CCEA A Level Biology 5.5 Genes and Patterns of Inheritance Exam Questions

About CCEA A Level Biology 5.5 Genes and Patterns of Inheritance

Section 5.5 of Unit A2 2 is classical genetics. You need to define genotype, phenotype, homozygous, heterozygous, gene, allele and locus, then use genetic diagrams and Punnett squares to solve monohybrid and dihybrid crosses, including dominance, codominance, multiple alleles, lethal combinations and test crosses. CCEA links Mendel's first and second laws to meiosis. You also need sex determination by X and Y chromosomes, dominant and recessive sex-linked inheritance, gene interaction including epistasis, and continuous variation from polygenes plus environment. Expect at least one multi-step genetics problem, often asking you to explain an unusual ratio or to compare observed and expected numbers of offspring.

What you need to know

• Key terms Genotype is the alleles present; phenotype is the observable characteristic, resulting from genotype and environment. Alleles are alternative forms of a gene at the same locus on homologous chromosomes.
• Monohybrid and Mendel's first law Each organism has two alleles for a gene and these segregate into separate gametes (law of segregation), as homologous chromosomes separate in meiosis. A heterozygous cross gives 3:1 with dominance; a test cross with a homozygous recessive reveals an unknown genotype.
• Dihybrid and Mendel's second law Alleles of different genes on different chromosomes assort independently, because of random orientation of homologous pairs in meiosis. AaBb x AaBb gives 9:3:3:1.
• Codominance, multiple and lethal alleles In codominance both alleles are expressed, for example ABO blood groups where IA and IB are codominant and IO recessive. Lethal combinations, such as homozygous yellow in mice, change 3:1 into 2:1.
• Sex linkage Females are XX and males XY. Recessive X-linked alleles (for example haemophilia and red-green colour blindness) show more often in males, who have only one X. Carrier females are heterozygous.
• Gene interaction and polygenes In epistasis one gene masks the expression of another, altering dihybrid ratios such as 9:3:4. Continuous variation results from many genes with additive effects, plus environmental influence.

Common mistakes in 5.5 Genes and Patterns of Inheritance

• Using letters with similar capitals and lower case, such as S and s, without a key. Choose distinct letters and always state what each allele represents.
• Showing gametes with two alleles of the same gene. Each gamete has one allele of each gene.
• Putting sex-linked alleles on the Y chromosome. Write them as superscripts on X, for example XH Xh, with Y carrying no allele.
• Saying a 3:1 ratio means exactly three out of four offspring will show the dominant trait. Ratios are probabilities; small samples will vary.
• Treating codominance as blending. Both alleles are fully expressed in the heterozygote, for example group AB.

Try three 5.5 Genes and Patterns of Inheritance exam-style questions