Genetic crosses, family trees and sex determination
A Punnett square predicts the chances of offspring inheriting a characteristic. It only takes a handful of words and a two by two grid, but the words must be used exactly.
Part 1 of 3: Learn it
In short
- A dominant allele (capital letter) shows even if there is only one copy; a recessive allele shows only with two copies.
- Genotype is the pair of alleles (such as Bb); phenotype is the characteristic you see.
- Females are XX and males XY, so each child has a 50% chance of being male.
Where this is in your specification
Spec points: AQA 4.6.1.6 and 4.6.1.8 (Combined Trilogy 4.6.1.6 and 4.6.1.8), Edexcel Topic 3, OCR B5.1
| Board | Topic: Inheritance, variation and evolution |
|---|---|
| AQA 8461 | 4.6 (8461); 4.6 (8464) |
| Edexcel 1BI0 | Topics 3 and 4 |
| OCR J247 | B5 and B6 |
| Eduqas C400 | 7.1 to 7.4 |
| Cambridge IGCSE 0610 | 16.1 to 16.4, 17.1 to 17.4, 18.1 to 18.3, 21.3 |
| National 5 C807 75 | Cell biology: 3 DNA and the production of proteins, 5 Genetic engineering; Multicellular organisms: 3 Reproduction, 4 Variation and inheritance; Life on Earth: 6 Evolution of species |
The key words
| Word | Meaning |
|---|---|
| Allele | a version of a gene |
| Dominant | shows in the phenotype even with only one copy |
| Recessive | only shows when there are two copies |
| Homozygous | two identical alleles, such as BB or bb |
| Heterozygous | two different alleles, such as Bb |
| Genotype | the alleles an organism has |
| Phenotype | the characteristic that results |
Punnett squares
- Write each parent's genotype and split it into the alleles in their gametes.
- Put one parent's gametes across the top and the other's down the side.
- Fill each box with one allele from the top and one from the side.
- Count the boxes to give the ratio, the fraction or the percentage asked for.
Inherited disorders and family trees
Cystic fibrosis is caused by a recessive allele, so a person can be a carrier (heterozygous) without having the disorder. Polydactyly (extra fingers or toes) is caused by a dominant allele, so one copy is enough.
In a family tree, if two parents without a condition have a child with it, the condition must be recessive and both parents must be carriers.
Sex determination
One of the 23 pairs of human chromosomes decides sex. Eggs always carry an X; sperm carry either an X or a Y. A Punnett square of XX × XY gives two XX and two XY, so a 1 : 1 ratio.
What is the genotype of someone who is homozygous recessive for a gene with alleles R and r?
Show the answer
rr.
Part 2 of 3: See it worked
Worked examples
Example 1
Two pea plants, both Tt, are crossed. T (tall) is dominant over t (short). Find the expected ratio of tall to short plants.
- Gametes from each parent: T and t
- Boxes: TT, Tt, Tt, tt
- Tall: TT, Tt, Tt (3 boxes); short: tt (1 box)
Answer: 3 tall : 1 short.
Example 2
A woman who is a carrier of cystic fibrosis (Ff) has a child with a man who is ff. What is the probability the child has cystic fibrosis?
- Gametes: F and f from the mother; f and f from the father
- Boxes: Ff, Ff, ff, ff
- 2 of the 4 boxes are ff
Answer: 1/2, or 50%.
Common mistakes
- Treating a 3 : 1 ratio as "3 out of 3". It means 3 out of every 4.
- Thinking a 25% chance means exactly one in four children will be affected. Each child is a separate event.
- Mixing up genotype and phenotype.
- Using different letters for the two alleles of one gene, such as B and t. Use the same letter in capital and lower case.
What is the chance that a baby will be a boy?
Show the answer
50%, because half of the sperm carry a Y chromosome.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
What is the genotype of someone who is homozygous recessive for a gene with alleles R and r?
rr.
What is the chance that a baby will be a boy?
50%, because half of the sperm carry a Y chromosome.
Two parents are both Bb. What fraction of their children are expected to be bb?
1/4 (one box out of four).
Jobs that use this
- Geneticist (s'ouvre dans un nouvel onglet)
- GP (s'ouvre dans un nouvel onglet)
- Biomedical scientist (s'ouvre dans un nouvel onglet)
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Ces fiches sont en anglais car elles suivent les programmes d'examen britanniques.
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses