BIOLOGY · Stage 9 · Chapter 11: Human Genetics and Development
Biology 11.3 Boy or girl? Sex inheritance in humans — X and Y Chromosomes — Alhussan International Academy - Biology (Stages 7-9) Revision Notes & Topical Questions
5 min read 5 parts Lesson 3 of 9
Lesson summary
11.3 Boy or girl? Sex inheritance in humans — X and Y Chromosomes — Alhussan International Academy — Biology (Stages 7–9) notes: Explain how sex is determined by the X and Y chromosomes and use a Punnett square to show the probability of having a boy or girl.Inheritance — at fert…
- Gene copies
- Gene number
- Sex chromosomes
- Deciding the sex
- Independence
- Sex-linked characteristics
Section 1 of 5Chromosomes from each parent
- Inheritance
- at fertilisation the embryo gets half of its chromosomes from the egg (its mother) and half from the sperm (its father).
- Each gamete contains 23 chromosomes and each cell of the embryo contains 46.
- Gene copies
- chromosomes come in pairs that carry the same genes, so you get one copy of each gene from your mother and one from your father.
- Gene number
- each of the 46 chromosomes carries around 1000 genes.
Section 2 of 5Chromosomes and sex
- Autosomes
- 22 of the 23 pairs have two chromosomes of the same size and shape with matching genes, in both males and females.
- Sex chromosomes
- the 23rd pair determines whether a person is female or male.
- The X chromosome is relatively large.
- The Y chromosome is much smaller with just a few genes.
- Females
- they have two X chromosomes (XX).
- Males
- they have one X and one Y chromosome (XY).
- The SRY gene on the Y chromosome makes the embryo develop testes.
Section 3 of 5Gametes carry one of each pair
- Meiosis
- gametes are made by meiosis, which halves the chromosome number, so each gamete has one chromosome from each pair.
- Eggs
- women's cells are XX, so every egg contains an X chromosome.
- Sperm
- men's cells are XY, so half the sperm carry an X chromosome and half carry a Y chromosome.
- Deciding the sex
- the sex of the baby depends on which sperm fertilises the egg.
- All sperm have an equal chance, so it is a matter of chance.
- An X sperm gives XX, a girl; a Y sperm gives XY, a boy.
Section 4 of 5Punnett square for sex
| X (mother) | X (mother) | |
|---|---|---|
| X (father) | XX girl | XX girl |
| Y (father) | XY boy | XY boy |
- Probability
- each child has a 1 in 2 (50%) chance of being a girl and 1 in 2 (50%) of being a boy.
- Independence
- every pregnancy has the same chances, so having three boys does not make a girl more likely next time.
- Ratio
- the ratio of boys to girls in a population is about 1:1, though a small sample may differ by chance.
Section 5 of 5Other points
- The father's sperm decides the sex, because the mother can only give an X.
- Sex-linked characteristics
- some characteristics, such as red-green colour blindness, are controlled by genes on the X chromosome.
- Males have only one X, so a single faulty allele shows up.
- Females must inherit the faulty allele on both X chromosomes, so more males than females are colour blind.
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