How is the sex of a baby determined, and how are genetic disorders inherited?
How the X and Y sex chromosomes determine sex, using a genetic cross to show the equal chance of a boy or girl, and how genetic disorders such as cystic fibrosis are inherited from carrier parents.
A focused CCEA GCSE Biology answer on sex determination and genetic disorders, covering the X and Y chromosomes, the genetic cross showing equal chances of a boy or girl, and how disorders such as cystic fibrosis are inherited from carriers.
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What this dot point is asking
CCEA wants you to explain how the X and Y chromosomes determine sex, use a genetic cross to show the equal chance of a boy or girl, and explain how genetic disorders such as cystic fibrosis are inherited from carrier parents.
Sex chromosomes
The genetic cross for sex
All of a mother's eggs carry an X chromosome. Half of a father's sperm carry an X and half carry a Y. So whether a baby is a boy or a girl depends on which sperm fertilises the egg.
Genetic disorders
If both parents are carriers, a Punnett square shows a 1 in 4 (25 percent) chance that a child has the disorder, a 2 in 4 chance of being a carrier, and a 1 in 4 chance of being unaffected.
Examples in context
- Example 1. Why a recessive disorder can skip generations
- Cystic fibrosis can appear in a child whose parents are both healthy, because both parents can be carriers (Ff) who never show the disorder. The faulty allele is hidden in the carriers and only causes the disorder when a child inherits two copies (ff). This is why a recessive disorder can seem to skip generations, reappearing when two carriers have children, and it is a classic CCEA genetic-cross question.
- Example 2. Using genetic counselling
- A couple who know that cystic fibrosis runs in their families may seek genetic counselling. By working out whether each parent is a carrier, a counsellor can use a Punnett square to explain the 1 in 4 risk of an affected child. The couple can then make informed decisions, perhaps choosing testing during pregnancy. This shows how understanding the inheritance of a recessive disorder has real, practical uses, which CCEA expects you to be able to discuss.
- Example 3. What cystic fibrosis does and why it is recessive
- Cystic fibrosis is caused by a faulty recessive allele of a gene that controls the movement of salt and water across cell membranes. People with two faulty copies (ff) produce thick, sticky mucus that clogs the lungs and the pancreas, causing breathing and digestion problems. A carrier (Ff) has one working copy of the gene, and that single working allele makes enough normal protein, which is why the carrier is healthy and the disorder is recessive. This connects the inheritance pattern back to genes and proteins: a recessive disorder appears only when there is no working copy of the gene at all. CCEA may ask you to link the genetics to the effect on the body.
Try this
Q1. What are the sex chromosomes of a human male? [1 mark]
- Cue. XY.
Q2. Two carriers (Ff) of a recessive disorder have a child. What is the chance the child is affected? [1 mark]
- Cue. 1 in 4 (25 percent).
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20204 marksUse a genetic diagram to explain why there is an equal chance of a baby being a boy or a girl.Show worked answer →
Four marks for the chromosomes, the gametes and the cross.
Females have two X chromosomes (XX) and males have one X and one Y (XY).
The mother's eggs all carry an X chromosome. The father's sperm carry either an X or a Y, in equal numbers.
A Punnett square crossing XX with XY gives offspring XX, XX, XY, XY.
So half the offspring are XX (girls) and half are XY (boys), giving a 50 percent chance of each.
Markers reward XX female and XY male, the father's sperm being X or Y, the correct cross, and the 1 to 1 (50 percent) ratio.
CCEA 20194 marksCystic fibrosis is caused by a recessive allele. Two carrier parents have a child. Explain the chance the child has cystic fibrosis.Show worked answer →
Four marks for carrier genotypes, the cross and the probability.
Cystic fibrosis is recessive, so a person with the disorder is homozygous recessive (ff), and a carrier is heterozygous (Ff) and does not have the disorder.
Both parents are carriers (Ff). A Punnett square of Ff x Ff gives FF, Ff, Ff, ff.
The ff child has cystic fibrosis, which is 1 out of the 4 boxes.
So there is a 1 in 4 (25 percent) chance the child has cystic fibrosis, and a 2 in 4 chance of being a carrier.
Markers reward carriers being Ff, the affected child being ff, the Punnett square, and the 1 in 4 (25 percent) chance.
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Sources & how we know this
- CCEA GCSE Biology specification — CCEA (2017)