Chapter 8 · Heredity

NCERT Class 10 Science Chapter 8 Intext Questions (Page 133)

All 4 questions of NCERT Class 10 Science Chapter 8 Intext Questions (Page 133) — Heredity — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   How do Mendel's ​experiments ​show that ​traits may be ​dominant ​or recessive?
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   All $F_1$ plants were tall but one quarter of $F_2$ were short, so tall is dominant and the hidden short is recessive.

   Mendel crossed a pure-breeding tall pea plant with a pure-breeding short one. Every plant of the first generation, F1, was tall, and not one was of medium height, so the two parental traits had not blended and only one of them was seen. He then let these F1 tall plants self-pollinate and counted the second generation. F2 was not all tall: about one quarter of the plants were short, giving a ratio of three tall to one short. The short trait had therefore not been lost in F1. It had only been hidden, and it reappeared as soon as a plant received the short factor from both parents. This shows that a plant carries two copies of the factor for a character, and that when the two copies differ, one of them is expressed and the other is not. The version that is expressed, tallness $T$, is called the dominant trait, and the version that stays hidden in its presence, shortness $t$, is called the recessive trait. Both $TT$ and $Tt$ plants are tall while only $tt$ plants are short, which is exactly why a recessive trait can skip a generation and then return.

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2. Q2
   How do Mendel's ​experiments ​show that ​traits ​are ​inherited ​independently?
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   In a two-character cross, F2 showed new combinations (tall wrinkled, short round) in a $9:3:3:1$ ratio, so the traits separate independently.

   Mendel crossed pea plants that differed in two characters at once, a tall plant bearing round seeds with a short plant bearing wrinkled seeds. Every F1 plant was tall with round seeds, so tallness and round seed shape are the dominant traits and each F1 plant is $TtRr$. He then let the F1 plants self-pollinate. The F2 generation contained the two parental combinations, tall with round seeds and short with wrinkled seeds, but it also contained two combinations that neither parent had shown: tall plants with wrinkled seeds and short plants with round seeds. The four classes appeared in the ratio 9 : 3 : 3 : 1. If height and seed shape had been tied to each other and passed on as a package, these new combinations could never have appeared, because every offspring would have been either tall with round seeds or short with wrinkled seeds. Their appearance shows that the pair of factors controlling height separates without reference to the pair controlling seed shape when gametes are formed, so the two can recombine freely in the zygote. The same point can be read from the ratio one character at a time: 12 tall to 4 short is 3 : 1, and 12 round to 4 wrinkled is also 3 : 1, so the ratio for either character is unchanged by what the other character does. Mendel's count for the seed shape and seed colour version of this cross, 315 round yellow, 108 round green, 101 wrinkled yellow and 32 wrinkled green out of 556 seeds, is the same 9 : 3 : 3 : 1.

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3. Q3
   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?
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   No: the same family arises whether A is dominant over O or O over A, so it cannot show which is dominant.

   No, this information is not enough. Dominance can only be decided by seeing which version of a trait is expressed in a person who carries both versions, and this one family does not identify such a person with certainty, because both possibilities produce exactly the family that was observed. Suppose A is dominant over O. Then O is recessive, so the mother showing blood group O must be $OO$ and the daughter must be $OO$ as well; the daughter's second O must have come from her father, so the father is $AO$ and still shows blood group A. The family fits. Now suppose O is dominant over A. Then A is recessive, so a person showing blood group A can carry no O at all and the father must be $AA$; the mother showing blood group O may be $AO$; the father passes A to every child, so the daughter is $AO$, and because O is dominant she shows blood group O. The family fits equally well. Since the same father, mother and daughter arise on either assumption, and both even predict that about half the couple's children would be of each blood group, the observation cannot separate the two. To settle the matter one would need information that one of the possibilities forbids, from more children or from more generations: a child of blood group A born to two parents of blood group O would prove that A is recessive, and a child of blood group O born to two parents of blood group A would prove that O is recessive.

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4. Q4
   How is ​the ​sex of ​the child ​determined in ​human beings?
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   The fertilising sperm decides the sex: an $\text{X}$ sperm gives a girl ($\text{XX}$) and a $\text{Y}$ sperm a boy ($\text{XY}$).

   The sex of the child is decided by the sex chromosomes it inherits, and in practice by the one it receives from its father. A human body cell carries 23 pairs of chromosomes. Twenty-two of these pairs are the same in men and in women; the twenty-third is the pair of sex chromosomes. A woman has two X chromosomes, XX, a perfect pair, while a man has a mismatched pair, one normal-sized X and one short Y, so he is XY. Every germ cell receives one chromosome of each pair, so every egg the mother makes carries an X, whereas the father makes two kinds of sperm in equal numbers, half carrying an X and half carrying a Y. If a sperm carrying X fertilises the egg the zygote is XX and the child is a girl; if a sperm carrying Y fertilises it the zygote is XY and the child is a boy. Since the mother can supply nothing but an X, she cannot influence the outcome: every child inherits an X from her whether it is a boy or a girl, and the sex depends entirely on whether the fertilising sperm carried the X or the Y. Because the two kinds of sperm are produced in equal numbers, about half the children born are girls and half are boys.

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