document.write( "Question 169072: Red-green colorblindness in humans is sex-linked and recessive. A colorblind man (jack) marries a normal woman (mary) whose mother was colorblind. Jack and Mary produce a son who is colorblind and a normal daughter. The genotypes of the 5 individuals involved are: \n" ); document.write( "

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The gene for the colourblindness is carried on the X chromosome.
\n" ); document.write( "As males only have one X chromosome they only need to have the gene on the one X chromosome for colour blindness to be displayed.
\n" ); document.write( "Females have two X chromosomes. As colour blindness is recessive then they need to have it on both of the X chromosomes.
\n" ); document.write( "I am using X for normal vision and x for colourblind vision
\n" ); document.write( "Jack has the genotype xY (He is colourblind)
\n" ); document.write( "Mary's mum has the genotype xx (She is colourblind)
\n" ); document.write( "Mary has the genotype Xx (The normal gene from her father and the colourblindness gene from her mother)
\n" ); document.write( "Son has the genotype xY (he is colourblind. The colourblindness gene came from Mary)
\n" ); document.write( "Daughter has the genotype xX (The colourblindness gene from Jack and the normal gene from Mary.)
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