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 DeterminationAutosomes:

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/genes/207.asp

Explain the pedigree chart

ReviewThe 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 colour-blindness 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)