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Sex Linkage and Polygenic Inheritance. Higher Human Biology. Lesson Aims. To revise sex chromosomes To examine effects of sex-linked genes To look at polygenic inheritance. Normal Body Cells. In the nucleus of every body cell there are 46 chromosomes
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Sex Linkage and Polygenic Inheritance Higher Human Biology
Lesson Aims • To revise sex chromosomes • To examine effects of sex-linked genes • To look at polygenic inheritance
Normal Body Cells • In the nucleus of every body cell there are 46 chromosomes • 22 homologous pair and one pair of sex chromosomes
Red Green Colour Blindness • Inability to distinguish between red and green • A red green colour blind person does not see the number 29 on the right • In humans normal vision is completely dominant to red-green colour blindness
Genetics of Colour Blindness • Normal vision C • Red-green colour blindness c • Heterozygous females are called carriers • Work out the genotypes of the following family tree
Answers • Carrier mother XCXc • Nomal father XCY • Normal daughter XCXC • Carrier daughter XCXc • Normal son XCY • Colour-blind son XcY
Haemophilia • Haemophiliacs cannot make the blood clotting protein Factor VIII. • It caused by a recessive allele carried on the X but not the Y chromosome • Hence is sex-linked
Muscular Dystrophy • Skeletal muscles loose their normal structure and fibrous tissue develops in their place • Caused by a recessive allele carried on the X chromosome and is sex-linked
Polygenic Inheritance • Polygenic inheritance is a characteristic showing continuous variation and is controlled by the alleles of more than one gene • The more genes involved the more intermediate phenotypes that can be produced • The effects of the genes are additive (each dominant allele of each gene adds a contribution towards the characteristic controlled by the gene)
Examples include skin colour, height, weight Polygenic Inheritance in Humans
Effect of Environment • Many of these characteristics are influenced by the environment. • Polygenic inheritance + environmental factors = phenotypic characteristic which shows a wide range of continuous variation and a normal pattern of distribution.
Facts you need to know • P6 from “sex-linked genetics problems…” • to p7 “weight or height or skin colour…”