Transcript Sex Inheritance and linkage

Sex Inheritance and linkage
WALT
• The genetic information for your gender is
carried on two specific chromosomes in
human these are X and Y
• The Y chromosome is smaller than the X
and so carries fewer genes
• Genetic disorders carried on the X
chromosome are more likely to effect
males
Sex Determination
Autosomes:
22 pairs of chromosomes
which are identical in both sexes
(ordinary paired chromosomes which are
not sex chromosomes)
Sex chromosomes (heterosomes):
the pair (23rd)which determine sex of
the organism
Homogametic sex: XX (female)
Heterogametic sex: XY (male)
Sex determination
• In humans Females have two XX
chromosomes and are homogametic
• Males have one X and one Y chromosome
and are heterogametic
• In humans about 114 boys are born for
every 100 girls
• By puberty these numbers are equal
• In the 1990’s it was determined that the Y
chromosome only carries 60 genes
• This includes a male determining gene called
SRY
• All embryo’s are female unless the active SRY
imposed maleness on it
• Androgen Insensitivity Syndrome is a condition
where a genetically male child develops as a
female as the receptors for male hormones do
not work properly
Abnormal inheritance of sex
chromosomes
Genotype
Gender
Syndrome
Physical
Traits
XXY
male
Klinefelter
syndrome
sterility, small
testicles, breast
enlargement
XYY
MALE
XXY syndrome
normal male traits
although usually
taller
XO
female
Turner syndrome
sex organs don't
mature at
adolescence,
sterility, short
stature
XXX
Female
Triple X syndrome tall stature,
learning
disabilities, limited
fertility
Sex Linkage
• Sex linkage refers to the carrying of genes
on the sex chromosomes.
• These genes determine body characters
and have nothing to do with sex.
• Human
females
have
two
XX
chromosomes and therefore two alleles
• For males however as the Y chromosome
is smaller not all the alleles are paired
Pictures of X and Y chromosomes
Sex linked genetic disorders
• This results in males suffering from
genetic disorders carried on the X
chromosome
• The X chromosome contains between 900
and 1,400 genes compared to just 60 on
the Y chromosome
• Examples of sex linked disorders are
haemophilia, colour blindness, Duchene
muscular dystrophy and SCID.
Haemophilia inheritance
• Genes found on the X-chromosome cause
haemophilia, so the disorder usually
affects only boys.
• In some cases, the disorder appears 'out
of the blue', following a genetic change in
the unfertilised egg.
Haemophilia
• In other cases, the affected boy inherits
haemophilia from his carrier mother.
• A carrier woman has a one-in-four chance of
having an affected son, and a one-in-four
chance of having a carrier daughter.
• About a third of carrier women have mild
symptoms themselves.
• http://www.sciencemuseum.org.uk/exhibitions/ge
nes/207.asp
Explain the pedigree chart
Review
The X chromosome carries many such genes.
The Y chromosome has very few.
• Features linked on the Y chromosome will only
arise in heterogametic sex (XY)
• Features linked on the X chromosome may arise
in either sex
• e.g. white eye colour is a sex-linked gene in
fruitfly, haemophilia and red-green colourblindness in man
Role reversal?
• Sex determination does differ in other
organisms:
• Male being XX, female being XY,
e.g. birds, most reptiles, some fish,
butterflies
OR
• Male being XO, female being XX,
e.g. some insects (grasshopper)