Sex Inheritance and Sex Linkage

Female - **   Male - XY      Haemophilia:

X chromosome = longer - for most of X chromosome > no equivalent homologous portion on the Y chromosome

Characteristics controlled by recessive alleles on the non-homologous portion of the X chromosome will occur more frequently in males. No homologous portion on the Y chromosome that might have the dominant allele - in which the recessive allele would not be expressed.

X chromosome -carries many genes - haemophilia - blood clots slowly - condition almost entrely confined to males. Causes - recessive allele - alters DNA nucleotides - so they do not code for the protein required for the clotting process.

Gametes always represented attached to the X chromosome because they are linked to the X chromosome. No equivalent allele on the Y chromosome - so do not carry gene for clotting protein.  E.g. XH XhTherefore males always inherit the condition from their mother. May be a carrier: carry the allele without showing any signs of the character in their phenotype. Cannot pass disease on to their sons but can pass onto daughters (who would become carriers).

Trace inheritance of sex-linked charcaters:

* Male represented by a square

* Female represented by a cirlce

* Shading within each shape represents presence of character in phenotype

* Dot within the circle represents woman with normal phenotype, who carries the defective allele