Genetics and Gene Technology

Lessons

Senior 3 · Biology

Genetics and Gene Technology

5 lessons

Lesson 4 of 5

Inheritance of sex and sex-linked characteristics

25 min

Why mostly boys?

Eye clinics test colour vision with plates of coloured dots that hide a number. Most people read the number at once. A person with red–green colour blindness sees the reds and greens as similar and may see no number at all. When such tests are given to large groups of school pupils, far more boys than girls fail them. Two other facts make the puzzle sharper. A colour-blind man almost never has a colour-blind son unless the mother also carries the allele, but his daughter's sons often are colour-blind. The characteristic seems to pass from grandfather to grandson through a daughter who sees normally.

The explanation lies in the two chromosomes that decide whether a baby is a boy or a girl.

Definition

Autosomes: the chromosomes that are the same in males and females. A human body cell has 22 pairs (44 autosomes).

Sex chromosomes: the pair that decides sex. Human females have two X chromosomes (XX); males have one X and one Y (XY).

Sex-linked gene: a gene carried on a sex chromosome. In humans almost all are on the X chromosome, so they are also called X-linked genes.

Carrier: a person who has one copy of a recessive allele, and so does not show it, but can pass it on. Example: a woman with genotype $\mathrm{X^H X^h}$ for haemophilia.

How sex is inherited

A human body cell has 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes. The X chromosome is large and carries many genes. The Y chromosome is small and carries few genes, but one of them starts the development of testes in the embryo, so an XY embryo develops as a male.

In meiosis the sex chromosomes separate like any other pair. Every egg receives one X. A sperm receives either an X or a Y, in equal numbers.

Worked example

Worked example 1: boy or girl? Show that a couple's chance of having a girl is the same as their chance of having a boy, and say which parent's gamete decides it.

Each child has a $\frac{1}{2}$ chance of being a girl (XX) and $\frac{1}{2}$ of being a boy (XY). The egg always carries X, so the sperm decides: an X-carrying sperm gives a girl, a Y-carrying sperm gives a boy. A mother who has only daughters, or only sons, is not the cause; in some families mothers have been blamed for this, and biology shows the blame is wrong. It is chance, and the father's sperm, not the mother's egg, carries the deciding chromosome.

Genes on the X chromosome

A girl has two X chromosomes, so she has two alleles of every X-linked gene, just as for autosomes. A boy has only one X; his Y has no matching locus for most of those genes. So a boy has one allele of each X-linked gene, and whatever that allele is, it shows, even if it is recessive.

To write X-linked genotypes, put the allele as a superscript on the X. For haemophilia, $\mathrm{X^H}$ is the normal allele and $\mathrm{X^h}$ the recessive allele for haemophilia. The Y carries no allele, so it is written Y alone.

Genotype	Phenotype
$\mathrm{X^H X^H}$	female, normal clotting
$\mathrm{X^H X^h}$	female, normal clotting, carrier
$\mathrm{X^h X^h}$	female with haemophilia (rare)
$\mathrm{X^H Y}$	male, normal clotting
$\mathrm{X^h Y}$	male with haemophilia

Definition

Haemophilia: an X-linked recessive condition in which the blood clots very slowly, because one of the clotting factors is missing or does not work (factor VIII in haemophilia A, factor IX in haemophilia B). Small cuts and bruises can bleed for a long time, and bleeding into joints is painful. It is treated by injecting the missing clotting factor.

Red–green colour blindness: an X-linked recessive condition in which some of the light-sensitive cone cells of the retina (Unit 10) do not work normally, so red and green are hard to tell apart.

Worked example

Worked example 2: haemophilia. A woman who is a carrier of haemophilia marries a man with normal clotting. What is the chance that (a) a daughter is a carrier, (b) a son has haemophilia, (c) their next child is a son with haemophilia?

Mother $\mathrm{X^H X^h}$ × father $\mathrm{X^H Y}$ .

The daughters are $\mathrm{X^H X^H}$ and $\mathrm{X^H X^h}$ : half of the daughters are carriers, $\frac{1}{2}$ . No daughter has haemophilia, because each receives $\mathrm{X^H}$ from her father.
The sons are $\mathrm{X^H Y}$ and $\mathrm{X^h Y}$ : half of the sons, $\frac{1}{2}$ , have haemophilia.
Of all four boxes, one is $\mathrm{X^h Y}$ : the chance that the next child is a son with haemophilia is $\frac{1}{4}$ .

Read the question carefully: "a son has haemophilia" ( $\frac{1}{2}$ ) and "the next child is a son with haemophilia" ( $\frac{1}{4}$ ) are different questions.

Worked example

Worked example 3: colour blindness through three generations. Use $\mathrm{X^B}$ for normal colour vision and $\mathrm{X^b}$ for red–green colour blindness. Explain the family in Figure 1.

Figure 1. A family tree (pedigree) for red–green colour blindness. Squares are males, circles are females, and a horizontal line joining a male and a female shows a couple; their children hang below.

The colour-blind grandfather (I) is $\mathrm{X^b Y}$ . He gives his Y to his son and his $\mathrm{X^b}$ to his daughter. So his son has normal vision ( $\mathrm{X^B Y}$ , with $\mathrm{X^B}$ from his mother) and his daughter is a carrier ( $\mathrm{X^B X^b}$ ).
The carrier daughter (II) passes $\mathrm{X^b}$ to half of her children. A son who receives it is colour-blind ( $\mathrm{X^b Y}$ ), because his Y has no allele to mask it.
Her daughter has normal vision, but may be $\mathrm{X^B X^B}$ or $\mathrm{X^B X^b}$ : written $\mathrm{X^B X^?}$ .

So the allele passed from grandfather to grandson through a daughter who saw normally. A father never passes an X-linked allele to his son.

Warning

Do not give the Y an allele. Write $\mathrm{X^h Y}$ , never $\mathrm{X^h Y^h}$ . Also, do not use capital and small letters without the X (such as Hh) for a sex-linked gene: the examiner needs to see which allele is on which chromosome, and that a male has only one.

Punnett Squares and Sex-Linked Traits (updated), from the Amoeba Sisters. Watch how the alleles are written on the X, and why a male with one recessive X-linked allele shows the trait.

Tip

Sons get their X from their mother. To work out a son's X-linked phenotype, look only at his mother. To know whether a daughter can be affected, you need both parents: she must receive the recessive allele from each, so her father must be affected.

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Practice: sex and sex linkage

Give the sex chromosomes in a body cell of (a) a woman, (b) a man. How many autosomes are in a human body cell?
Explain, with a Punnett square, why the sex of a baby is decided by the father's gamete. What does this show about blaming a mother for having only daughters?
Explain why haemophilia is much more common in males than in females.
A man with haemophilia marries a woman who is homozygous for normal clotting. Give the genotypes of their children. What fraction of their sons will have haemophilia?
A couple who both have normal colour vision have a colour-blind son. Give the parents' genotypes. From which parent did the son receive the allele?
A woman with red–green colour blindness marries a man with normal colour vision. Predict the phenotypes of their sons and of their daughters.
Can a girl be colour-blind? State the genotypes her parents would need.
For the couple in Worked example 2, which is larger: the chance that their next baby is a boy, or the chance that their next son has haemophilia? Explain.

Show answersHide answers

(a) XX; (b) XY. There are 44 autosomes (22 pairs).
The mother's eggs all carry X; half of the father's sperm carry X and half carry Y. The sperm that fertilises the egg decides: X gives XX (girl), Y gives XY (boy). The mother does not decide the sex, so blaming her is wrong.
A male has one X, so one recessive allele $\mathrm{X^h}$ is enough to cause haemophilia. A female needs two recessive alleles, one from each parent, which is much less likely.
$\mathrm{X^h Y} \times \mathrm{X^H X^H}$ : daughters $\mathrm{X^H X^h}$ (all carriers); sons $\mathrm{X^H Y}$ . None of the sons has haemophilia: they receive Y from their father.
Mother $\mathrm{X^B X^b}$ (carrier), father $\mathrm{X^B Y}$ . The son is $\mathrm{X^b Y}$ ; his X came from his mother.
$\mathrm{X^b X^b} \times \mathrm{X^B Y}$ : all sons $\mathrm{X^b Y}$ , colour-blind; all daughters $\mathrm{X^B X^b}$ , normal vision but carriers.
Yes. She must be $\mathrm{X^b X^b}$ : her father must be colour-blind ( $\mathrm{X^b Y}$ ) and her mother a carrier or colour-blind.
They are equal, both $\frac{1}{2}$ . The chance of a boy is $\frac{1}{2}$ ; among sons, half are $\mathrm{X^h Y}$ , also $\frac{1}{2}$ . (The chance that the next child is a son with haemophilia is $\frac{1}{4}$ .)

Key points

Humans have 22 pairs of autosomes and one pair of sex chromosomes: XX female, XY male. The sperm decides the sex, with a 1 : 1 ratio.
X-linked genes: females have two alleles, males one. A single recessive X-linked allele shows in a male.
Haemophilia and red–green colour blindness are X-linked recessive, so they are more common in males; females are usually carriers.
A son's X comes from his mother; a father passes his X to every daughter and to no son.

Sources and credits

Photo: Ishihara colour blindness test, London, England, 1948, Science Museum, London, Wellcome Collection, via Wikimedia Commons, licensed under CC BY 4.0.
Video: Punnett Squares and Sex-Linked Traits (updated), Amoeba Sisters, embedded from YouTube.

Written by Ganzaa: the text, the tables, the Punnett squares, the family tree (Figure 1), the worked examples and the practice questions.

1Why mostly boys?

2How sex is inherited

3Genes on the X chromosome

Why mostly boys?

How sex is inherited

Genes on the X chromosome