Genetics

Meiosis

State that meiosis is a reduction division in terms of diploid and haploid numbers of chromosomes.

Define homologous chromosomes.

Outline the process of meiosis, including pairing of chromosomes followed by two divisions, which results in four haploid cells.

Explain how the movement of chromosomes during meiosis can give rise to genetic variety in the resulting haploid cells.

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

Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down's syndrome (trisomy 21).: The recognition of Down's syndrome in a person is not required. Translocation of part of chromosome 21 possibly resulting in Down's syndrome is not required.

State Mendel's law of segregation.

Explain the relationship between Mendel's law of segregation of meiosis.

State Mendel's law of independent assortment.

Explain the relationship between Mendel's law of independent assortment and meiosis.

Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.

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.

Chromosomes, genes, alleles and mutations

State that eukaryotic chromosomes are made of DNA and protein.

State that in karyotyping, chromosomes are arranged in pairs according to their structure.: Karyotyping can be done by using enlarged photocopies of chromosomes.

Describe one application of karyotyping.

Define gene, allele and genome.: Gene -- a heritable factor that controls a specific characteristic.; Allele -- one specific form of a gene, differing from other alleles by one or a few bases only and occupying the same gene locus as other alleles of the gene.; Genome -- the whole of the genetic information of an organism.

Define gene mutation.: The terms point mutation or frameshift mutation will not be used.

Explain the consequence of a base substitution mutation in relation to the process of transcription and translation, using the example of sickle cell anemia.: GAG has mutated to GTG causing glutamic acid to be replaced by valine, and hence sickle cell anemia. The relationship between the frequency of the sickle cell allele and the distribution of malaria should be discussed.

Theoretical Genetics

Define: genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross.: Genotype--the alleles possessed by an organism.; Phenotype--the characteristics of an organism.; Dominant allele--an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state.; Recessive allele--an allele that only has an effect on the phenotype when present in the homozygous state.; Codominant alleles--pairs of alleles that both affect the phenotype when present in a heterozygote. (The terms incomplete and partial will no longer be used.); Locus--the particular position on homologous chromosomes of a gene.; Homozygous--having two identical alleles of a gene.; Heterozygous--having two different alleles of a gene.; Carrier--an individual that has a recessive allele of a gene that does not have an effect on their phenotype.; Test cross--testing a suspected heterozygote by crossing it with a known homozygous recessive. (The term backcross is no longer used.)

Construct a Punnett grid.

Construct a pedigree chart.

State that some genes have more than two alleles (multiple alleles).

Describe ABO blood groups an an example of codominance and multiple alleles.

Outline how the sex chromosomes determine gender by referring to the inheritance of X and Y chromosomes in humans.

State the some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.

Define sex linkage.

State two examples of sex linkage.: Examples from any species where the female is the homogametic sex can be used, although humans will be referred to most commonly.; Colour blindness and hemophilia--both these conditions are produced by a recessive sex-linked allele on the X chromosome. X^b and X^h is the notation for the alleles concerned. The corresponding dominant alleles are X^B and X^H.

State that a human female can be homozygous or heterozygous with respect to sex-linked genes.

Explain that female carriers are heterozygous for X-linked recessive alleles.

Calculate and predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance.

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 analyzing 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.

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.

Autosomal Gene Linkage

State the difference between autosomes and sex chromosomes.

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.

Deduce the genotypes or phenotypes of individuals in pedigree charts.: For dominant and recessive alleles upper-case and lower-case letters respectively should be used. Letters representing alleles should be chosen with care to avoid confusion between upper and lower case.; For codominance, the main letter should relate to the gene and the suffix to the allele, both upper case. For example, red and white codominant flower colours should be represented as C^R and C^W respectively. For sickle cell anemia, Hb^A is normal and Hb^S is sickle cell.