Practise meiosis, variation and inheritance using chromosome diagrams, genetic crosses and probability while interpreting linkage, sex linkage, codominance and selection.
The house mouse, Mus musculus, has a diploid number of 40 chromosomes.
Fig. 2.1 shows 6 of these chromosomes.
Fig. 2.1
Identify one pair of homologous chromosomes on Fig. 2.1 by drawing circles around two chromosomes.
Fig. 2.2 shows the banding pattern of chromosome pair 11 of M. musculus. The banding pattern is obtained by staining.
Fig. 2.2
Explain why chromosomes, such as those in Fig. 2.2, are described as a homologous pair.
State the number of chromosomes that are present in M. musculus spermatozoa.
M. musculus produces gametes by meiosis. These gametes are genetically different.
There is random fusion of gametes at fertilisation.
Explain why meiosis is important in the life cycle of M. musculus, apart from producing genetically different gametes.
Explain how the random fusion of gametes leads to the expression of rare, recessive alleles.
A mutation causing coats of mice to be woolly in appearance is in a gene located on chromosome 11. The mutation causes a very shortened polypeptide product. Mice with the woolly coat phenotype have longer fur than mice with normal coats.
The inheritance of the woolly coat characteristic was investigated.
Draw a genetic diagram to show a cross between two heterozygous parents with normal coats.
Use the symbols A and a for the alleles.
parental genotypes
gametes
offspring genotypes
offspring phenotypes
Meiosis is an important process that contributes to genetic variation in a population.
Fig. 4.1 shows a pair of homologous chromosomes during prophase I of meiosis.
Fig. 4.1
On Fig. 4.1, use the letter C with a label line to identify where crossing over occurs.
Describe how crossing over produces genetic variation in a population.
Wing pattern in the butterfly species Heliconius melpomene is controlled by genes on autosomal chromosomes.
The gene for banding pattern on the upper wing has two alleles:
- a dominant allele coding for a full band
- a recessive allele coding for a broken band.
The gene for ray pattern on the lower wing has two alleles:
- a dominant allele coding for rays
- a recessive allele coding for no rays.
Scientists crossed a butterfly that was homozygous dominant for both genes with a butterfly that was homozygous recessive for both genes. The scientists wanted to check whether the phenotypic ratio for offspring in the F2 generation agreed with the expected phenotypic ratio of 9:3:3:1.
The results of this genetic cross are shown in Fig. 4.2.
Fig. 4.2
Explain the term F1 generation.
Suggest two reasons why phenotypic ratios in the F2 generation do not always match the expected ratios.
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.