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It has long been recognized that one of the advantages of sexual reproduction, consisting of the fusion between a male and a female gamete derived from two different individuals, is the increased genetic diversity of the offspring resulting in improved adaptability to environmental changes (Maynard Smith, 1978).

The different genetic strategies: The existence of functional male and female individuals is essential for reproduction in mammals as well as the majority of species (though not all – parthenogenetic lizards are known; Cole, 1984) in other vertebrates. It might be supposed that the mechanisms of sex differentiation would be basically similar in all vertebrates. Instead of this, several variations on a theme exist for achieving what is effectively the same end. In both mammals and birds, sex is determined by specific, visibly distinguishable chromosomes. In mammals, males (the heterogametic sex) have an X and a Y chromosome, while females (homogametic) have two