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4.3 Theoretical Genetics 4.3.1 Definitions 4.3.2 Monohybrid Crosses Monohybrid genetic crosses: genetics involving one gene. • Example: Pea plants and the texture of their seed coats. • The characteristic of seed coat texture is controlled by one gene with two alleles. • The seed coat can be either smooth or rough. • Smooth coat is dominant to rough coat. • One parent is homozygous dominant and the other is homozygous recessive. 4.3.2 Monohybrid Crosses • F1 Cross = F1 (heterozygote) x F1 (heterozygote) • Phenotype = Smooth coat seed x Smooth coated seed: 4.3.3 Multiple Alleles • • • • Some genes have more than two alleles. An individual can only possess two alleles. The population may contain many alleles for a given gene. Multiple alleles increases the number of different phenotypes. • Multiple alleles can be dominant, recessive or codominant to each other. • Example: Rabbit coat colour(C) has four alleles which have the dominance hierarchy: C > cch > ch> c • This produces 5 phenotypes, Dark(C_) , Chinchilla (cchcch), light grey (cchch ,cchc), Point restricted (ch ch, chc) and albino (cc) 4.4.4 Codominance and Multiple Alleles • The ABO blood group system is an example of both a multiple allele and codominance condition. • There are three alleles the base letter = I stands for immunoglobulin • IA and IB are codominant to each other. Both these alleles are dominant to i • The Allele hierarchy is IA = IB > i 4.3.5 Sex Chromosomes • Gender in humans is controlled by the 23rd pair of chromosomes. • XX is female and XY is male. • Female - two X chromosomes: one inherited from the father the other from the mother. They are both the longer chromosomes • Male - one X chromosome inherited from the mother and the much shorter Y chromosome inherited form the father. • The image represents the difference in the XX and XY combination. 4.3.6 Sex Chromosomes and Genes Male: Some genes are present on the X-chromosome but missing on the shorter Y. • The image of the male 23rd pair of homologous chromosomes represents the size difference in the two chromosomes. • In the non-homologous region of the Xchromosome, a male will only have one allele for any gene in this region. • Genes in the homologous region have two alleles per gene and function just as other genes already described. Female: The complete length of the X-chromosome has a homologous pair on the other Xchromosome. • Genes on the x-chromosome of female therefore have two alleles just like another gene on the other chromosomes. 4.3.7 Sex Linkage • Genes on the non-homologous region of the X chromosome are said to be sex linked. • Phenotypes associated with recessive alleles are more common in males • The recessive allele (a) is found on the nonhomologous region of the X-chromosome. • Males only get one allele for this gene. • Males have a 50% chance of being recessive. • Female have a lower risk (33.3 %) since they always receive 2 alleles. • 'Recessive' males can pass on this condition ( Xchromosome) to the 'daughter'. • Cannot pass these conditions to the 'sons' as they pass the y-chromosome with no alleles. 4.3.8 Colorblindness