer: Trait B is likely to have arisen earlier because it is present in more members of the population. Trait A is new and has spread to only 10% of the population.

Answer: Depending on the nature of variations, different individuals would have different kinds of advantages. For example, Bacteria variants that can withstand heat are more likely to survive a heat wave than non-variant bacteria that cannot tolerate heat. Thus, variations in a population of a species help in survival of a species.

Answer: Mendel selected true breeding tall (TT) and dwarf (tt) pea plants. Then, he crossed these two plants. The seeds formed after fertilization were grown and these plants that were formed represent the first filial or F1 generation. All the F1 plants obtained were tall. Then, Mendel self-pollinated the F1 plants and observed that all plants obtained in the F2 generation were not tall. Instead, one-fourth of the F2 plants were short. From this experiment, Mendel concluded that the F1 tall plants were not true breeding. They were carrying traits of both short height and tall height. They appeared tall only because the tall trait is dominant over the dwarf trait.

Answer: Mendel took two pairs of alternate expressions of two traits and carried out dihybrid crosses by crossing them. The traits that appeared in the first generation were termed as dominant. When he used these F1 progeny to generate F2 progeny by self-pollination plants of different types were produced. In some plants both the traits were dominant, while in some plants both were recessive and some plants exhibited mixed traits. This indicates that traits are inherited independently.

Answer: This information is not enough. This is because each individual is carrying two alleles. The recessive trait can occur only when alleles are similar. If blood group A is dominant and O is recessive, then daughter can have blood group O only when both recessive alleles occur together in mother, and father has one allele of O and other of A.

Answer: In human beings, the females have two X chromosomes and the males have one X and one Y chromosome. Therefore, the females are XX and the males are XY. The gametes receive half of the chromosomes. The male gametes have 22 autosomes and either X or Y sex chromosome. Types of male gametes are 22+X or 22+Y. However, since the females have XX sex chromosomes, their gametes can only have X sex chromosome. There is only one type of female gamete that is 22+X. Thus, the mother provides only X chromosomes. The sex of the baby is determined by the type of male gamete (X or Y) that fuses with the X chromosome of the female.

Answer: (c) TtWW

Answer: This information is not sufficient. We need data of at least three generations to determine if a trait is dominant or recessive.

Answer: Dogs have a certain set of genes that govern coat colour. There are a minimum of eleven known sequence series (A, B, C, D, E, F, G, M, P, S, T) that influence the colour of a dog. A dog inherits one copy from each of its parents.

As an example, within the B series, a dog is genetically black or brown. Assume that one parent is homozygous black (BB), whereas the other parent is homozygous brown (bb). In this case, all the offspring are going to be heterozygous (Bb).

Since black (B) is dominant, all the offspring are going to be black. However, they are going to have each B and b alleles. If such heterozygous pups are crossed, they are going to produce 25 homozygous blacks (BB), 15 heterozygous black (Bb), and 25 homozygous brown (bb) offspring.

Answer: In human beings, every somatic cell of the body contains 23 pairs of chromosomes. Out of these 23 pairs, the first 22 pairs are known as autosomes and the remaining one pair is known as sex chromosomes represented by X and Y. Females have two X chromosomes and males have one X chromosome and one Y chromosome. The gamete receives half of the chromosomes. Therefore, the male gametes have 22 autosomes and either X or Y chromosomes. The female gamete, on the other hand, has 22 autosomes and X chromosomes.

During reproduction, the male and female gametes fuse and thus the progeny receives 22 autosomes and one X or Y chromosome from male parent and 22 autosomes and one X chromosome from the female parent.