Topic 8 Genetics

8.1 Meiosis (2h)
Describe the behaviour of the chromosomes in the phases of meiosis.
Students will be expected to know the names of the phases. The subdivisions of prophase I will not be required.
Outline the process of crossing over and the formation of chiasmata.
Cross reference with 8.3.2.
Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.
Cross reference with 3.2.4. The number of different types of gametes produced is 2n (where n=haploid number).
Define recombination.
Recombination -- the reassortment of genes or characters into different combinations from those of the parents.
Recombination occurs for linked genes by crossing over and, for unlinked genes, by chromosome assortment.
State Mendel's law of independent assortment.
Explain the relationship between Mendel's law of independent assortment and meiosis.
8.2 Dihybrid Crosses (2h)
Calculate and predict the genotypic and phenotypic ratios of offspring of dihybrid crosses involving unlinked autosomal genes.
Identify which of the offspring in dihybrid crosses are recombinants.
Recombination has often been restricted to linked genes but it also applies to non-linked situations. For example, in the cross tall, white [Ttrr] with short, red [ttRr], the F1 will contain four different phenotypes -- tall, white [Ttrr}, shor red [ttRr], tall, red [TtRr] and short, white [ttrr]. The tall red and the short, white are the recombinants.
Outline the use the the chi-squared test in analysing monohybrid and dihybrid crosses using given values.
Students should appreciate that the test can be used to establish whether an observed ratio differs significantly from the expected one.
8.3 Autosomal Gene Linkage (2h)
State the difference between autosomes and sex chromsomes.
Explain how crossing over in prophase I (between non-sister chromatids of a homologous pairs) can result in an exchange of alleles.
The fact that crossing over does not occur in male Drosophila will not be expected.
Define linkage group.
Explain an example of a cross between two linked genes.
Alleles are usually shown side-by-side in dihybrid crosses eg TtBb. In representing crosses involving linkage it is more common to show them as vertical pairs.
This format will be used in examination papers, or candidates will be given sufficient information to allow them to deduce which alleles are linked.
There are several advantages arising from this format. The line(s) can be taken to represent the chromosome(s) thereby indicating linkage visually. Also, the linked alleles and the cross-over allele combinations are clear. In a side-by-side format it is impossible to tell which allele is linked to which.
Identify which of the offspring in such dihybrid crosses are recombinants.
8.4 Polygenic Inheritance (1h)
Define polygenic inheritance.
Explain that polygenic inheritance can contribute to continuous variation using two examples. One example must be human skin colour.
Human melanin production seems to be controlled by three or four genes. Dealing with all four genes at once is unwieldy and the principle can be explained clearly enough using two genes.