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Blueprint of Life
Topic 12: Sex Linked Genes
Biology in Focus, HSC Course
DOT POINT(s)
 describe the inheritance of sex-linked genes, and alleles
that exhibit co-dominance and explain why these do not
produce simple Mendelian ratios
 describe the work of Morgan that led to the
understanding of sex linkage
Introduction
Mendelian ratios of inheritance apply only in situations where
conditions are similar to those studied by Mendel. However, if
genes do not assort independently or do not show dominance,
Mendel’s ratios are not obtained.
homeschoolersresources.blogspot.com
Introduction
Two examples where these deviations from Mendel’s ratios have
been seen are in sex-linked inheritance and co-dominance,
which is what we are going to talk about today.
en.wikipedia.org
Sex-linkage
Every cell in the human body contains 23 pairs of chromosomes:
22 pairs of autosomes (chromosomes that code for general traits
within the body) and 1 pair of sex chromosomes
ghr.nlm.nih.gov
Sex-linkage
Sex chromosomes carry genes
that determine the sexual
characteristics of a person and
therefore influence whether
they are male or female. Sex
chromosomes in an individual
may differ from each other in
size and shape (in contrast to
autosomes, where homologous
chromosomes all have the same
appearance).
www.psmicrographs.co.uk
Sex-linkage
In humans:
 Females have a pair of similar sex
chromosomes— their genotype is
represented as XX (described as
homogametic).
 Males appear to be different
(heterogametic) and their genotype is
represented as XY. In human males,
one sex chromosome is a typical X
chromosome, but the other is shorter
and seems to be missing part of its
‘arms’, resulting in a ‘Y’ shape
leavingbio.net
Sex Determination
The offspring of most animals have an equal (50%) chance of
being male or female. This is determined by the following
mechanisms during the life cycle:
■ the segregation of sex
chromosomes during meiosis
■ the transfer of one sex
chromosome to each gamete
■ the fusion of gametes during
fertilisation.
www.sodahead.com
Sex Determination
During meiosis, the sex chromosomes segregate, just like any
other homologous pair of chromosomes, and only one of each
chromosome pair passes into a gamete.
www.biology.iupui.edu
Sex Determination
Since a female has 44 autosomes + XX, when the chromosome
number is halved during gamete formation, each female gamete
(egg cell) receives 22 autosomes + X. In males, half the gametes
(sperm cells) receive 22 autosomes + X and the other half receive
22 autosomes + Y. . It is therefore the male gamete in humans that
determines the sex of the offspring.
www.bbc.co.uk
Sex Determination
Fertilisation follows and the
combination of sex chromosomes
in the zygote dictates the sex of the
child: a zygote that inherits an X
chromosome from both the
mother and father will be female
(XX). A zygote that receives an X
chromosome from the mother and
a Y chromosome from the father
will be male (XY). In humans,Y
carries the testis-determining gene
and therefore, if present, the Y
assures the child will be male.
toppersguide17.blogspot.com
Sex Determination
In some cases, non-disjunction
of chromosomes occurs during
meiosis (the chromatids do not
separate), resulting in the
incorrect number of
chromosomes in the offspring
(e.g. 45 or 47). If this occurs in
the sex chromosomes and only
one X chromosome is present,
the child will be female. If the
individual is XXY, it will be
male.
www.zazzle.ca
Sex Determination
Turner Syndrome occurs when only one
X chromosome is present. There are
characteristic physical abnormalities,
such as short stature, swelling, broad
chest, low hairline, low-set ears, and
webbed necks. Girls with Turner
syndrome typically experience gonadal
dysfunction (non-working ovaries),
which results in amenorrhea (absence of
menstrual cycle) and sterility.
www.doctortipster.com
Sex Determination
Concurrent health concerns are
also frequently present, including
congenital heart disease,
hypothyroidism (reduced hormone
secretion by the thyroid), diabetes,
vision problems, hearing concerns,
and many autoimmune diseases.
Finally, a specific pattern of
cognitive deficits is often observed,
with particular difficulties in
visuospatial, mathematical, and
memory areas.
en.wikipedia.org
Sex Determination
XYY syndrome is an abnormal number
of the sex chromosomes in which a
human male receives an extra Ychromosome, giving a total of 47. This
produces a 47,XYY karyotype, which
occurs in 1 in 1,000 male births. Some
medical geneticists question whether the
term "syndrome" is appropriate for this
condition because its clinical phenotype
is normal and the vast majority (an
estimated 97% in Britain) of 47,XYY
males do not know their karyotype.
powerofthegene.com
Sex-linked Genes
The larger sex chromosome (X
in humans) may also carry a few
genes that code for non-sexual
body characteristics. These
genes are termed sex-linked
genes, since they are physically
linked to the sex chromosome
and are inherited together with
the sexual traits.
www.biologycorner.com
Sex-linked Genes
Sex-linked genes in males and
females tend to differ in their
inheritance patterns, since
males lack one X chromosome
and therefore have only one
allele for each sex-linked gene,
rather than a pair of alleles (as is
present in females).
feistyhome.phpwebhosting.com
Sex-linked Genes
Mendel’s experiments did not
show sex-specific effects and so
sex-linked inheritance shows a
deviation from Mendel’s
expected ratios. (Today, some
scientists wonder whether
Mendel was just fortunate in
selecting only nonlinked traits, or
if he perhaps simply disregarded
results that did not ‘fit’ his
ratios.)
chubbyriceball.wordpress.com
Thomas Morgan
When Thomas Hunt Morgan, an
American cytologist, began his
now famous genetics
experiments in 1910, the
paradox that faced scientists was:
If chromosomes are the basis of
inheritance, why do the number of
traits that separate during meiosis
exceed the number of chromosomes?
bio.as.uky.edu
Thomas Morgan
Morgan was initially sceptical about aspects of both Mendelian
inheritance and the chromosome theory and so it is ironic that it
was his experiments on the fruit fly, Drosophila melanogaster, that
provided the significant evidence needed by scientists before they
could finally accept Sutton and Boveri’s chromosome theory of
inheritance (that more than one factor (trait) was present on each
chromosome).
www.taxateca.com
Thomas Morgan
Morgan’s experiments showed
without any doubt that:
■ the gene for eye colour in
fruit flies is located on the X
chromosome, and
■ hereditary factors can be
exchanged between the X
chromosomes of an individual.
www.brain.riken.go.jp
Thomas Morgan
The results of Morgan’s studies
led to a greater understanding of
how genes are arranged on
chromosomes and how genetic
material can be exchanged
(during crossing over) in
meiosis. He was awarded the
Nobel Prize for Physiology or
Medicine in 1933.
www.nobelprize.org
Thomas Morgan
The Nobel Prize is a set of annual
international awards bestowed in a
number of categories by Scandinavian
committees in recognition of cultural
and/or scientific advances. The will of
the Swedish philanthropist inventor
Alfred Nobel established the prizes in
1895. The prizes in Physics, Chemistry,
Physiology or Medicine, Literature,
and Peace were first awarded in 1901.
en.wikipedia.org
Morgan’s Experiment
Morgan was working with Drosophila melanogaster (fruit flies),
which normally have red eyes. He discovered a mutant male fruit
fly that had white eyes.
www.nobelprize.org
Morgan’s Experiment
Morgan began with a series of
crosses in the typical Mendelian
sequence to determine if the
gene for white eyes would show
a Mendelian pattern of
inheritance:
■ Cross 1: He cross-bred purebreeding parents to obtain F1
hybrid offspring. Morgan crossed
a white-eyed male and a purebred (homozygous) redeyed
female.
Morgan’s Experiment
Cross 2: He then crossed the F1
hybrid offspring to obtain the F2
generation. Morgan was
expecting a Mendelian 3:1 ratio
but, surprising to him, his
results showed something
different—more than 80% of
the flies had red eyes and less
than 20% had white eyes; in
addition, most flies with white
eyes were male. At first, he
thought that perhaps female flies
could not have white eyes.
www.gizmodo.com.au
Morgan’s Experiment
■ Cross 3: He performed a typical ‘test cross’ to investigate this
hypothesis. He crossed a white-eyed male with a hybrid red-eyed
female. His results showed in the F2 that both females and males
could have white eyes. He would have to come up with another
hypothesis to explain the unexpected ratios in his second cross.
www.gizmodo.com.au
Morgan’s Experiment
He arrived at his next (and this time correct) hypothesis that the
white eye characteristic is ‘sex limited’ (Morgan’s words) and is
carried on the X chromosome. He followed this with subsequent
genetic crosses that proved that red eyes were in fact sex-linked
(carried on a sex chromosome).
library.thinkquest.org
Explaining Symbols
Since the inheritance of a sexlinked gene is being studied, the
alleles of that gene (e.g. R = red
eye colour, r = white eye
colour) and the type of
chromosome on which it is
carried (X or Y) must be shown
in the genetic cross. The letters
R and r are assigned in keeping
with the symbols used to show
dominant and recessive alleles of
a gene.
www.iq.poquoson.org
Explaining Symbols
These are then written as a superscript to the chromosome on
which they are attached:
www.gizmodo.com.au
Morgan’s Experimental Significance
Morgan’s work was significant
because it provided evidence for
the chromosomal basis of sexlinkage: his experiments showed
that the gene for eye colour is
linked to other genes that
determine the sex of Drosophila
melanogaster —that is, that the
gene for eye colour is sex-linked.
This paved the way for many
more experiments to show
linkage on all chromosomes.
suite101.com
Human Example: Haemophilia
The gene that codes for a certain bloodclotting factor is normally
situated on the X chromosome. In males who lack an allele of this
gene, haemophilia results as they have no similar gene on the Y
chromosome to compensate and so they are unable to produce the
clotting factor.
research.fuseink.com
Human Example: Haemophilia
In females lacking one allele of
this gene, another may be present
since they have two X
chromosomes. If the allele
present is not defective, they do
not have the disease haemophilia
and are said to be ‘carriers’ of the
disease, since they can pass their
defective gene on their X
chromosome to their sons (who
would be afflicted) or to their
daughters (who could then be
carriers).
simple.wikipedia.org
Human Example: Haemophilia
Daughters will be affected only
if they inherit a pair of
defective alleles from both
parents—one defective allele
on each of their X
chromosomes. Colour
blindness in humans is also an
X-linked trait. Students who
study the HSC Option:
Communication will need to
know about colour blindness.
www.ehc.eu
Activity/Homework
-Students to complete The work of Tomas Hunt Morgan
Worksheet (CD)
a
Extension
-Handout 12.1.4 Additional Information of Sex linked genes in
humans
a