These are detailed Revision Notes for Topic 6.1.2 of OCR A-Level Biology (Patterns of Inheritance and Variation). They were written by me using a combination of the textbook and class notes. I will also be uploading the other topics and creating bundles.
Topics Included:
- Variation and inherita...
Patterns of Inheritance and Variation
20.1 Variation and Inheritance
20.1 Variation and inheritance
- Chlorosis is when plants do not produce the normal amount of
chlorophyll, this makes the leaves look pale or yellow and reduces 20.2 monogenetic inheritance
the plants’ ability to photosynthesise. 20.3 dihybrid inheritance
- Most of the time chlorosis is due to environmental factors such as:
o Lack of light – plants turn off their chlorophyll production 20.4 phenotypic ration
to conserve resources 20.5 evolution
o Mineral deficiencies – iron is needed as a cofactor for the
20.6 speciation and artificial selection
enzymes that make chlorophyll, magnesium is at the heart
of the chlorophyll molecule.
o Viral infections – viruses can interfere with the metabolism of cells; this often means that they
can no longer support the synthesis of chlorophyll.
- Body mass is caused by a number of different environmental and genetic factors.
Sources of Variation:
- Meiosis
o Crossing over (chiasmata formation) at prophase I, resulting in the exchange of alleles between
non-sister chromatids, so new combinations of alleles on these chromosomes.
o Independent assortment of homologous chromosomes ant metaphase I.
o Independent assortment of chromatids at metaphase II
- Sexual Reproduction
o Random mating
o Random fertilisation of sperm and eggs
- New variation
o Mutations of genes or chromosomes
Types of variation:
- Discontinuous variation
o Clear cut, qualitative differences
o Variation falls easily into different categories.
o Cause by different alleles of a single gene locus.
o Individual alleles have a large effect on the phenotype.
o Different gene loci have different effects on the phenotype.
- Continuous variation
o Quantitative differences
o Can’t put large groups into easy classes as differences between individual values may be small.
o Get a range of values
o A large number of loci may have a combined effect on a phenotype (polygenic).
o Different alleles at a gene locus have a small effect. Different gene loci have an additive effect
on the phenotype.
o Genes are on different chromosomes (unlinked)
o Environmental influence is often quite large
20.2 Monogenetic Inheritance
- Genetic Cross
1. State the phenotype of both parents.
2. State the genotype of both parents.
3. State the gametes of each parent
4. Use a Punnett Square to show the results of random fusion.
5. State the proportion of each genotype of the offspring
6. State the corresponding phenotype for each of the possible genotypes.
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