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A human karyotype - female
A human karyotype - male
Sex determination in the fruit fly
Sex determination depends on the
ratio
between number of X
chromosomes
and sets of autosomes
Results when there is
nondisjunction
of chromosomes during meiosis
Sex chromosome non-disjunction in
humans
XO – Turner syndrome
Sex chromosome non-disjunction in
humans
XXY – Kleinfelter syndrome
Achieving balance of genetic traits that are sex-linked
in mammals - Barr bodies
A calico kitten
Pattern of inheritance of X and Y chromosomes determines sex
Pattern of inheritance of traits that are Sex (X) linked - recess
Pattern of inheritance of traits that are Sex (X) linked - recessive
Non-disjunction can
affect pattern of
inheritance of sex-inked
traits
Pedigree analysis – sex linked traits
what to look for:
Recessive trait:
1. More males that females affected
2. Affected sons are usually born to unaffected
mothers, zig-zag pattern – from grandfather to
grandson through an unaffected female.
3. Approximately 50% of a carrier female are
affected
4. It is never passed from father to son
5. All daughters of affected fathers are carriers
Pedigree analysis – sex linked traits
what to look for:
Dominant trait:
1. Both males and females are affected; often
more females than males
2. Does not skip a generation. Affected sons
must have an affected daughter; affected
daughters must have either an affected mother or
an affected father.
3. Affected fathers will pass the trait on to all their
daughters.
4. Affected mothers ( if heterozygous) will pass
on the trait to approximately 50% of their
daughters and 50% of their sons.
X-linked dominant
inheritance
Faulty enamel
Reminder
If you are working with pedigrees and probability,
and the trait is autosomal (dominant or recessive)
the probability of inheriting the trait is
independent from the sex of the individual. If the
trait is sex linked the probability of inheriting the
trait includes the sex of the individual.