Which one of the following chromosomes has a mismatched pair in a normal human male ?

Correct — C, (c) X-chromosome. A normal human body cell carries 46 chromosomes as 23 pairs. Twenty-two of those pairs are AUTOSOMES, and in both sexes the two members of each autosomal pair are homologous — the same size, the same shape, the same banding pattern and the same sequence of gene loci. The twenty-third pair is the sex-chromosome pair, and it is the only one whose two members can differ. In a normal female it is XX, a matched pair. In a normal MALE it is XY, and the Y is a much smaller chromosome carrying far fewer genes than the X, so the two partners are unlike each other in size and in gene content — a mismatched, or heteromorphic, pair. Of the four options only the X-chromosome belongs to that pair; chromosomes 21, 18 and 13 are autosomes and are matched in a normal male exactly as they are in a normal female. Note the word 'normal' in the stem: it rules out the trisomies these three numbers are famous for, so their pairing cannot be the abnormality being asked about.

Why the others are wrong

• (a)Chromosome number 21 — Chromosome 21 is an AUTOSOME, and in a normal human male it is present as a matched homologous pair, one copy inherited from each parent. It is offered because it is the most famous chromosome number in school biology: an extra third copy of it produces trisomy 21, or Down syndrome. But that is a numerical abnormality — three chromosomes where there should be two — not a mismatched pair, and in any case the stem specifies a NORMAL male, which excludes it. Chromosome 21 is also the smallest human chromosome, which sometimes leads candidates to associate 'small' with 'mismatched'; the mismatch in a male is between X and Y, not between the two copies of 21.
• (b)Chromosome number 18 — Chromosome 18 is an autosome and is present as a matched pair in a normal male. Its exam reputation comes from trisomy 18, Edwards syndrome, again an extra copy rather than a mismatched partner, and again ruled out by the word 'normal' in the stem. Nothing about chromosome 18 differs between the sexes. This option, like the other two autosome options, is testing whether a candidate can separate two distinct ideas that share the vocabulary of chromosomes: an abnormal NUMBER of chromosomes, and a normal pair whose two members are structurally unlike.
• (d)Chromosome number 13 — Chromosome 13 is an autosome, matched in a normal male. It is the third of the well-known trisomy chromosomes — trisomy 13 is Patau syndrome — and it is here to complete the set 21, 18, 13 so that a candidate recalling those three conditions has three plausible-looking options to choose between and must instead notice that none of them answers the question asked. Autosomal pairs are homologous in both sexes; only the sex-chromosome pair can be heteromorphic, and it is heteromorphic only in the male.

The human karyotype is 46 chromosomes: 44 autosomes in 22 homologous pairs, plus one pair of sex chromosomes. A normal female is 44 + XX, a normal male 44 + XY. Homologous means the two members of a pair carry the same gene loci in the same order, which is what allows them to align and exchange material during meiosis. The X and Y do not satisfy that for most of their length — the X is a large chromosome with well over a thousand genes, the Y is small and gene-poor, carrying the SRY gene that triggers male development along with genes for spermatogenesis — so in the male the twenty-third pair is heteromorphic and pairs only at the short pseudoautosomal regions at the tips. Two consequences follow and are examined constantly. First, sex is determined by the father: the mother's egg always carries an X, while a sperm carries either X or Y, so which sperm fertilises the egg decides the sex of the child. Second, X-linked recessive conditions such as haemophilia and red-green colour blindness show up far more often in males, because a male has only one X and no second copy to mask a defective allele — he is described as hemizygous for X-linked genes. Set against this, an abnormality of chromosome NUMBER — trisomy 21, 18 or 13, or the sex-chromosome aneuploidies such as XXY (Klinefelter) and XO (Turner) — is a different category of fact altogether, and this question is built to see whether the two categories are kept apart.

Key facts

• A normal human cell has 46 chromosomes: 22 pairs of autosomes plus one pair of sex chromosomes.
• A normal female is 44 + XX; a normal male is 44 + XY.
• All 22 autosomal pairs are homologous — matched in size, shape and gene loci — in both sexes.
• The X and Y chromosomes differ greatly in size and gene content, so the sex-chromosome pair is mismatched (heteromorphic) only in the male.
• The Y chromosome carries the SRY gene, whose expression sets development on the male path.
• Sex in humans is determined by the sperm, since the egg always carries an X while a sperm carries X or Y.
• Males are hemizygous for X-linked genes, which is why X-linked recessive conditions such as haemophilia and red-green colour blindness are far commoner in males.
• Trisomy 21 is Down syndrome, trisomy 18 Edwards syndrome and trisomy 13 Patau syndrome — all abnormalities of chromosome NUMBER, not of pair matching.

Common traps

• Reading past 'normal' in the stem and answering with a trisomy chromosome. In a normal male all three autosome options are perfectly matched pairs.
• Confusing an abnormal NUMBER of chromosomes with a normally mismatched PAIR. They are different ideas that share vocabulary.
• Picking chromosome 21 because it is the smallest human chromosome. Size relative to other chromosomes is irrelevant; the mismatch is between the two partners of one pair.
• Thinking the X-chromosome is mismatched in females too. In XX it is a matched pair; the mismatch exists only in the male.
• Attributing the sex of a child to the mother. The egg always contributes an X.

• practice — not a real PYQ

  How many pairs of autosomes are present in a normal human somatic cell ?

  • (a)21
  • (b)22
  • (c)23
  • (d)44

  Answer(b) 22

• practice — not a real PYQ

  Red-green colour blindness is far more common in men than in women because

  • (a)the gene concerned lies on the Y chromosome, which only men have
  • (b)men have a single X chromosome and no second copy to mask the recessive allele
  • (c)the condition is caused by an extra copy of chromosome 21
  • (d)women inherit the gene only from their mothers

  Answer(b) men have a single X chromosome and no second copy to mask the recessive allele