Chromosomes X and YChromosomes X and Y do not truly make up a homologous pair. They act similarly in their roles,but they are not homologous (the same). The X chromosome in humans is much longer than the Y chromosome and also contains many more genes.These genes are said to be sex-linked, due to the fact they are present in one of the sex chromosomes. During fertilization, when the opposing homologous chromosomes come together, the smaller Y chromosome offers no dominance against the ‘extra’ X chromosomes as indicated below.The arrows indicate sex-linkedgenes in the X chromosome. In this homologous pairing, all thosegenes are dominant, because there are no opposing genes in the Y chromosome to offer dominance.So when the organism has an XY chromosome complement (i.e. a male), these sex-linked genes are freely expressed in the organism phenotype.Sex-Linked CharacteristicsThese sex-linked genes on the X chromosome display a number of characteristics. The followingare just some examples of phenotypes as a result of these genes in expression;Red-Green color blindnessHemophilia– A condition which prevents the clotting of the bloodMore information on sex-linkedcharacteristics and how they are passed on from generation to generation will be available in new areas of the site soon.The next tutoriallooks at genetic mutations and the consequences as a result of them.
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