Inherited Change

Genetic information is transmitted from generation to generation to maintain the continuity of life. In sexual reproduction, meiosis introduces genetic variation so that offspring resemble their parents but are not identical to them. Genetic crosses reveal how some features are inherited. The phenotype of organisms is determined partly by the genes they have inherited and partly by the effect of the environment. Genes determine how organisms develop and gene control in bacteria gives us a glimpse of this process in action. 

Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts.

Learning outcomes

Candidates should  be able to:

16.1 Passage of information from parent to offspring 

Diploid organisms contain pairs of homologous chromosomes. The behaviour of maternal and paternal chromosomes during meiosis generates much variation amongst individuals of the next generation. 

a) explain what is meant by homologous pairs of chromosomes 

b) explain the meanings of the terms haploid and diploid and the need for a reduction division (meiosis) prior to fertilisation in sexual reproduction 

c) outline the role of meiosis in gametogenesis in humans and in the formation of pollen grains and embryo sacs in flowering plants 

d) describe, with the aid of photomicrographs and diagrams, the behaviour of chromosomes in plant and animal cells during meiosis, and the associated behaviour of the nuclear envelope, cell surface membrane and the spindle (names of the main stages are expected, but not the sub-divisions of prophase) 

e) explain how crossing over and random assortment of homologous chromosomes during meiosis and random fusion of gametes at fertilisation lead to genetic variation including the expression of rare, recessive alleles

16.2 The roles of genes in determining the phenotype 

Patterns of inheritance are explained by using genetic diagrams. The results of genetic crosses are analysed statistically using the chisquared test. 

Studies of human genetic conditions have revealed the links between genes, enzymes and the phenotype. 

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

b) use genetic diagrams to solve problems involving monohybrid and dihybrid crosses, including those involving autosomal linkage, sex linkage, codominance, multiple alleles and gene interactions (the term epistasis does not need to be used; knowledge of the expected ratio for various types of epistasis is not required. The focus is on problem solving) 

c) use genetic diagrams to solve problems involving test crosses 

d) 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) (see Mathematical requirements) e) explain that gene mutation occurs by substitution, deletion and insertion of base pairs in DNA and outline how such mutations may affect the phenotype 

f) outline the effects of mutant alleles on the phenotype in the following human conditions: albinism, sickle cell anaemia, haemophilia and Huntington’s disease 

g) explain the relationship between genes, enzymes and phenotype with respect to the gene for tyrosinase that is involved with the production of melanin

16.3 Gene control 

Some genes are transcribed all the time to produce constitutive proteins; others are only ‘switched on’ when their protein products are required. 

a) distinguish between structural and regulatory genes and between repressible and inducible enzymes 

b) explain genetic control of protein production in a prokaryote using the lac operon 

c) explain the function of transcription factors in gene expression in eukaryotes 

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