Inheritance and genetic variation

Learning objectives

Define gene, allele, locus, genotype, phenotype, dominant, recessive, homozygous and heterozygous, and explain Mendel's law of segregation in terms of the separation of alleles during meiosis

Predict the outcome of a monohybrid cross using a Punnett grid, express the result as a ratio or a probability, and use a test cross to determine an unknown genotype

Explain codominance and multiple alleles using the ABO blood group system, and construct genetic diagrams for crosses involving them

Explain sex determination by the X and Y chromosomes and sex linkage using haemophilia or red–green colour blindness, and interpret a pedigree chart to deduce the mode of inheritance of a condition such as cystic fibrosis or Huntington's disease

(HL) Predict the outcome of a dihybrid cross between heterozygous parents using the law of independent assortment, identify the 9:3:3:1 ratio, and explain how linked genes on the same chromosome give a different ratio in which recombinants arise by crossing over

Explain continuous variation in characteristics such as human height as the result of polygenic inheritance together with environmental effects, and (HL) apply a chi-squared test to the observed and expected offspring numbers of a genetic cross