Cambridge IGCSE Biology 17 Inheritance Exam Questions

About Cambridge IGCSE Biology 17 Inheritance

Inheritance explains how characteristics are passed from parents to offspring in Cambridge IGCSE Biology. Core candidates learn chromosomes, genes and alleles, the inheritance of sex, mitosis and meiosis in outline, and how to use genetic diagrams and Punnett squares for monohybrid crosses. Extended candidates add how the base sequence codes for amino acids in protein synthesis, haploid and diploid nuclei, stem cells, test crosses, codominance with ABO blood groups, and sex-linked characteristics such as colour blindness. Examiners award marks for correctly laid out genetic diagrams with parental phenotypes, genotypes, gametes, offspring genotypes and phenotypes, and ratios.

What you need to know

• 17.1 Chromosomes and genes A chromosome is a thread-like structure of DNA carrying genetic information in the form of genes. A gene is a length of DNA that codes for a protein. An allele is an alternative form of a gene. Sex is determined by XX (female) or XY (male) chromosomes.
• 17.1 Protein synthesis (Extended) The sequence of bases in a gene determines the sequence of amino acids in a protein. mRNA carries a copy of the gene from the nucleus to ribosomes in the cytoplasm, where amino acids are joined in the correct order.
• 17.2 Mitosis Nuclear division giving rise to genetically identical cells. It is used for growth, repair of damaged tissues, replacement of cells and asexual reproduction. Extended: chromosomes are copied before division, and stem cells are unspecialised cells that divide by mitosis and can become specialised.
• 17.3 Meiosis A reduction division in which the chromosome number is halved from diploid to haploid, producing genetically different cells. It produces gametes.
• 17.4 Monohybrid inheritance Learn genotype, phenotype, homozygous, heterozygous, dominant and recessive. A heterozygous cross (Aa x Aa) gives a 3:1 phenotypic ratio; Aa x aa gives 1:1. Extended: a test cross with a homozygous recessive identifies an unknown genotype.
• 17.4 Codominance and sex linkage (Extended) In codominance both alleles are expressed in the heterozygote, for example ABO blood groups with alleles IA, IB and Io (IA and IB are codominant, Io recessive). A sex-linked characteristic is controlled by a gene on the X chromosome, so it is more common in males, for example red-green colour blindness.

Common mistakes in 17 Inheritance

• Using 'gene' and 'allele' as if they mean the same thing. A gene codes for a protein; alleles are different versions of that gene.
• Leaving out the gametes in a genetic diagram. Show gametes clearly, circled or labelled.
• Saying a 3:1 ratio means exactly three of every four offspring will show the dominant trait. It is a probability, and real numbers vary.
• Writing that meiosis produces identical cells. Meiosis produces genetically different haploid cells; mitosis produces identical cells.
• Using different letters for alleles of the same gene, such as T and s. Use one letter, upper case for dominant and lower case for recessive.

Try three 17 Inheritance exam-style questions