1. 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

2. 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)

3. 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

4. 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

5. 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

6. 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

7. Pictures of X and Y chromosomes

8. 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.

9. 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.

10. 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.

11. Explain the pedigree chart

12. 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 colour-blindness in man

13. 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)