OCR GCSE Biology A B5.1 Inheritance Exam Questions

About OCR GCSE Biology A B5.1 Inheritance

B5.1 explains how characteristics are passed from parents to offspring. On the OCR Gateway Biology A course it opens Topic B5 Genes, inheritance and selection, assessed in Paper 2 (Foundation) or Paper 4 (Higher). You learn the language of genetics, how to use Punnett squares, how sex is determined and how meiosis produces gametes. It builds on DNA from B1.2 and mitosis from B2.1, and leads into natural selection in B5.2 and genetic engineering in B6.2. Examiners regularly set genetic crosses with ratios or probabilities, ask you to compare mitosis and meiosis, and test the difference between genotype and phenotype.

What you need to know

• Key terms The genome is the entire genetic material of an organism. A gene is a section of DNA coding for a protein; alleles are different versions of the same gene. Genotype is the combination of alleles and phenotype is the characteristic that results.
• Dominant and recessive A dominant allele is expressed when present once; a recessive allele is only expressed when two copies are present. Homozygous means two identical alleles, heterozygous means two different alleles.
• Genetic crosses Use Punnett squares to predict offspring genotypes and phenotypes. A cross between two heterozygous parents (Aa x Aa) gives a 3 : 1 ratio of dominant to recessive phenotypes.
• Sex determination Humans have 23 pairs of chromosomes; one pair are sex chromosomes. Females are XX and males are XY, so there is a 50% chance of each sex at fertilisation.
• Meiosis Meiosis is cell division that produces four genetically different haploid gametes, each with half the chromosome number. At fertilisation the diploid number is restored and genetic variation increases.
• Variation and mutation Variation is caused by genes, the environment or both. Mutations are random changes in DNA; most have no effect, but some alter the protein made and can change the phenotype.

Common mistakes in B5.1 Inheritance

• Mixing up genotype and phenotype. Genotype is the alleles (for example Bb); phenotype is the visible or measurable characteristic (for example brown eyes).
• Giving a ratio instead of a probability when asked for a probability. A 1 in 4 chance can be written as 0.25 or 25%, while a ratio is written as 3 : 1.
• Saying that each child of two carrier parents will definitely follow the ratio. Ratios are probabilities for each separate fertilisation, not guarantees.
• Stating meiosis produces two identical cells. Meiosis produces four genetically different cells; mitosis produces two identical cells.
• Writing that the mother determines the sex of a child. The father's sperm carries either an X or a Y chromosome.

Try three B5.1 Inheritance exam-style questions