Download Sex-Linked Traits (x-linked traits)

Sex Determination
- Sex is determined by the 23rd pair of chromosomes – called
the sex chromosomes
- Chromosome pairs #1-22 are called autosomes
- Females have two X chromosomes while males have an X and
a Y, ie, the 23rd pair in males is NOT homologous – the Y
chromosome is shorter and does not carry the same genetic
information as the X
- There are no matching alleles on males’ 23rd pair
Chances of having a male or female child:
P1
XX
X
Y
X
XX
XY
X
XX
XY
50% males
50% females
x
XY
Sex-Linked Traits (x-linked traits)
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-
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some traits seem to occur in males more often than females (Ex:
Colour blindness (types), hemophilia, muscular dystrophy, male pattern
baldness … why?)
Alleles for these diseases are carried on the X chromosomes, so if a
male receives a defective X from his mother he will get the disease
because he does not have a compensating allele on his Y
A woman would have to get two defective copies in order to have the
disease (Note: these diseases are usually recessive)
How males and females get an X-linked disease:
Alleles:
Xr – colour blind
XR – normal vision
P1:
XRXr
x
Xr
Y
XR
XRXr
XRY
Xr
XrXr
XrY
XrY
25% XRXr – Carrier female (does not have the condition)
25% XrXr – female with the condition (2 copies of the defective
allele)
25% XRY - normal male (1 copy of normal allele)
25% XrY - male with the condition (1 copy of defective allele)