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B12.4 Inheritance in Action

GCSE Combined Science · AQA · Chapter B12: Reproduction

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: alleles, dominant and recessive, genotype and phenotype, Punnett squares, ratios and probability. 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 gamete, chromosome, gene, allele, dominant, recessive, homozygous, heterozygous, genotype and phenotype
  • Recall that most characteristics are controlled by several genes, but some by a single gene
  • Complete a Punnett square and interpret it; construct your own crosses (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 (version) of the same gene, e.g. R and r.
  • Dominant: An allele that is always expressed, even if only one copy is present. Written as a capital letter.
  • Recessive: An allele that is only expressed if two copies are present (no dominant allele). Lower-case letter.
  • Homozygous: Two identical alleles of a gene, e.g. RR or rr.
  • Heterozygous: Two different alleles of a gene, e.g. Rr.
  • Genotype: The alleles an organism has, e.g. Rr.
  • Phenotype: The characteristic that is expressed, e.g. round seeds.

Genes and alleles

Some characteristics are controlled by a single gene, for example seed shape in peas, attached or free ear lobes, and fur colour in mice. But most characteristics, such as height, are the result of multiple genes interacting (and often the environment as well).

Each gene may have different forms called alleles. Chromosomes come in pairs, so you have two alleles of each gene: one from each parent. The alleles present make up the genotype; they 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 one (R and r). Choose letters that look different in both cases, e.g. R/r or T/t rather than S/s.

Dominant and recessive

In pea plants, the allele for round seeds (R) is dominant and the allele for wrinkled seeds (r) is recessive.

  • RR; Description homozygous dominant; Phenotype round seeds
  • Rr; Description heterozygous; Phenotype round seeds (one R is enough)
  • rr; Description homozygous recessive; Phenotype wrinkled seeds

Key fact: An organism showing the recessive phenotype must be homozygous recessive (rr). A dominant phenotype could be RR or Rr.

Punnett squares

A Punnett square shows all the possible ways the gametes can combine. Put one parent's gametes across the top and the other's down the side, then fill in each box. All students should be able to complete one; (Higher tier) you must also be able to construct your own.

Genotypes: 1 RR : 2 Rr : 1 rr. Phenotypes: 3 round : 1 wrinkled. Probability of a wrinkled seed = 1/4 = 0.25 = 25%.

Worked example: a heterozygous plant with a wrinkled plant (Rr × rr)

Gametes: R or r; and r or r. Offspring: Rr, Rr, rr, rr.
Ratio 1 round : 1 wrinkled, so the probability of wrinkled = 2/4 = 1/2 = 50%.

Answer: 1 : 1

Exam tip: Each fertilisation is a separate chance event. A 3 : 1 ratio does not mean that every group of four seeds has exactly three round ones.

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

An Rr × Rr cross gives 480 seeds. How many do you expect to be wrinkled?
Wrinkled = 1 in every 4: 480 × 1/4 = 120. Round = 480 − 120 = 360.

Answer: 120 wrinkled, 360 round

Worked example: comparing real results with the ratio

Total = 273 + 87 = 360. Ratio = 273 ÷ 87 = 3.14 : 1, close to the expected 3 : 1.
Expected: 360 × 3/4 = 270 round and 90 wrinkled. The real results differ a little because fertilisation is random.

Answer: about 3 : 1, as predicted

Common mistakes

  • Using different letters for alleles of one gene, e.g. R and w. Use one 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 (1 : 2 : 1) when the question asks for the phenotype ratio (3 : 1).
  • Expecting real numbers to match the ratio exactly. Bigger samples get closer to it.

Revision questions

Try these, then check your answers below each one.

Question 1. In pea plants, round seeds (R) are dominant to wrinkled seeds (r). Which genotype gives wrinkled seeds? (Choose from: rr, Rr, RR, Rr or rr.)

Answer: rr

Working: Wrinkled is recessive, so it only shows when both alleles are recessive: rr.

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

Answer: False

Working: Most characteristics, like height, result from many genes interacting (and the environment). Only a few, like attached ear lobes, are controlled by a single gene.

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

Answer: 50 %

Working: Offspring: Ee, Ee, ee, ee. 2 out of 4 are ee, so 50% show the recessive phenotype.

Related lessons

Continue in the full course

This lesson is section B12.4 (AGCSC1204) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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