13.7 Inheritance in Action

A free GCSE Biology lesson for AQA (8461) from OBA School: alleles, dominant and recessive, genotype and phenotype, Punnett squares and ratios. 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

• Use the terms allele, dominant, recessive, homozygous, heterozygous, genotype and phenotype
• Recall that most characteristics are controlled by several genes, but some by a single gene
• Complete and construct Punnett squares for single-gene crosses (construct: Higher tier)
• Express the outcome of a cross as a ratio, a probability, a fraction or a percentage
• Use direct proportion to predict numbers of offspring and compare them with real results

Key words

• Allele: A different form (variant) of the same gene, e.g. B and b.
• Dominant: An allele that is always expressed, even if only one copy is present (capital letter).
• Recessive: An allele that is only expressed if two copies are present, i.e. no dominant allele (lower-case letter).
• Homozygous: Two identical alleles of a gene, e.g. BB or bb.
• Heterozygous: Two different alleles of a gene, e.g. Bb.
• Genotype: The alleles an organism has, e.g. Bb.
• Phenotype: The characteristic that is expressed, e.g. black fur.

Genes and alleles

Some characteristics are controlled by a single gene, for example fur colour in mice and red-green colour blindness in humans. But most characteristics, such as height or skin colour, are the result of multiple genes interacting (and often the environment too).

Each gene may have different forms called alleles. Because chromosomes come in pairs, you have two alleles for each gene: one from each parent. The alleles present (the genotype) work at a molecular level, by coding for proteins, to produce the characteristics that are expressed (the phenotype).

Exam tip: Use one letter per gene: a capital for the dominant allele and the same letter in lower case for the recessive allele (B and b). Pick letters that look different in both cases, e.g. B/b rather than S/s.

Dominant and recessive

In mice, the allele for black fur (B) is dominant and the allele for brown fur (b) is recessive.

• BB; Description homozygous dominant; Phenotype black fur
• Bb; Description heterozygous; Phenotype black fur (one B is enough)
• bb; Description homozygous recessive; Phenotype brown fur

Key fact: If an organism shows the recessive phenotype, you know its genotype straight away: it must be homozygous recessive (bb). A dominant phenotype could be BB or Bb.

Punnett squares

A Punnett square shows all the possible combinations of gametes. Write one parent's gametes across the top and the other's down the side, then fill in each box. (Higher tier: you must be able to construct these yourself.)

Worked example: two heterozygous black mice (Bb × Bb)

Gametes: B or b from each parent.

• B; B BB; b Bb
• b; B Bb; b bb

Genotypes: 1 BB : 2 Bb : 1 bb. Phenotypes: 3 black : 1 brown. Probability of a brown pup = 1/4 = 0.25 = 25%.

Worked example: a heterozygous mouse with a brown mouse (Bb × bb)

Gametes: B or b; and b or b. Offspring: Bb, Bb, bb, bb.
Ratio 1 black : 1 brown, so probability of brown = 2/4 = 1/2 = 50%.

Answer: 1 : 1

Exam tip: Each fertilisation is a separate chance event. A 3 : 1 ratio does not mean every litter of four has exactly three black pups.

Ratios, probability and direct proportion

Use the ratio to predict numbers: the expected number is in direct proportion to the total number of offspring.

Worked example: predicting numbers

A Bb × Bb cross gives 64 offspring. How many do you expect to be brown?
Brown = 1 out of every 4: 64 × 1/4 = 16. Black = 64 − 16 = 48.

Answer: 16 brown, 48 black

Worked example: comparing real results with the ratio

Total = 152 + 48 = 200. Ratio = 152 ÷ 48 = 3.17 : 1, close to the expected 3 : 1.
Expected: 200 × 3/4 = 150 red and 50 yellow. Real results differ a little because fertilisation is random.

Answer: about 3 : 1, as predicted

Common mistakes

• Writing alleles with different letters, e.g. B and w. Use the same letter: capital for dominant, lower case for recessive.
• Mixing up genotype (the letters) and phenotype (what you see).
• Thinking "dominant" means "more common". A dominant allele can be rare.
• Giving the genotype ratio when the question asks for the phenotype ratio (1 : 2 : 1 vs 3 : 1).
• Expecting real numbers to match the ratio exactly. Larger samples get closer to it.

Revision questions

Try these, then check your answers below each one.

Question 1. In mice, black fur (B) is dominant to brown fur (b). Which genotype gives brown fur? (Choose from: BB, Bb, bb.)

Answer: bb

Working: Brown is recessive, so a brown mouse must be bb.

Question 2. True or false? Most human characteristics, such as height, are controlled by a single gene.

Answer: False

Working: Most characteristics result from multiple genes interacting. A few, like red-green colour blindness, are single-gene.

Question 3. A plant with genotype Tt is crossed with a plant with genotype tt. What percentage of the offspring would you expect to show the recessive phenotype?

Answer: 50 %

Working: Offspring Tt, Tt, tt, tt: 2 out of 4 are tt, so 50%.

Related lessons

• Previous lesson: 13.6 Gene Expression and Mutation
• Next lesson: 13.8 More About Genetics
• In this chapter: 13.1 Types of Reproduction
• In this chapter: 13.2 Cell Division in Sexual Reproduction
• In this chapter: 13.3 The Best of Both Worlds

Continue in the full course

This lesson is section 13.7 (AGCBH1307) 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.