16 Inheritance
Genetic information is transmitted from generation to generation to maintain the continuity of life. In sexual
reproduction, meiosis
... [Show More] 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 that they have inherited and partly by the effect of the environment. Genes determine how
organisms develop; gene control in bacteria gives us a glimpse of this process in action.
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)
16.2 The roles of genes in
determining the phenotype
continued
Learning outcomes
Candidates should be able to:
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 [Show Less]