SEX-LINKED ALLELES
• The Y chromosome is very short and has very
few alleles on it other than those responsible
for maleness.
• The X chromosome is much longer and can
carry many other alleles along its length in
addition to those for femaleness.
• Only a very small part of the X and Y
chromosomes can pair up during Prophase 1
of meiosis and no crossing over occurs.
SEX-LINKED ALLELES
• The alleles that are carried on the nonhomologous part of an X-chromosome are
called sex-linked alleles.
SEX-LINKED ALLELES: How are sexlinked alleles inherited?
• Men have only one X chromosome. They only
have one of each sex-linked allele on the X
chromosome.
• Women have two X chromosomes. They have
two sex-linked alleles, one on each X
chromosome.
SEX-LINKED ALLELES: How are sexlinked alleles inherited?
• Alleles on the X chromosome are inherited by
both males and females but
* a male only inherits one X chromosome
and so can only receive one of the sexlinked alleles.
* a female inherits two X chromosomes
and so receives two of the sex-linked
alleles which can be homozygous or
heterozygous.
EXAMPLES OF SEX-LINKED DISEASES
• Cleft palate; Diabetes insipidus; Red-Green
Colour Blindness; Haemophilia; Muscular
dystrophy
• Sex-linked diseases occur more often in males.
This is because they only have one X
chromosome. A male will either inherit a
normal gene for a sex-linked allele or the
disease causing gene.
EXAMPLES OF SEX-LINKED DISEASES
• If the disease causing gene is recessive, a female
may inherit two normal genes, a normal gene and
a disease causing gene or two disease causing
genes.
• This means the female will only suffer from the
disease if she inherits two recessive disease
causing alleles.
• If the female has a normal allele and a disease
causing allele she will not suffer from the disease
but is referred to as a carrier.
EXAMPLES OF SEX-LINKED DISEASES
SEX-LINKED DISEASE: Red-Green
Colour Blindness
• A sex-linked gene on the X chromosome
enables humans to distinguish between
green, yellow, orange and red.
• If this gene mutates all the colours appear the
same.
• The dominant allele codes for normal vision.
• The recessive allele codes for red-green colour
blindness.
SEX-LINKED DISEASE: Red-Green
Colour Blindness
SEX-LINKED DISEASE: Haemophilia
• A rare, genetically determined condition of
frequent, excessive bleeding as the blood
clots very slowly.
• The dominant allele allows blood to clot.
• The recessive allele does not allow blood to
clot.
• Alleles are sex-linked alleles located on the Xchromosome. There is no gene for blood
clotting on the Y chromosome.
SEX-LINKED DISEASE: Haemophilia
• Possible genotypes for males:
XHY – normal male
XhY _ haemophiliac male
• Possible genotypes for females:
XHXH – normal female
XHXh – carrier female
(XhXh– haemophiliac female : no record of this
happening)
SEX-LINKED DISEASE: Haemophilia
SEX-LINKED DISEASE: Haemophilia
SEX-LINKED DISEASE: Haemophilia
GENETIC COUNSELLING
• Genetic counsellors are qualified professionals
part of a healthcare team, who provide
information and support to families who have
family members with birth defects or genetic
disorders.
• Genetic counsellors study family and medical
histories to analyse inheritance patterns and the
risks of a genetic disorder recurring.
• Genetic counsellors also give options on how to
best manage a genetic disorder and refer families
to social support structures.
GENETIC COUNSELLING
• See in your textbook some of the issues that could
be discussed with a genetic counsellor Pg 3.3.19
GENE TESTING
• Gene tests are new, sophisticated techniques
used to test for genetic disorders by directly
examining the DNA molecule.
• Amniocentesis is one of investigating the DNA
of a foetus.
GENE TESTING: Amniocentesis
GENE TESTING
ADVANTAGES
DISADVANTAGES
1.Clarify a diagnosis so correct
treatment can be given
1.Laboratory errors occur
2. Families can decide not to have
children with devastating diseases
2.Uncertainties w.r.t. test interpretations
as tests only give a probability for a
disorder
3. Identify people at high risk for
conditions that can be prevented
3. Current lack of cures for many of these
diseases
4. Anxiety caused by the results of the
test
5.Risks of discrimination and social
stigmatisation
FAMILY TREE or PEDIGREE DIAGRAM
• A family tree is used to investigate the pattern
of inheritance of a particular characteristic in
a family.
• A family tree shows the phenotypes and
genotypes of several generations in a family.
FAMILY TREE or PEDIGREE DIAGRAM
• A family tree can be used to predict if a couple
is likely to pass a genetic disorder to their
children.
FAMILY TREE or PEDIGREE DIAGRAM
• When trying to work out the genotypes of
individuals in a family tree use these steps:
1. Fill in the genotype of all the individuals which
have the recessive phenotype. They are
homozygous recessive.
2. Then look at all the individuals which have the
dominant phenotype. By looking at their parents
and/or offspring decide whether they are
heterozygous dominant or homozygous
dominant.
FAMILY TREE or PEDIGREE DIAGRAM
• http://www.biologycorner.com/worksheets/p
edigrees_practice.html
Go to the above web page and do these
activities.
FAMILY TREE or PEDIGREE DIAGRAM
• Remember when using a pedigree diagram for
a sex-linked allele e.g. red-green colour
blindness; haemophilia, these alleles are only
inherited on the X chromosome!!
FAMILY TREE or PEDIGREE DIAGRAM: Haemophilia is a
recessive sex-linked allele on the X chromosome