Higher Human Biology
Unit 1: Cell Function and
Inheritance
Chapter 12: Sex Linkage and
Polygenic Inheritance
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Mrs Smith: Ch12 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
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Polygenic Inheritance
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The language – Lots........
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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
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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
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• 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
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Polygenic Inheritance
Sex-linked genes.
• The X and Y
chromosomes
behave as a
homologous pair
at meiosis.
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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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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 sexlinked genes and cannot offer
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dominance to them. Polygenic Inheritance
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Symbols
• In crosses and family
trees involving sexlinked gees, the sex
chromosomes are
represented by the
symbols X and Y and the
alleles of the sex-linked
gene by appropriate
superscripts.
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Polygenic Inheritance
Sex-linked
genes
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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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Polygenic Inheritance
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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 CX C
X CX c
X cX c
X CY
X cY
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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%)
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Polygenic Inheritance
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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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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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Polygenic Inheritance
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Puzzle 1
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Polygenic Inheritance
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Puzzle 1 - Answer
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Puzzle 2
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Puzzle 2 - Answer
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Puzzle 3
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Answer: Puzzle 3
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Puzzle 4
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Puzzle 4 - Answer
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Puzzle 5
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Puzzle 5 - Answer
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Puzzle 6
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Answer puzzle 6
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Puzzle 7
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Puzzle 7 - Answer
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Puzzle 8
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Puzzle 8 - Answer
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Puzzle 9
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Puzzle 9 - answer
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Puzzle 10
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Puzzle 10 - Answer
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Puzzle 11
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Puzzle 11 Answer
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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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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 xchromosome) but not the Y
chromosome.
• The haemophiliac allele (Xh)is
recessive to the normal allele (XH).
• 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.
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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
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Family Tree of Haemophilia
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Puzzle 1
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Puzzle 1- Answer
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Muscular Dystrophy
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• 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 sexlinked.
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Linkage and
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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
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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)
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Task: Torrance pg 91 Qu’s 1-3
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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
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HISTOGRAM. Mrs Smith: Ch12 Sex Linkage and
Polygenic Inheritance
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Polygenic Inheritance
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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
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genes.
Polygenic Inheritance
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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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Polygenic Inheritance
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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
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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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