Chapter 12: Sex Linkage and Polygenic Inheritance

Higher Human Biology Unit 1: Cell Function and Inheritance Chapter 12: Sex Linkage and Polygenic Inheritance Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Learning intentions; • To revise sex chromosomes • To examine effects of sex-linked genes • To look at polygenic inheritance Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

The language – Lots........ Polygenic Inheritance Sex chromosomes Carriers Continuous variation Normal distribution Sex-linked Haemophilia Superscripts Allele Discontinuous variation Autosomes Additive Homozygous Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

The stuff you Need to know! • Sex-linked inheritance and the effects of the presence of genes on the X-chromosome and not on the Y-chromosome. • Polygenic inheritance leading to characteristics Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Normal Body Cells • In the nucleus of every body cell there are 46 chromosomes • 22 homologous pair (AUTOSOMES) and one pair of sex chromosomes Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Female and Male Sex Chromosomes In the female, the sex chromosomes make up a fully homologous pair, the X chromosomes. In the male, the sex chromosomes make up a pair consisting of an X and a much smaller Y, which is homologous to only part of the X chromosome. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Sex-linked genes. • The X and Y chromosomes behave as a homologous pair at meiosis. However, the X chromosome differs from the Y chromosome in that the larger X carries many genes not present on the smaller Y. These genes are said to be sex-linked. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

At fertilisation • When an X chromosome meets a Y chromosome at fertilisation, each sex-linked gene on the X chromosome becomes expressed in the phenotype of the human male produced. • This is because his Y chromosome does not possess alleles of any of these sex-linked genes and cannot offer dominance to them. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Symbols Sex-linked genes • In crosses and family trees involving sex-linked gees, the sex chromosomes are represented by the symbols X and Y and the alleles of the sex-linked gene by appropriate superscripts. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

X-linked recessive disorder - 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 (C) is completely dominant to red-green colour blindness (c) Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Genetics of Colour Blindness • Normal vision C • Red-green colour blindness c • These are the alleles are sex-linked because... • Heterozygous females are called carriers (Cc) Although they are unaffected themselves there is a 1 in 2 chance (50%) chance that they will pass the allele on to each of the offspring. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Five possible genotypes for normal and red-green colour blindness Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Work out the genotypes of the following family tree Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Answers • Carrier mother XCXc • Nomal father XCY • Normal daughter XCXC • Carrier daughter XCXc • Normal son XCY • Colour-blind son XcY Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Why is colour blindness more common in males? • Red green colour blindness is rare in females since 2 recessive alleles must be inherited. • It is more common in males where only one is needed. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Colour blindness problem set http://www.biology.arizona.edu/human_bio/problem_sets/color_blindness/01q.html Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 1 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 1 - Answer Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Answer: Puzzle 3 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Answer puzzle 6 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 9 - answer Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 11 Answer Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Haemophilia: mutated form of factor VIII in platelets • Haemophiliacs cannot make the blood clotting protein Factor VIII. • This is a problem with blood clotting. So, if a tissue is damaged and blood vessels are broken, bleeding continues for longer than normal. • Some bleeding is obvious such as when the skin is cut or broken. Others are less easy to spot like bleeding into or around the joints. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

X-linked recessive disorder -Haemophilia • It caused by a recessive allele carried on the X (e.g. The gene is located on the non-homologous region of the x-chromosome) but not the Y chromosome. • The haemophiliac allele (Xh)is recessive to the normal allele (XH). • Hence is sex-linked. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

More common in males than females • Haemophilia is more common in men than women. • Fequency in britian is 1:5000 • Males inherit the allele from their mother and develop the disease. • Since (until recently) the prognosis for survival was poor and haemophiliac males did not survive to pass on the allele to their daughters (its on the X-chromosome). Therefore females with haemophilia where rare. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

There is now treatment -FYI • ‘Clotting factor concentrates’ revolutionised haemophilia care allowing patients to travel, have jobs, and live full and independent lives. • Transfusion with whole blood and plasma. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

When the father is normal and the mother is an unaffected carrier Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Family Tree of Haemophilia Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 1- Answer Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Muscular Dystrophy • Duchenne Muscular Dystrophy is the most common form of this disease. • Sufferers are severely disabled from an early age. • The normally die without passing allele onto the next generation. • Afects 1:3000 male infants. • 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. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Family Tree of Muscular Dystrophy • In this family the allele survives from female carrier to female carrier. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

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) Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Task: Torrance pg 91 Qu’s 1-3 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Chapter 12: Sex Linkage and Polygenic Inheritance