Download Sex Linkage - The Grange School Blogs

Monday 31st October
COPY LO:
To describe and explain sex
linkage
Starter
Explain how the sex of a human is
determined
BATS
• Describe how sex is determined
genetically
• Explain what is meant by sex
linkage
• Describe how sex linked
diseases such a haemophilia are
inherited
How many chromosomes do
humans have?
23 pairs - 22 of
these have partners
that are identical in
appearance, whether
in male or female
The remaining pair
are the sex
chromosomes
In females the 2 sex chromosomes appear the
same and are called the X chromosomes
In the human male - there
is a single X
chromosome like that in
the female
but the second one is
smaller in size and
shaped differently
This is the Ychromosome
Hand out sheet ‘Sex Linkage’ - go through first
paragraph - draw in sex chromosomes.
Explain Karyotype preparation and
amniocentesis
- use sheets + sex linkage sheet
Unlike other features in an organism , sex
is determined by chromosomes rather
than genes
• female Gametes egg cells all
contain X
chromosomes
• male Gametes sperm - 1/2 have
an X chromosome
and 1/2 have a Y
chromosome
Draw a genetic diagram and punnet square to
show the inheritance of sex (Fig 1 Page 118)
A gene is describes as being sexlinked if it is found on one of the
sex chromosomes.
There are many genes
controlling features on
these chromosomes. The
most common are
red/green colour
blindness, haemophilia and
Duchennes muscular
dystrophy.
Explain why characteristics that are
controlled by recessive alleles will
appear more frequently in males
Because there
is no
homologous
portion on the
Y-chromosome
that might have
a dominant
allele
The X chromosome carries many
genes - e.g. haemophilia? - what is
this?
•
•
•
•
Blood clots slowly
May be slow and
persistent internal
bleeding
Lethal if not treated
Why are diseases
like this selected
against?
About 1 in 20 000 people in Europe
have the gene
•
Although there are
a few haemophilic
females nearly all
are men
One cause of haemophilia - a
recessive allele that has altered DNA
nucleotides - mutation - so the
required protein is not coded for.
•
•
This results in the
person not being
able to produce
one of the blood
clotting proteins.
How are
haemophiliacs
treated?
How do we draw sex linked genetic
diagrams?
We use capital letters and lower
case letters to represent
dominant and recessive as
before but we attach these to
capital X
e.g
H
X
and
h
X
Why do we not attach them to the
capital Y in the same way?
Human sex linkage : Haemophilia sheet
Copy fig 3 - top of page 119
•
•
Remember the
boys X
chromosome
ALWAYS comes
from their mother
as the Y can only
come from a man
Read Hint and
Examiners tip on
Page 119
So - the defective allele must always
come from the mother - She will either
have the disease or be a ‘carrier’
•
Explain why
haemophiliac
males cannot
pass on the
haemophilia
allele to their
sons but they
can to their
daughters
Go through rest of sheet and do
colour blind example.
Pedigree charts - rules
•
•
•
•
Male = square
Female = circle
shading = presence
of a character in the
phenotype
dot in a circle =
woman has a normal
phenotype but
carries the defective
allele
Q 1-5 Bottom page 119
•
•
Application Page
120
Royal Family
haemophilia sheet
Other useful Sources
http://www.youtube.com/watch?v=NgXOLU3uiHg - summary of Mendelian inheritance
http://www.biologymad.com/master.html?http://www.biologymad.com/A2Biology.htm
The best A Level Biology Website
References
Glenn Toole, Susan Toole (2008). A2 Biology. Cheltenham: Nelson