OxfordAQA International A Level Biology 3.3.6 Inheritance Exam Questions

About OxfordAQA International A Level Biology 3.3.6 Inheritance

Inheritance in Unit 3 covers how alleles are passed on and how to predict the results of crosses. You need to use fully labelled genetic diagrams for monohybrid and dihybrid crosses, including crosses involving codominant alleles, multiple alleles, sex linkage, autosomal linkage and epistasis. You also need to use the chi-squared test to decide whether differences between observed and expected ratios are significant. This builds on meiosis from Unit 1 and leads to allele frequencies and evolution. Examiners award marks for setting out genetic diagrams clearly, defining symbols, showing gametes and stating ratios.

What you need to know

• Key terms The genotype is the genetic make-up of an organism; the phenotype is the expression of the genotype and its interaction with the environment. Alleles can be dominant, recessive or codominant, and individuals can be homozygous or heterozygous. Some genes have multiple alleles, such as the ABO blood group gene.
• Monohybrid and dihybrid crosses A cross between two heterozygotes for one gene gives a 3:1 ratio of phenotypes. A dihybrid cross between two double heterozygotes gives 9:3:3:1 when the genes are on different chromosomes.
• Sex linkage Genes on the X chromosome show sex linkage. Males have only one X, so a recessive X-linked allele is always expressed in males, which is why conditions such as haemophilia are more common in males.
• Autosomal linkage Genes on the same autosome are linked and tend to be inherited together. Offspring ratios differ from 9:3:3:1, and recombinant phenotypes appear only when crossing over separates the alleles.
• Epistasis Epistasis is when one gene affects or masks the expression of another gene. It gives modified dihybrid ratios such as 9:3:4 or 12:3:1.
• Chi-squared test Chi-squared = sum of (O - E)^2 / E. Compare the value with the critical value at p = 0.05 for the right degrees of freedom (number of classes - 1). If it is less than the critical value, the difference is due to chance.

Common mistakes in 3.3.6 Inheritance

• Not defining symbols or not showing gametes. Examiners expect a key to alleles, parental genotypes, gametes and offspring genotypes and phenotypes.
• Writing sex-linked alleles without the X. Use notation such as X^H X^h and X^h Y.
• Using the number of categories as degrees of freedom. Degrees of freedom are the number of categories minus one.
• Saying a chi-squared value below the critical value proves the hypothesis. It means there is no significant difference, so any difference is due to chance.

Try three 3.3.6 Inheritance exam-style questions