Practise meiosis, variation and inheritance using chromosome diagrams, genetic crosses and probability while interpreting linkage, sex linkage, codominance and selection.
In sexual reproduction, meiosis occurs to produce haploid gametes from a diploid cell.
Explain the meaning of the terms haploid and diploid.
Describe the behaviour of chromosomes during the main stages of meiosis I in animal cells. prophase I
metaphase I
anaphase I
telophase I
Cystic fibrosis is an autosomal recessive genetic disease. People with cystic fibrosis have a homozygous recessive genotype.
Explain the meaning of the terms homozygous and recessive.
homozygous
recessive
The lac operon of prokaryotes contains a group of structural genes that are under the same control and are transcribed together.
Another operon found in prokaryotes is the trp operon.
Fig. 6.1 summarises the structure and control of the trp operon.
Fig. 6.1
Describe the differences in structure and control between the lac operon and the trp operon.
Suggest why structural genes in operons are transcribed together.
is an example of a structural gene and is an example of a regulatory gene. Describe the differences between the functions of structural genes and regulatory genes.
codes for the enzyme tryptophan synthase.
Tryptophan synthase catalyses the formation of the amino acid tryptophan.
Explain why tryptophan synthase is an example of a repressible enzyme.
Control of gene expression in eukaryotes is more complex than in prokaryotes.
In plants, the control of gene expression can involve plant hormones, such as gibberellin, and proteins known as JAZ and MYC.
Describe how gibberellin activates genes in plant cells.
Transcription of some plant genes is prevented when JAZ proteins bind to other proteins known as MYC.
When JAZ proteins are broken down, MYC proteins are free to bind to DNA. This allows transcription to begin.
State the term that is used to describe proteins such as MYC proteins.
Many different genes are involved in the production of pigments in mammals. One example is the TYR gene.
In humans the TYR gene is located on chromosome 11.
Fig. 6.1 shows the homologous pair for chromosome 11.
Fig. 6.1
Homologous chromosomes have the same genes located at the same loci. State one other feature shared by homologous chromosomes.
Albinism in humans can be caused by recessive mutations of the TYR gene.
Explain why a person who is homozygous recessive for the TYR gene shows albinism.
Fur colour in rabbits involves a number of different genes. Some of these genes interact.
Gene 1 has two alleles, B and b.
- The dominant allele, B, results in black fur.
- The recessive allele, b, results in brown fur.
Gene 2 has two alleles, F and f.
- The dominant allele, F, codes for a protein that allows the expression of gene 1.
- The recessive allele, f, codes for a protein that does not allow the expression of gene 1, resulting in white fur.
The two genes are on different pairs of autosomes.
Complete the genetic diagram for a cross between two black rabbits that are heterozygous for both genes and show the ratio of possible offspring phenotypes.
parental phenotypes
parental genotypes
black × black
BbFf × BbFf
Punnett square
ratio of offspring phenotypes: