Sex Inheritance and Sex Linkage

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  • Created by: J.E.C.
  • Created on: 28-01-14 09:44

Inheritance of Haemophilia

Female - XX   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).

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Pedigree charts

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 

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