Chapter 12: Sex Linkage and Polygenic Inheritance



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Chapter 12: Sex Linkage and Polygenic Inheritance

  • Higher Human Biology
  • Unit 1: Cell Function and Inheritance
  • 05/09/18
  • 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
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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

The language – Lots........

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  • 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
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  • 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

  • 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.
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  • 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.
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  • 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.
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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

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.
  • Sex-linked genes
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  • 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)
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  • 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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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Five possible genotypes for normal and red-green colour blindness

  • Genotype
  • Phenotype
  • XCXC
  • Female with normal colour vision
  • XCXc
  • Female (carrier) with normal colour vision.
  • XcXc
  • Female with colour blindness (very rare e.g. 0.5%)
  • XCY
  • Male with normal colour vision
  • XcY
  • Male with colour blindness more common (8%)
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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Work out the genotypes of the following family tree

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Answers

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

Puzzle 1

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 1 - Answer

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 2

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 2 - Answer

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 3

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Answer: Puzzle 3

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 4

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 4 - Answer

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 5

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 5 - Answer

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 6

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Answer puzzle 6

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 7

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 7 - Answer

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 8

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 8 - Answer

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 9

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 9 - answer

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 10

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 10 - Answer

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 11

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 11 Answer

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  • 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.
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  • 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.
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  • 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.
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  • 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.
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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

When the father is normal and the mother is an unaffected carrier

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Family Tree of Haemophilia

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 1

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Puzzle 1- Answer

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  • 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.
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  • 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.
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  • 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)
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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Task: Torrance pg 91 Qu’s 1-3

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

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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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

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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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance
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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance
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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

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 genes.
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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

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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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

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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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

Task: Torrance pg 93 Qu’s 1+2

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  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

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)
  • 05/09/18
  • Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance


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