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Cellular Control And Variation Notes

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Explain the terms allele, locus, phenotype, genotype, dominant, codominant and recessive. Allele: One of the different alternative forms of a particular gene that determines the way a characteristic is expressed. Locus: The position of a gene within a DNA molecule Phenotype: The characteristic of an individual which results from the genes the organism possess and the environment in which it has developed Genotype: The genetic make-up of an organism. The genotype described the organism in terms of the alleles it contains. Eg. TT, Tt or tt. Dominant: An allele is said to be dominant if it is always expressed in the phenotype of an organism. Codominant: Where two different alleles of a gene are present (heterozygote) and both have an effect on the phenotype.

Explain the terms linkage and crossing over. Linkage: The presence of two genes on the same chromosome, so that they tend to be inherited together and do not assort independently. Crossing over: The exchange of alleles between chromatids of homologous chromosomes as a result of chiasma formation during prophase of meiosis.

Explain how meiosis and fertilisation can lead to variation through the independent assortment of alleles. Crossing over between non-identical sister chromatids (Prophase 1) During prophase 1 of meiosis, homologous chromosomes pair (form a bivalent). Exchange of sections of chromatids occurs during this stage which results in new allele combinations. Independent assortment of homologous chromosomes (Metaphase 1) In metaphase 1 the chromosomes assort independently. The homologous pairs of chromosomes align themselves on the equator

of the spindle; each pair can assort themselves in either of two ways. Independent assortment of chromatids (Metaphase 2) The arrangement of chromatids at Metaphase II will affect the combination of chromosomes in the daughter cells. Genetically different gametes fuse together randomly during fertilisation. When two gametes fuse during fertilisation it results in the bringing together of two different gene combinations. The zygotes will develop into adults showing variation.

Use genetic diagrams to solve problems involving sex linkage and codominance: Sex linkage: The sex chromosomes do not just carry genes determining sex but numerous others related to other features. In mammals the X chromosome pair of the female is homologous along the whole length but the Y chromosome of the male is homologous with only a small portion of the X chromosome. The Y chromosome is shorter and has fewer genes. Genes located on homologous regions behave like autosomal genes on other chromosomes. Genes located on the non-homologous region of the X chromosome will always be expressed, even if they are recessive, because there's nothing to suppress them on the Y chromosome. e.g. Haemophilia XH XH Normal


Xh Xh







Xh Y-

Normal M

Sufferer M

Codominance: Occasionally a dominant allele may not completely suppress its contrasting allele. Therefore the heterozygote is phenotypically distinguishable from the two homozygous phenotypes. Two alleles are said to be codominant if both are expressed in the phenotype of the heterozygote. e.g. Roan Cattle.

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