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Sex-linked traits
Applied Biology sections
3-8 & 3-9
Pp 80-83
Sex chromosomes
• Humans- 23rd pair of chromosomes- X,Y
• Eggs- Have an X chromosome.
• Sperm- Half have an X chromosome, half
have a Y chromosome.
– Fathers determine gender(sex) of offspring.
• Males- XY
• Females- XX
• 50% chance of having a girl or boy
offpring.
Sex-linked traits
• Any trait controlled by a gene on the X or
Y chromosome ( sex chromosomes).
• Most sex linked traits are on the X
chromosome.
• Some human examples:
– Hemophilia- blood does not clot –bleeding
results.
– Colorblindness- can’t tell the difference
between certain colors.
Sex-linked recessive disorders
• Why do we see the disorders more often
in males?
• Since males have only one X chromosome
if it has the recessive gene for the disorder
they have the disorder.
• For females to have the disorder they
must inherit two X chromosomes (one
from mom and one from dad) with the
recessive gene.
Continued….
• Females can be carriers– They are heterozygous
– Have one recessive allele
– Do not have the disorder
– but can pass on the recessive allele to
offspring.
• Mothers pass trait on to sons.
• In a Pedigree- a chart showing inheritance of
traits over several generations there will be more
males with the disorder.
Pedigree
• Chart used to record and trace inherited traits in
a family over generations.
• Key– Squares- males
– Circles- females.
– Completely shaded shapes- have the trait.
– Half-shaded-carriers- do NOT show the trait.
– Parents-connected by horizontal lines
(marriage line)
– Children beneath them.
– Generations – Roman Numerals ( I, II, III etc)
– Individuals -Numbered from left to right in
each generation.
Sex-linked recessive pedigree
skips generations , see more often in males.