XY Sex Determination System — NEET Biology
📚 Practice ConceptThese observations led to the investigation of a number of species to understand the mechanism of sex determination. In a number of other insects and mammals including man, XY type of sex determination is seen where both male and female have same number of chromosomes. Among the males an X-chromosome is present but its counter part is distinctly smaller and called the Y-chromosome. Females, however, have a pair of X-chromosomes. Both males and females bear the same number of autosomes. Hence, the males have autosomes plus XY, while female have autosomes plus XX. In human beings and in Drosophila the males have one X and one Y chromosome, whereas females have a pair of X-chromosomes besides autosomes.
Which of the following statements regarding the sex determination and outcome of the XX genotype are NOT correct? S1: A zygote with XX chromosomes in humans will develop into a female. S2: In Drosophila, an individual with XX chromosomes will develop as a female. S3: In birds, an XX chromosomal complement (analogous to ZZ) would correspond to a male phenotype. S4: The XX genotype universally determines the female sex across all animal species with chromosomal sex determination. S5: In grasshoppers, individuals with an XX genotype are females.
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Correct answer: C — Only S4 is not correct.
S1 is correct. In humans (XY system), XX individuals develop as females. S2 is correct. In Drosophila (XY system), XX individuals develop as females. S3 is correct. In birds (ZW system), males have ZZ chromosomes. If we consider XX as analogous to ZZ in this context, it would correspond to a male phenotype. S4 is not correct. The XX genotype does not universally determine the female sex across all animal species. For example, in birds, females are ZW and males are ZZ. An XX genotype as a literal chromosomal pair is not part of their standard sex determination, and even if it were, the system is different. Therefore, stating it 'universally determines' is false. S5 is correct. In grasshoppers (XO system), females have XX chromosomes. Therefore, only statement S4 is NOT correct.
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