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13.8 More About Genetics

GCSE Biology · AQA · Chapter 13: Reproduction

A free GCSE Biology lesson for AQA (8461) from OBA School: sex determination, and using family trees to work out how characteristics are inherited. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Recall that human body cells have 23 pairs of chromosomes, including one pair of sex chromosomes
  • Explain how sex is determined, using a Punnett square
  • Read a family tree and decide whether a characteristic is caused by a dominant or a recessive allele
  • Work out genotypes and probabilities from family trees

Key words

  • Sex chromosomes: The pair of chromosomes that decides sex: XX in females, XY in males.
  • X chromosome: The larger sex chromosome. Females have two, males have one.
  • Y chromosome: The smaller sex chromosome, found only in males.
  • Family tree: A diagram that shows how a characteristic has been passed through generations of a family.
  • Carrier: A person who is heterozygous for a recessive disorder: they carry one faulty allele but do not have the disorder.

Chromosomes and sex

Ordinary human body cells contain 23 pairs of chromosomes. 22 pairs control characteristics only. One pair carries the genes that determine sex: the sex chromosomes.

  • Female; Sex chromosomes in body cells XX; Sex chromosome in gametes every egg has an X
  • Male; Sex chromosomes in body cells XY; Sex chromosome in gametes half the sperm have X, half have Y

Key fact: So it is the sperm that decides the sex of the baby: an X sperm gives a girl, a Y sperm gives a boy.

Inheriting sex: a Punnett square

Treat the sex chromosomes just like alleles. The mother's gametes go down the side, the father's across the top.

  • X (egg); X (sperm) XX girl; Y (sperm) XY boy
  • X (egg); X (sperm) XX girl; Y (sperm) XY boy

Worked example: probability of a boy

From the square: 2 out of 4 offspring are XY. Probability of a boy = 2/4 = 1/2 = 50%. Ratio 1 boy : 1 girl.
A couple already have three daughters. The probability that the next baby is a boy is still 50%: each fertilisation is a separate event.

Answer: 0.5 (50%) every time

Worked example: two in a row (maths skill)

What is the probability that a couple's next two children are both girls?
Multiply the separate probabilities: 1/2 × 1/2 = 1/4.

Answer: 1/4 (25%)

Reading family trees

A family tree uses squares for males and circles for females. Shaded symbols show people with the characteristic. A horizontal line joins parents; lines down lead to their children.

Worked example: dominant or recessive?

Look for two unaffected parents with an affected child. People 1 and 2 are unaffected, but their son 4 is affected.
So 1 and 2 must each carry a hidden allele. A hidden allele is recessive.
Using N (unaffected) and n (condition): 4 is nn, so 1 and 2 are both Nn (carriers).

Answer: caused by a recessive allele

Exam tip: Two unaffected parents with an affected child → recessive. Two affected parents with an unaffected child → dominant.

Probabilities from a family tree

Worked example: the next child

In family tree A, people 5 and 6 are unaffected but their daughter 8 is affected (nn). So 5 and 6 are both Nn.
Nn × Nn gives NN, Nn, Nn, nn. Probability that their next child is affected = 1/4 = 25%.
Probability that the next child is an affected boy = 1/4 × 1/2 = 1/8.

Answer: 25% affected; 1/8 affected and male

Common mistakes

  • Saying the mother decides the sex of the baby. All eggs carry X; the sperm carries X or Y.
  • Thinking that after three girls a boy is "due". Each baby has a 50% chance of being a boy.
  • Deciding a condition is dominant because it appears in many people. Use the pattern of parents and children instead.
  • Forgetting that an unaffected parent of an affected child (recessive condition) must be a carrier.

Revision questions

Try these, then check your answers below each one.

Question 1. What are the sex chromosomes in a human male body cell? (Choose from: XX, XY, YY.)

Answer: XY

Working: Males have one X and one Y chromosome.

Question 2. Human body cells contain 23 pairs of chromosomes. How many of these pairs control characteristics only, and not sex?

Answer: 22

Working: 23 − 1 = 22 pairs control characteristics only.

Question 3. True or false? The sex of a baby is decided by the egg cell.

Answer: False

Working: Every egg carries an X. Sperm carry X or Y, so the sperm decides the sex.

Related lessons

Continue in the full course

This lesson is section 13.8 (AGCBH1308) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.

 

 

 

 

 

 

 

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