Higher Human Biology Unit 1: Cell Function and Inheritance

Chapter 12: Sex Linkage and Polygenic Inheritance

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Learning intentions; • To revise sex chromosomes • To examine effects of sex-linked genes • To look at polygenic inheritance 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 2

The language – Lots........

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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 4 02/05/2020 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

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Female and Male Sex Chromosomes 02/05/2020 • 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 6

Sex-linked genes.

• The X and Y chromosomes behave as a homologous pair at meiosis.

02/05/2020 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 7 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 02/05/2020 Mrs Smith: Ch12 Sex Linkage and dominance to them.

Polygenic Inheritance 8

Symbols • 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

.

02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance Sex-linked genes 9

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) 10 02/05/2020 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.

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Five possible genotypes for normal and red-green colour blindness

Genotype X C X C X C X c X c X c X C Y X c Y Phenotype

Female with normal colour vision Female (carrier) with normal colour vision.

Female with colour blindness (very rare e.g. 0.5%) Male with normal colour vision Male with colour blindness more common (8%) 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 12

Work out the genotypes of the following family tree 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 13

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 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 14

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.

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Colour blindness problem set http://www.biology.ari

zona.edu/human_bio/p roblem_sets/color_blin dness/01q.html

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Puzzle 1 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 17

Puzzle 1 - Answer 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 18

Puzzle 2 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 19

Puzzle 2 - Answer 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 20

Puzzle 3 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 21

Answer: Puzzle 3 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 22

Puzzle 4 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 23

Puzzle 4 - Answer 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 24

Puzzle 5 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 25

Puzzle 5 - Answer 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 26

Puzzle 6 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 27

Answer puzzle 6 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 28

Puzzle 7 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 29

Puzzle 7 - Answer 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 30

Puzzle 8 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 31

Puzzle 8 - Answer 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 32

Puzzle 9 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 33

Puzzle 9 - answer 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 34

Puzzle 10 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 35

Puzzle 10 - Answer 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 36

Puzzle 11 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 37

Puzzle 11 Answer 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 38

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. 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 39

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 (X h )is recessive to the normal allele (X H ).

• Hence is sex-linked.

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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. 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 41

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.

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When the father is normal and the mother is an unaffected carrier 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 43

Family Tree of Haemophilia 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 44

Puzzle 1 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 45

Puzzle 1- Answer 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 46

02/05/2020 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 47 Polygenic Inheritance

Family Tree of Muscular Dystrophy • In this family the allele survives from female carrier to female carrier.

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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) 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 49

Task: Torrance pg 91 Qu’s 1-3 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 50

Polygenic Inheritance Discontinuous Variation.

• A characteristic shows

DISCONTINUOUS VARIATION

if it can be used to divide up the members of a species into two distinct groups.

• e.g. Can roll tongue vs Cannot roll tongue.

• Such information is often presented in a

bar chart

.

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Polygenic Inheritance Continuous Variation.

• A characteristic shows

CONTINUOUS VARIATION

when it varies amongst the members of a species in a smooth continuous way from one extreme to another, and does not fall into distinct groups.

• e.g. Height – varies from very small to very tall. Also, skin colour, weight • Such information is often presented in a

HISTOGRAM.

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The additive effect of polygenic genes • The genes involved in polygenic inheritance are transmitted from generation to generation via meiosis in the normal way.

• What makes them different from other genes is that their effects are ADDITIVE. That is each dominant allele of each gene adds a contribution towards the characteristic controlled by the Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 55

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.

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Many characteristics which show continuous variation (e.g. Height, foot size, etc.) are influenced by the environment • They are dependant on favourable environmental conditions for their full phenotypic expression.

• For example, regardless of how many dominant alleles for height that a person inherits, he or she will not reach their full potential for height without consuming an adequate diet during childhood and adulthood.

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Task: Torrance pg 93 Qu’s 1+2 02/05/2020 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance 58

Essay Question Guide to H-Grade essays pg 60 With named examples, discuss how inheritance of sex linked traits differ from that of autosomal traits.

(15)

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