In human beings, the chromosomes that determine birth of a normal female child are
- (a)one X chromosome from mother and one X chromosome from father
- (b)one X chromosome from mother and one Y chromosome from father
- (c)two X chromosomes from mother and one X chromosome from father
- (d)one X chromosome and one Y chromosome from father and one X chromosome from mother
Correct — A, one X chromosome from mother and one X chromosome from father. A human cell carries 23 pairs of chromosomes, of which 22 pairs are autosomes and one pair is the sex chromosomes — XX in a female and XY in a male. Gametes carry one of each pair, so every egg carries an X, because that is all the mother has to give, while sperm are of two kinds, half carrying an X and half a Y. A female child therefore has to receive an X from the mother, which is automatic, and an X from the father, which is the fifty-fifty event. That gives XX, and only option (a) states it. The consequence worth stating plainly is that the sex of the child is decided entirely by which sperm fertilises the egg, so it is the father's contribution, not the mother's, that varies.
- (b)one X chromosome from mother and one Y chromosome from father — That combination is XY, which produces a male child. The stem asks for a female, so the paternal contribution must be an X-bearing sperm, not a Y-bearing one.
- (c)two X chromosomes from mother and one X chromosome from father — Three sex chromosomes in total, and two of them from one parent. A gamete carries only one chromosome of each pair, so a mother cannot contribute two X chromosomes to a normal child; XXX is a chromosomal abnormality, not the normal female pattern.
- (d)one X chromosome and one Y chromosome from father and one X chromosome from mother — Again three sex chromosomes, this time XXY, which is Klinefelter syndrome and arises from an error in chromosome separation. The stem specifies a normal female child, so only two sex chromosomes can be present.
Human sex determination is of the XX-XY type. The female is homogametic, carrying XX and producing only X-bearing eggs; the male is heterogametic, carrying XY and producing X-bearing and Y-bearing sperm in roughly equal numbers. Maleness is triggered by the SRY gene on the short arm of the Y chromosome, which sets the undifferentiated gonad on the path to becoming a testis. Because the paternal gamete is the variable one, the theoretical ratio of male to female births is one to one.
Two of the four options can be struck out before any biology is recalled, simply by counting: options (c) and (d) list three sex chromosomes, and a normal individual has two. That leaves a straight choice between XX and XY, and the stem's word 'female' settles it. This item carries real social weight, which is why examiners keep returning to it — the widespread belief that a woman is responsible for the sex of her child is biologically backwards, since every egg carries an X and only the sperm can carry a Y. Worth adding that other groups do it differently: birds use a ZZ-ZW system in which the female is the heterogametic sex, and in crocodiles and many turtles sex is set by the temperature at which the eggs incubate, not by chromosomes at all.
- Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes, XX in females and XY in males.
- Every egg carries an X chromosome; sperm carry either an X or a Y in roughly equal proportions.
- The sex of the child is therefore determined by the father's gamete, giving a theoretical one-to-one sex ratio.
- The SRY gene on the Y chromosome triggers development of the testis and hence the male pathway.
- Not all animals use XX-XY: in birds the female is ZW and the male ZZ, and in many reptiles sex is determined by incubation temperature.
Because every egg carries an X, the mother's contribution to the sex of the child is fixed and the father's is the one that changes.
- Believing the mother determines the sex of the child — she cannot, since all her eggs carry X.
- Overlooking that options listing three sex chromosomes describe abnormalities, not a normal child.
- Confusing the number of chromosome pairs, 23, with the number of chromosomes, 46.
As this combination item, as an assertion-and-reason on who determines the sex of the child, or through sex-linked inheritance problems on haemophilia and colour blindness.
Assertion (A): In human beings, the females play a major role in determining the sex of the offspring. Reason (R): Women have two ‘X’ chromosomes.
- (a) Both A and R are true, and R is the correct explanation of A
- (b) Both A and R are true, but R is not a correct explanation of A
- (c) A is true, but R is false
- (d) A is false, but R is true
Answer(d) A is false, but R is true
The same biology used to correct the same misconception. Women do carry two X chromosomes, which is exactly why they cannot determine the sex of the child — every egg they produce carries an X, so the variation has to come from the sperm.
- practice — not a real PYQ
The total number of chromosomes in a normal human somatic cell is
- (a)23
- (b)44
- (c)46
- (d)48
Answer(c) 46 — that is 23 pairs, of which 22 pairs are autosomes and one pair is the sex chromosomes.
- practice — not a real PYQ
In human beings, the sex of the child is determined by
- (a)the chromosome carried by the egg
- (b)the chromosome carried by the sperm that fertilises the egg
- (c)the mother's diet during pregnancy
- (d)the number of autosomes in the zygote
Answer(b) the chromosome carried by the sperm that fertilises the egg — an X-bearing sperm gives a girl and a Y-bearing sperm a boy, while every egg carries an X.