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NCERT Class 10 Science Heredity: In-text questions (page 133) Solutions

Step-by-step working for every question of In-text questions (page 133) in NCERT's Class 10 Science book, chapter “Heredity”, written by Shishya. Each question is in our own short words; open the NCERT chapter for the exact wording and figures.

1. In-text (page 133) Q1

   How do Mendel's experiments show that traits may be dominant or recessive?

   1. Mendel crossed pure tall pea plants with pure short pea plants to create the F1 generation.
   2. All the offspring in the F1 generation were tall, showing that the tall trait was expressed while the short trait was hidden.
   3. This indicates that the tall trait is dominant because it masks the expression of the short trait in the hybrid offspring.
   4. When the F1 plants were self-pollinated to produce the F2 generation, both tall and short plants appeared again.
   5. The reappearance of the short trait in the F2 generation proves that the recessive trait was present but hidden in the F1 generation.
   6. Thus, Mendel's experiments demonstrate that dominant traits are expressed in the F1 generation while recessive traits reappear in the F2 generation.

   Answer: Mendel's experiments show that traits may be dominant or recessive because the dominant trait is expressed in the F1 generation while the recessive trait remains hidden but reappears in the F2 generation.

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2. In-text (page 133) Q2

   How do Mendel's experiments show that traits are inherited independently?

   1. Mendel crossed pure tall pea plants with pure dwarf pea plants to get F1 hybrids.
   2. He then self-crossed the F1 hybrids to obtain the F2 generation.
   3. In the F2 generation, he observed that tall and dwarf traits appeared in a 3:1 ratio.
   4. He also tracked seed shape and seed color simultaneously in the same crosses.
   5. He found that the ratio of tall to dwarf plants was independent of the ratio of round to wrinkled seeds.
   6. This showed that the inheritance of one trait did not influence the inheritance of another trait.
   7. Thus, Mendel concluded that different traits are inherited independently of each other.

   Answer: Mendel's experiments show that traits are inherited independently because the segregation of one pair of traits (like height) did not affect the segregation of another pair (like seed shape) in the F2 generation.

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3. In-text (page 133) Q3

   A man with blood group A and a woman with blood group O have a daughter with blood group O. Is this enough to say whether A or O is dominant? Why?

   1. Test both possibilities against the family.
   2. If A were dominant: the father could carry one A and one O copy (AO); the mother is OO; the daughter can get O from each parent and be OO, blood group O. This fits.
   3. If O were dominant: the father, showing A, would have two A copies (AA) and give the daughter an A; the mother, showing O, gives an O; the daughter (AO) would then show O, the dominant one. This also fits.
   4. Since the observation fits both ideas, one family with one child cannot tell us which trait is dominant; we would need data from many families and their children.

   Answer: No. The daughter having blood group O fits both “A is dominant” and “O is dominant”, so this information alone cannot show which trait is dominant.

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4. In-text (page 133) Q4

   How is the sex of a child determined in human beings?

   1. Human beings have 23 pairs of chromosomes, where 22 pairs are autosomes and 1 pair is sex chromosomes.
   2. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).
   3. During gamete formation, the female egg always carries an X chromosome.
   4. The male sperm can carry either an X chromosome or a Y chromosome.
   5. If a sperm carrying an X chromosome fertilizes the egg, the zygote becomes XX and develops into a female.
   6. If a sperm carrying a Y chromosome fertilizes the egg, the zygote becomes XY and develops into a male.
   7. Thus, the sex of the child is determined by the type of sperm that fertilizes the egg.

   Answer: The sex of a child is determined by the type of sperm (X or Y) that fertilizes the egg.

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