Sex determination in humans and many other mammals is genetic, with the presence or absence of the Y chromosome determining maleness or femaleness. The SRY gene on the Y chromosome causes embryonic gonads to develop into testes rather than ovaries. In birds and some fish, the ZW sex determination system is used instead of XY, with females being ZW and males ZZ. Genes located on the sex chromosomes are called sex-linked genes and can be inherited in sex-specific patterns. Nondisjunction of sex chromosomes during meiosis can sometimes result in individuals with abnormal sex chromosome complements.

15. The inheritance of X-linked genes P generation parent 1 x parent 2 Parental phenotype Female, red - eyes Male, white eyes Parental genotype X w+ X w+ X w Y Gametes all X w+ ½ X w , ½ Y F 1 generation F 1 genotype ½ X w+ X w , ½ X w+ Y F 1 phenotype ½ female, red eyes ½ male, red eyes Interbreeding F 1 generation parent 1 x parent 2 F 1 phenotype Female, red eyes Male, red eyes F 1 genotype X w+ X w X w+ Y F 1 Gametes ½ X w+ , ½ X w ½ X w+ , ½ Y

16. (F 1 x F 1 )

17. Reciprocal cross P generation parent 1 x parent 2 Parental phenotype Female white - eyed Male red - eyed Parental genotype X w X w X w+ Y Gametes all X w ½ X w+ , ½ Y F 1 generation F 1 genotype ½ X w+ X w , ½ X w Y F 1 phenotype ½ female red - eyed ½ male white - eyed In terbreeding F 1 generation parent 1 x parent 2 F 1 phenotype Female red - eyed Male white - eyed F 1 genotype X w+ X w X w Y F 1 Gametes ½ X w+ , ½ X w ½ X w , ½ Y

18. (F 1 x F 1 )

20. Sex-linked inheritance in chickens

21. (F1 x F1)

26. Primary nondisjunction

27. Secondary nondisjunction