CCEA GCSE Biology 2.4 Genome, Chromosomes, DNA and Genetics Exam Questions

About CCEA GCSE Biology 2.4 Genome, Chromosomes, DNA and Genetics

Genome, Chromosomes, DNA and Genetics is a core Unit 2 topic in CCEA GCSE Biology. It moves from the structure of DNA and how genes code for proteins, through mitosis and meiosis, to inheritance using genetic diagrams. You also need to explain how sex is determined, describe inherited conditions, discuss genetic screening and explain how genetic engineering is used to make human insulin in bacteria. It links to reproduction, variation and natural selection. Examiners frequently set Punnett square questions with ratios or probabilities, ask for differences between mitosis and meiosis, and test the steps of genetic engineering in a logical order.

What you need to know

• DNA and genes DNA is a double helix of two strands held together by complementary base pairs: A with T and C with G. A gene is a section of DNA that codes for a particular protein; the sequence of bases determines the order of amino acids. The genome is all the genetic material of an organism.
• Chromosomes Human body cells contain 46 chromosomes in 23 pairs (diploid). Gametes contain 23 single chromosomes (haploid), so the diploid number is restored at fertilisation.
• Mitosis and meiosis Mitosis produces two genetically identical diploid cells for growth, repair and asexual reproduction. Meiosis happens in the testes and ovaries and produces four genetically different haploid gametes, giving variation.
• Monohybrid inheritance Key terms: allele, dominant, recessive, homozygous, heterozygous, genotype and phenotype. A cross between two heterozygotes (Aa x Aa) gives a 3 : 1 ratio of dominant to recessive phenotype. Use a Punnett square and state ratios, percentages or probabilities clearly.
• Sex determination and genetic conditions Females are XX and males XY, so there is a 50% chance of a boy or girl. Cystic fibrosis is caused by a recessive allele, so a child must inherit two copies. Down's syndrome is caused by an extra copy of chromosome 21.
• Genetic screening Embryos or fetuses can be tested for genetic conditions, and adults can be tested to see if they carry a faulty allele. Issues include the risk of miscarriage from some tests, decisions about termination and who should have access to the results.
• Genetic engineering The human insulin gene is cut out using restriction enzymes. A bacterial plasmid is cut with the same enzyme, the gene is inserted and joined using ligase, and the plasmid is put back into a bacterium. The bacteria multiply in a fermenter and produce human insulin.

Common mistakes in 2.4 Genome, Chromosomes, DNA and Genetics

• Confusing gene and allele. A gene codes for a characteristic; alleles are different versions of the same gene.
• Saying meiosis produces two cells. Meiosis produces four genetically different haploid cells.
• Giving the probability of a recessive phenotype in each child as changing with earlier children. Each pregnancy is independent, so the probability stays the same.
• Writing genotypes with different letters for each allele, such as A and b. Use the same letter in upper and lower case.
• Stating that sperm decide sex because they are XY. Each sperm carries either an X or a Y chromosome.

Try three 2.4 Genome, Chromosomes, DNA and Genetics exam-style questions