OxfordAQA International GCSE Biology 3.5 Inheritance Exam Questions

About OxfordAQA International GCSE Biology 3.5 Inheritance

Inheritance in OxfordAQA International GCSE Biology explains how genetic information is stored, copied and passed on. You compare sexual and asexual reproduction, mitosis and meiosis, and learn about stem cells, therapeutic cloning and how tumours form. The genetics part covers DNA, genes, alleles, Mendel's work and genetic diagrams, then inherited disorders such as cystic fibrosis, polydactyly and sickle cell anaemia, and Down's syndrome. The section ends with cloning methods, genetic engineering and GM crops. Examiners often ask you to complete Punnett squares and give probabilities, compare mitosis with meiosis, and make balanced judgements about stem cells and GM crops.

What you need to know

• Reproduction Sexual reproduction involves the fusion of gametes, mixing genetic information and producing variation. Asexual reproduction needs only one parent, has no fusion of gametes and produces genetically identical clones.
• Mitosis and meiosis Mitosis is used for growth and replacement: DNA is copied and the cell divides once to give two identical cells. Meiosis happens in reproductive organs: DNA is copied and the cell divides twice to give four gametes with one set of chromosomes. Human body cells have 23 pairs of chromosomes; females are XX and males XY.
• Stem cells and cancer Embryonic and adult bone marrow stem cells can differentiate into many cell types; therapeutic cloning gives stem cells with the patient's own genes so they are not rejected. Tumours are uncontrolled cell growth: benign tumours do not invade tissue, malignant ones spread in the blood to form secondary tumours. Carcinogens include tobacco smoke chemicals, asbestos, UV and X-rays.
• DNA and genes DNA is a double helix made of four bases. A gene is a section of DNA coding for one protein; a sequence of three bases codes for one amino acid. Base names and pairing are not required.
• Genetic diagrams Use the terms allele, dominant, recessive, homozygous, heterozygous, genotype and phenotype. Construct Punnett squares and family trees and give outcomes as ratios, fractions or percentages. Mendel's work on peas came before anyone linked inherited factors to chromosomes.
• Genetic disorders Polydactyly is caused by a dominant allele; cystic fibrosis and sickle cell anaemia by recessive alleles, so carriers are heterozygous. Down's syndrome is caused by an extra chromosome.
• Cloning Tissue culture uses small groups of plant cells; cuttings (separate Biology) give many identical plants; embryo transplants split an early embryo; adult cell cloning puts a body cell nucleus into an egg with its nucleus removed, then an electric shock starts division before the embryo is implanted.
• Genetic engineering Enzymes cut out the required gene, which is inserted into a vector (bacterial plasmid or virus) that carries it into target cells, often at an early stage of development. GM crops resist insects or herbicides and give higher yields; concerns include effects on wild flowers, insects and human health.

Common mistakes in 3.5 Inheritance

• Saying meiosis produces two cells. It involves two divisions and produces four genetically different gametes.
• Writing genotypes with different letters for each allele, such as F and c. Use one letter, capital for dominant and lower case for recessive, for example Ff.
• Stating that two carriers will have exactly one affected child out of four. 25% is a probability for each child, not a guaranteed outcome.
• Saying a gene codes for a characteristic directly. A gene codes for a specific protein, which then affects the characteristic.
• Confusing the definitions of clone and stem cell. A clone is genetically identical to its parent; a stem cell is an undifferentiated cell.

Try three 3.5 Inheritance exam-style questions