Sex Linked Inheritance

 
As you already know in the nucleus of every human cell there are 46 chromosomes. These exist as 22 pairs of homologous pairs of autosomes and one pair of sex chromosomes. The X chromosome is different from the Y chromosome in that it carries more genes that are not present on the Y chromosome. these are sex linked genes.
 

When the X and Y chromosomes meet at fertilisation each sex linked gene on the X chromosome is expressed in the phenotype as there is no opposition on the Y chromosome to offer dominance.

 
 
 
 
Example
 
The gene that codes for colour blindness is both sex linked and reccesive. Here are the five possible genotypes and phenotypes for normal colour vision and colour blindness
 
C - Normal Vision
c - Colour Blind
 
 Genotype  Phenotype
 XCXC  Female with normal colour vision
 XCXc  Female (carrier) with normal colour vision
 XcXc  Female with colour blindness
 XCY  Male with normal colour vision
 XcY  Male with colour blindness
 

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