Edexcel GCSE Biology Topic 3: Genetics Exam Questions

About Edexcel GCSE Biology Topic 3: Genetics

Topic 3 of Edexcel GCSE Biology is assessed on Paper 1. It covers how reproduction passes on genetic information, the structure of DNA, and how characteristics are inherited. GCSE Biology students also study protein synthesis, how variants affect phenotype, Mendel's experiments, codominance in blood groups and sex-linked inheritance. It links to mitosis in Topic 2 and to natural selection and genetic modification in Topic 4. Examiners regularly set genetic cross and family pedigree questions with probabilities and ratios, ask you to compare mitosis and meiosis, and test the DNA extraction practical.

What you need to know

• Reproduction and meiosis Sexual reproduction produces variation; asexual reproduction is quicker and needs one parent. Meiosis produces four genetically different haploid gametes, halving the chromosome number so that fertilisation restores the diploid number.
• DNA structure DNA is a polymer of nucleotides forming two strands in a double helix. Complementary base pairs (A with T, C with G) are joined by weak hydrogen bonds. A gene is a section of DNA that codes for a specific protein; the genome is all the DNA of an organism.
• Extracting DNA DNA can be extracted from fruit such as strawberries by mashing with a detergent and salt solution, filtering, then adding ice-cold ethanol so the DNA precipitates as white strands.
• Protein synthesis (Biology only) In transcription, RNA polymerase makes an mRNA copy of a gene in the nucleus. In translation, mRNA attaches to a ribosome, tRNA brings specific amino acids, and each triplet of bases (codon) codes for one amino acid. Variants in a gene can change the protein's shape and activity.
• Inheritance Learn chromosome, gene, allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype, gamete and zygote. Use Punnett squares and family pedigrees to predict outcomes of monohybrid crosses as ratios, percentages and probabilities. Mendel's work on pea plants established units of inheritance (Biology only).
• Sex determination and special cases Females are XX and males XY, giving a 50% chance of either sex. Biology only: ABO blood groups show multiple alleles and codominance; sex-linked recessive disorders such as colour blindness are carried on the X chromosome, so males are more often affected.
• Variation Variation is caused by genetic factors, environmental factors or both. Most mutations have no effect on phenotype, some affect it slightly and rarely one has a large effect. The Human Genome Project mapped the human genome and has uses in medicine.

Common mistakes in Topic 3: Genetics

• Saying meiosis produces identical cells. Meiosis produces four genetically different haploid cells.
• Treating a 3:1 ratio as a probability of 3/3. A 3:1 ratio means 3 in 4 (75%) show the dominant phenotype.
• Confusing codominance with incomplete dominance or saying blood group O is dominant. Alleles IA and IB are codominant and both are dominant to IO.
• Saying the father passes on the X chromosome to his sons. Sons receive the Y chromosome from their father, so X-linked disorders in sons come from the mother.
• Writing genotypes with different letters for dominant and recessive alleles. Use the same letter in upper and lower case, for example T and t.

Try three Topic 3: Genetics exam-style questions