Sex Linkage and Polygenic Inheritance Higher Human Biology
Dec 18, 2015
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
Female and Male 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
Family Tree of Haemophilia
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
Family Tree of Muscular Dystrophy
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)
Polygenic Inheritance
Polygenic Inheritance in Humans
• 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…”