Class 10 Science Chapter 8 Heredity NCERT Solutions 2026 PDF Download

Ans: Trait B is likely to have arisen earlier. In asexual reproduction, traits are passed on with very little variation. Since trait B is present in a larger percentage (60%) of the population, it indicates it has been replicating for a longer time than trait A (10%).

Ans: Variations help a species adapt to changing environmental conditions. For example, if a bacteria population living in temperate water faces a sudden heat wave due to global warming, most would die. However, if there were a few variants resistant to heat, they would survive and continue the species. Thus, variation prevents extinction.

Ans: Mendel crossed pure tall plants (TT) with pure short plants (tt). In the F1 generation, all offspring were Tall (Tt). This showed that the 'Tall' trait appeared while 'Short' was hidden, proving 'Tall' is Dominant and 'Short' is Recessive.

Ans: In a Dihybrid Cross (Round-Yellow seeds x Wrinkled-Green seeds), Mendel observed that the F2 generation contained new combinations (Round-Green and Wrinkled-Yellow). This proved that the inheritance of seed shape was independent of seed colour (Law of Independent Assortment).

Ans: No, this information is not enough.
Possibility 1: If A is dominant, Father could be AO and Mother OO → Child OO (Valid).
Possibility 2: If O is dominant, Father could be AA and Mother AO → Child AO (Phenotype O) (Valid).
However, scientifically, we know A is dominant over O, meaning the father is heterozygous (AO).

Ans: Females have a perfect pair of sex chromosomes (XX), while males have a mismatched pair (XY).
1. All eggs produced by females have an X chromosome.
2. Sperm produced by males can have either X or Y.
If a sperm carrying X fertilizes the egg, the child is a Girl (XX).
If a sperm carrying Y fertilizes the egg, the child is a Boy (XY).

Ans: A small population means a small gene pool. This limits variations, which are essential for survival. It also increases the chances of inbreeding depression and makes the entire population vulnerable to sudden diseases or environmental changes, leading to extinction.

Ans:

• Heredity: The transmission of characters/traits from parents to offspring.
• Variation: The differences in characters among individuals of the same species.

Ans: 1. Short life span (easy to study many generations).
2. Availability of contrasting traits (Tall/Short, Round/Wrinkled).
3. Self-pollinating but can be cross-pollinated artificially.

Ans: A Gene is a segment of DNA that provides instructions for the synthesis of a specific protein, which in turn controls a specific trait. Genes are located on Chromosomes inside the nucleus.

Ans:

• Genotype: The genetic constitution of an organism (e.g., TT or Tt).
• Phenotype: The observable physical characteristic (e.g., Tallness).

Ans: 3:1 (3 Dominant : 1 Recessive).

Ans: (a) F1 Genotype is Bb. Since B is dominant, colour is Blue.
(b) In F2, Ratio is 3 Blue : 1 White. Percentage of White = 25%.
(c) Genotypic ratio is 1 BB : 2 Bb : 1 bb. Ratio of BB:Bb is 1:2.

Ans: Human females produce only one type of egg (X chromosome). Males produce two types of sperm (50% X and 50% Y).
- Egg (X) + Sperm (X) → Girl (XX)
- Egg (X) + Sperm (Y) → Boy (XY)
Since the sex depends entirely on which sperm fertilizes the egg, the father is biologically responsible for the sex of the child, not the mother.

Ans: Mendel crossed Pure Round Yellow (RRYY) seeds with Pure Wrinkled Green (rryy) seeds.
F1: All Round Yellow (RrYy).
F2: Obtained by selfing F1.
Ratio: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green (9:3:3:1).

Ans: Acquired characters (like building muscles or learning a language) cause changes in somatic cells (body cells), not in the germ cells (DNA of sperm/egg). Only changes in germ cell DNA can be passed to offspring. For example, if a mouse's tail is cut, its offspring are still born with tails because the DNA for "tail formation" remains intact.

Ans: 1. Mixing of Genes: Sexual reproduction involves the fusion of DNA from two different parents.
2. Crossing Over: During gamete formation (meiosis), exchange of genetic material occurs between chromosomes.
Asexual reproduction involves only copying DNA from one parent, so variations occur only due to rare mutation errors.

Ans: A zygote is a fertilized egg cell that results from the union of a female gamete (egg) with a male gamete (sperm). It contains the complete set of chromosomes (46 in humans) required to form a new individual.

Ans: Genotype: Pp (Heterozygous).
Phenotype: Purple (since P is dominant).

Ans: Humans have 23 pairs of chromosomes (Total 46).
- 22 pairs are Autosomes.
- 1 pair is Sex Chromosomes (XX or XY).

Ans: It states that when two pairs of traits are combined in a hybrid, the segregation (separation) of one pair of characters is independent of the other pair of characters.

Ans: In the F1 generation of a cross between two pure-bred parents (Homozygous Dominant x Homozygous Recessive), all offspring receive one dominant allele and one recessive allele (Heterozygous). Due to the Law of Dominance, only the dominant trait is expressed, making them look uniform.