Punnett Squares 2- Sex-linked
traits
Year 10 Science
X vs Y
chromosome
• The X chromosome
typically has
around 900
protein-coding
genes, while the
Y chromosome has
only about 55100.
Expression of a
gene
• Gene expression is the
process by which the
instructions encoded in a gene
are used to produce a
functional gene product,
typically a protein or RNA
molecule.
• It's essentially the "turning on"
of a gene, allowing it to be
used to create a specific
product that performs a
function within a cell or
Carrier of a
gene
• A carrier of a gene is an individual
who carries a genetic mutation but
does not show symptoms or have
health problems due to the presence
of a normal, working copy of the
same gene.
• Essentially, they have one mutated
gene and one normal gene, so the
normal gene can compensate for the
mutated gene.
• This means they can pass the
mutated gene on to their children,
who might then develop a genetic
disorder if they inherit the mutated
gene from both parents.
Normal, Carrier and affected
offspring
• Normal offspring have
two normal alleles of
the same gene.
• Carrier offspring have
one normal allele and
one recessive allele
for a disease or
mutation. They express
the don’t express the
disease or mutation.
• Affected offspring have
two alleles that are
recessive for a disease
Colour-blindness is a sexlinked trait
• Colour-blindness is a
recessive gene.
• So, for a female to have
colour-blindness she would
have inherited two copies of
the recessive gene; one from
mum and one from dad.
• A male on the other hand only
has to inherit one copy of
the recessive gene from mum
as there is no gene on the Y
chromosome to try and
Colour-blindness
• In this Punnett square is the
mother the carrier or the father?
• In this Punnett square what
percentage of the offspring would
be expected to be normal?
a.
b.
c.
d.
25%
50%
75%
100%
• In this Punnett square what
percentage of the offspring would
you expect to be carriers?
a.
b.
c.
d.
25%
50%
75%
100%
Haemophilia is a sex-linked
trait
• Haemophilia is a
blood clotting
disorder where you
can bleed out as a
result of not being
able to create a
blood clot properly.
Haemophilia is a sex-linked
trait
• Haemophilia is more
common in males than
females as it is a
recessive gene that is
passed down on the X
chromosome.
• As males only have one X
chromosome, they only
need to inherit one copy
of the gene from mum to
be affected.
Haemophilia
• In this Punnett square is the mother the
carrier or the father?
XH
XH
Xh
XHXh
XHXh
Y
XHY
XHY
• In this Punnett square what percentage of
the offspring would be expected to be
normal?
a.
b.
c.
d.
25%
50%
75%
100%
• In this Punnett square what percentage of
the offspring would you expect to be
carriers?
a.
b.
c.
d.
25%
50%
75%
100%