1. Sex Linkage and Polygenic Inheritance Higher Human Biology

2. Lesson Aims To revise sex chromosomes To examine effects of sex-linked genes To look at polygenic inheritance

3. Normal Body Cells In the nucleus of every body cell there are 46 chromosomes 22 homologous pair and one pair of sex chromosomes

4. Female and Male Sex Chromosomes

5. 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

6. 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

8. Answers Carrier mother X C X c Nomal father X C Y Normal daughter X C X C Carrier daughter X C X c Normal son X C Y Colour-blind son X c Y

9. 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

10. Family Tree of Haemophilia

11. 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

12. Family Tree of Muscular Dystrophy

13. 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)

14. Polygenic Inheritance

15. Polygenic Inheritance in Humans Examples include skin colour, height, weight

16. Polygenic Inheritance in Humans

17. 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.

18. Facts you need to know P6 from “sex-linked genetics problems…” to p7 “weight or height or skin colour…”