16. Inherited change

16.1 Passage of information from parents to offspring

Learning outcomes Candidates should be able to:

1. explain the meanings of the terms haploid (n) and diploid (2n)

2. explain what is meant by homologous pairs of chromosomes

3. explain the need for a reduction division during meiosis in the production of gametes

4. describe the behaviour of chromosomes in plant and animal cells during meiosis and the associated behaviour of the nuclear envelope, the cell surface membrane and the spindle (names of the main stages of meiosis, but not the sub-divisions of prophase I, are expected: prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II and telophase II)

5. interpret photomicrographs and diagrams of cells in different stages of meiosis and identify the main stages of meiosis

6. explain that crossing over and random orientation (independent assortment) of pairs of homologous chromosomes and sister chromatids during meiosis produces genetically different gametes

7. explain that the random fusion of gametes at fertilisation produces genetically different individuals

16.2 The roles of genes in determining the phenotype

Learning outcomes Candidates should be able to:

1. explain the terms gene, locus, allele, dominant, recessive, codominant, linkage, test cross, F1, F2, phenotype, genotype, homozygous and heterozygous

2. interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of monohybrid crosses and dihybrid crosses that involve dominance, codominance, multiple alleles and sex linkage

3. interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of dihybrid crosses that involve autosomal linkage and epistasis (knowledge of the expected ratios for different types of epistasis is not expected)

4. interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of test crosses

5. use the chi-squared test to test the significance of differences between observed and expected results (the formula for the chi-squared test will be provided, as shown in the Mathematical requirements)

6. explain the relationship between genes, proteins and phenotype with respect to the:

• TYR gene, tyrosinase and albinism

• HBB gene, haemoglobin and sickle cell anaemia

• F8 gene, factor VIII and haemophilia

• HTT gene, huntingtin and Huntington’s disease

7. explain the role of gibberellin in stem elongation including the role of the dominant allele, Le, that codes for a functional enzyme in the gibberellin synthesis pathway, and the recessive allele, le, that codes for a non-functional enzyme

16.3 Gene control

Learning outcomes Candidates should be able to:

1. describe the differences between structural genes and regulatory genes and the differences between repressible enzymes and inducible enzymes

2. explain genetic control of protein production in a prokaryote using the lac operon (knowledge of the role of cAMP is not expected)

3. state that transcription factors are proteins that bind to DNA and are involved in the control of gene expression in eukaryotes by decreasing or increasing the rate of transcription

4. explain how gibberellin activates genes by causing the breakdown of DELLA protein repressors, which normally inhibit factors that promote transcription