OCR GCSE Biology B B1.2 How is genetic information inherited? Exam Questions

About OCR GCSE Biology B B1.2 How is genetic information inherited?

B1.2 explains how genetic information passes from parents to offspring. You need to describe how gametes are made by meiosis, use the terms allele, dominant, recessive, homozygous and heterozygous correctly, and draw genetic diagrams to predict the outcomes of crosses. Sex determination by the X and Y chromosomes is included. The topic builds on the genome ideas in B1.1 and leads into genetic testing in B1.3 and variation in B6. Examiners regularly ask for Punnett squares, interpretation of family trees and ratios or probabilities, and they reward precise use of genetic vocabulary.

What you need to know

• Gametes and fertilisation Gametes (sperm and egg cells in animals, pollen and egg cells in plants) contain one set of chromosomes. Fertilisation fuses two gametes to give a zygote with paired chromosomes.
• Meiosis Meiosis is a cell division that produces four genetically different gametes, each with half the number of chromosomes of the parent cell. This creates variation in offspring.
• Alleles Alleles are different versions of the same gene. A dominant allele is expressed if one copy is present; a recessive allele is expressed only if two copies are present.
• Homozygous and heterozygous Homozygous means two identical alleles (e.g. AA or aa); heterozygous means two different alleles (Aa). A heterozygous person with a recessive disorder allele is a carrier.
• Genetic crosses Punnett squares predict the probability of offspring genotypes and phenotypes. A cross between two heterozygotes (Aa x Aa) gives a 3 : 1 ratio of dominant to recessive phenotypes.
• Sex determination Human females are XX and males are XY. Every egg carries an X, half of sperm carry X and half carry Y, so each pregnancy has a 50% chance of a boy.
• Single genes and many genes A few characteristics, such as cystic fibrosis, are controlled by a single gene, but most, such as height, are controlled by many genes plus environmental factors.

Common mistakes in B1.2 How is genetic information inherited?

• Treating a predicted ratio as a guaranteed outcome. A 3 : 1 ratio gives probabilities for each child, so actual families can differ.
• Saying dominant alleles are more common. Dominance describes expression, not how often an allele occurs in a population.
• Drawing Punnett squares without showing parental gametes. Show the parent genotypes, the gametes and then the offspring to gain all the marks.
• Confusing meiosis and mitosis. Meiosis makes four genetically different gametes with half the chromosomes; mitosis makes two identical body cells.
• Writing that males decide sex because they are stronger or similar. The sperm decides sex because it carries either an X or a Y chromosome.

Try three B1.2 How is genetic information inherited? exam-style questions