Genes and Variation

Notes on genes and variation for the GCSE Edexcel Biology Exam. Content ranges from G to A*. 

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  • Created on: 01-05-13 18:50

Genetic Variation

There are 2 types of variation:

Environmental variation are aquired characteristics caused by environmental factors.

Genetic variations are inherited charateristics caused by mutation or reproduction.

Inheritied characteristics are controlled by genes, which are sections of chromosomes. Chromosomes are long molecules of DNA, that are wrapped around proteins. Every human has 23 pairs of chromosomes (46 althogether) , which come equally from sperm and an egg.

Genes allow cells to make proteins, which result in characterisitics such as eye colour.

We inherit our parents characteristics as they pass on on their genes to us through fertilization

Genetic variation between parents and offspring may be caused by:

- Crossing over. This is the exchange of parts of chromosomes and their genes between chromosome pairs.

- Gene mutations: Changes in the number of chromosomes or in the genes that chromosomes carry. 

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Genetic Terms

Locus : Each chromosome in  pair carries the same genes in exactly the same place.

However, chromosome pairs may have different alleles on each chromosome. 

Allele: A form of a gene. For example, the eye colour gene can come in many colours/forms. 

Homozygous: 'h0mo' = same, zygous = allele. The allele pairs are the same. 

Heterozygous: The alleles are different. 

An allele many be dominant or recessive. Dominant masks the effect of the resecessive.

Phenotype: All the characteristics that make up an individual.

Genotype: All the genes of an individual. 

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Pedigree charts

Pedigree charts show the way characteristics of a family pass from one generation to the next. 

They provide information about the purity of lineage of plants and animals. This is useful for farmers. 

The risk of someone inheriting a particular disorder can be predicted using pedigree analysis.

In Pedigree Charts, generations are indicated from the top using Roman Numerals, and members of each generation are numbered from the left. 

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