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
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).
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses