CIE A Level Biology Topic 16: Inheritance Exam Questions

About CIE A Level Biology Topic 16: Inheritance

Inheritance in the Cambridge 9700 syllabus explains how characteristics pass from one generation to the next. You need to describe meiosis and how independent assortment, crossing over and random fertilisation create variation. You must use genetic diagrams for monohybrid and dihybrid crosses, including codominance, multiple alleles, sex linkage, and autosomal linkage, and use the chi-squared test to compare observed and expected results. The topic also includes gene control: the lac operon in bacteria, transcription factors in eukaryotes and the role of gibberellin and DELLA proteins. Expect genetic diagram questions where every step must be shown clearly.

What you need to know

• Meiosis Meiosis produces four haploid, genetically different cells. Homologous chromosomes pair in prophase I and crossing over occurs at chiasmata; homologues separate in meiosis I and chromatids separate in meiosis II.
• Sources of variation Crossing over, independent assortment of homologous chromosomes at metaphase I and random fertilisation all create genetic variation.
• Genetic crosses Use correct genetic diagrams with parental phenotypes, genotypes, gametes, offspring genotypes and ratios. A dihybrid cross of two heterozygotes for unlinked genes gives 9 : 3 : 3 : 1.
• Sex linkage and linkage Sex-linked genes such as those for haemophilia are on the X chromosome, so males are affected more often. Autosomal linkage means genes on the same chromosome are inherited together unless crossing over separates them.
• Chi-squared test Chi-squared = sum of (O - E)^2 / E. Compare with the critical value at p = 0.05 with degrees of freedom = number of classes minus 1.
• Gene control In the lac operon, the repressor binds the operator unless lactose is present. In plants, gibberellin causes breakdown of DELLA proteins, allowing transcription factors to activate genes for growth.

Common mistakes in Topic 16: Inheritance

• Writing gametes with two alleles of the same gene; each gamete carries one allele of each gene.
• Forgetting to put sex-linked alleles as superscripts on X, for example X^H X^h.
• Saying a chi-squared value below the critical value proves the hypothesis; it means the difference is not significant and is due to chance.
• Confusing homologous chromosomes with sister chromatids.
• Assuming linked genes always give a 3 : 1 or 9 : 3 : 3 : 1 ratio; linkage changes expected ratios and recombinants appear in small numbers.

Try three Topic 16: Inheritance exam-style questions