Genetics (AHL)
10.1 Meiosis (2h)
10.1.1 Define homologous chromosomes.
10.1.2 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
.MEIOSIS
Prophase I
Metaphase I
10.1.3 Outline the process of crossing-over (cross reference 10.3.2).
The precise details of how and why crossing over takes place will not be required
.
10.1.4 Define chiasma.
10.1.5 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 3.3.3).
The number of different types of gamete produced is 2n (where n = haploid number)
10.1.6 Define recombination.
10.1.7 State Mendel's Second Law (Law of Independent Assortment).
10.1.8 Explain the relationship between Mendel's Laws and meiosis.
10.2 Dihybrid crosses (2h)
Notes: As with SSC students should be directed to choose letters representing alleles with care to avoid possible confusion between upper and lower case.
As with SSC genetics, students will be expected to apply their understanding of dihybrid crosses to organisms that may not be familiar to them
.10.2.1 Calculate and predict the genotypic and phenotypic ratios of offspring of dihybrid crosses involving unlinked autosomal genes.
10.2.2 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], short, red [ttRr], tall, red [TtRr], and short, white [ttrr]. The tall red and the short, white are the recombinants
.10.3 Autosomal gene linkage and gene mapping (2h)
10.3.1 State the difference between autosomes and sex chromosomes.
10.3.2 Explain how crossing over in Prophase I (between non-sister chromatids of a homologous pair) can result in an exchange of alleles.
.The fact that crossing-over does not occur in male Drosophila will not be expected
10.3.3 Define linkage group.
10.3.4 Explain an example of a cross between two linked genes.
On paper, alleles are usually represented side-by-side in dihybrid crosses e.g., TtBb. In representing crosses involving link-age it is more common to represent them as vertical pairs, that is:
TB
tb
This form %U 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 since the line(s) can be taken to represent the chromosome(s) thereby indicating link-age visually. The linked alleles are clear and the cross--over allele combinations are clear. In a side-by-side format indicated above it is impossible to tell which allele is linked to which. These are sometimes represented using a single line between alleles and occasionally writing them side-by- side: T B t b
.10.3.5 Identify which of the offspring in such dihybrid crosses are recombinants.
In a test cross of T B
t b the recombinants will be Tb and tB
tb tb
10.3.6 Analyse cross over value (COV) data to construct gene maps of up to four genes using two-point testcross data.
10-3.7 Define centimorgan.
10.3.6- These units (cM) are named in recognition of the pioneering work of Morgan
10.3.7 and Sturtevant. Loci positioned a long way apart on the same chromosome appear to behave independently because of the high frequency of crossing-over which occurs between them. Reference to differences between cytological and genetic map will not be required.
Enthusiastic students may wish to analyse three-point test cross data (but it will not be required)!
10.4 Statistical analysis (1h)
10.4.1 Analyse both monohybrid and dihybrid genetic crosses using the chi-squared test.
S (O-E)2X2 =
E
where: O = observed number; E = expected number;
n = number of different classes; (v) degrees of freedom = n - 1
In biology, a value of less than 5% probability is accepted as being significant. Students will not be required to remember the formula for the examinations
.10.5 Polygenic inheritance (2h)
10.5.1 Define polygenic inheritance.
10.5.2 Explain that polygenic inheritance can contribute to continuous variation, using three examples including human skin colour.
Human melanin production seems to be controlled by 3 or 4 genes. Dealing with all four genes at once is unwieldy but the principle can be explained clearly enough using two
.10.5.3 Explain how interaction between genes can cause modified Mendelian ratios in dihybrid crosses.
Few phenotypes result from the action of one gene. Most are due to a sequence of two or more genes, and thus the action of a gene earlier in the sequence can effect a gene later in the sequence. This is the basis of epistasis (not required). This assessment statement should be approached from a simple level involving only pairs of unlinked genes. Students should be able to recognise various modified Mendelian ratios such as 9 : 3 : 4 or 9 : 7. Reference could be made to the work of Bateson and Punnett
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10.6 Applications of genetics to agriculture and horticulture (2h)
10.6.1 Define inbreeding.
10.6.2 Define outbreeding.
10.6.3 Define interspecific hybridisation.
10.6.4 Define polyploidy.
10.6.5 Define F1 hybrid vigour.
10.6.6 Outline one example for each of the above terms.
10.6.7 Describe a total of three examples of the use of transgenic techniques in agriculture and/or horticulture.
.Most examples are at the trial stage but numbers should increase rapidly. Earliest ones involved transfer of human copy DNA (cDNA) into bacteria, e.g. human genes for insulin and factor VIII. Other examples include: a-1-antitrypsin (emphysema drug) genes into sheep, expressed as protein in milk; winter flounder fish gene to make tomatoes frost resistant; transfer of T toxin gene from Bacillus thuringiensis into tomato; tomato resistance to TMV (tobacco mosaic virus); glyphosate resistance; factor IX (human blood clotting) into sheep milk; VEF (viral enhancement factor) in plants for pest control; black spot resistance in roses. Consult modern textbooks on biotechnology for more
10.6.8 Discuss the ethical issues arising from the use of transgenic techniques.
Most groups have no ethical objections to the use of foods containing copy DNA of human origin. However it is a very sensitive issue
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10.6.9 Discuss the need to maintain the biodiversity of wild plants/ancient farm breeds as a reservoir of alleles which may have future value.