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…”