Chapter 12: Sex Linkage and Polygenic Inheritance

Chapter 12: Sex Linkage and Polygenic Inheritance

Higher Human Biology

Unit 1: Cell Function and Inheritance

20/04/23 1Mrs 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


The language – Lots........

20/04/23 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

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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


Normal Body Cells

• In the nucleus of every body cell there are 46 chromosomes

• 22 homologous pair (AUTOSOMES) and one pair of sex chromosomes


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.


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.


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.


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


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)


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.


Five possible genotypes for normal and red-green colour blindness

Genotype PhenotypeXCXC 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%)


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 XcY20/04/23 14Mrs 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.


Colour blindness problem sethttp://www.biology.arizona.edu/human_bio/problem_sets/color_blindness/01q.html


Puzzle 1


Puzzle 1 - Answer


Puzzle 2


Puzzle 2 - Answer


Puzzle 3


Answer: Puzzle 3


Puzzle 4


Puzzle 4 - Answer


Puzzle 5


Puzzle 5 - Answer


Puzzle 6


Answer puzzle 6


Puzzle 7


Puzzle 7 - Answer


Puzzle 8


Puzzle 8 - Answer


Puzzle 9


Puzzle 9 - answer


Puzzle 10


Puzzle 10 - Answer


Puzzle 11


Puzzle 11 Answer


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.


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.


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.


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.


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


Family Tree of Haemophilia


Puzzle 1


Puzzle 1- Answer


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.


Family Tree of Muscular Dystrophy

• In this family the allele survives from female carrier to female carrier.


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)


Task: Torrance pg 91 Qu’s 1-3


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.


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.




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.


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.


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.


Task: Torrance pg 93 Qu’s 1+2


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)

20/04/23 Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

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Chapter 12: Sex Linkage and Polygenic Inheritance