Q15 of 26 Page 1

Explain sex – determination in humans.

Sex-determination is the mechanism which determines the individual to be Male or Female based on the sex chromosome present.

Humans have 23pairs of chromosomes, of which one pair is sex chromosomes. Females have a perfect pair chromosomes, XX whereas male have an imperfect pair of sex chromosome XY.


Male produces 2 different types of gametes X and Y, while female produces only one type of gamete containing X chromosome.


The sex of the baby is determined by the type of the male gamete that fuses with the female gamete.


If the fertilizing sperm contains X chromosome, the baby produced will be girl and if the fertilizing sperm contains Y chromosome then the baby produced in boy.


Hence males are responsible for the gender of the newly formed babies.


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(i) How many true – breeding lines did Mendel select in pea plants for his experiments?

(ii) What is a true – breeding line?


(iii) Give two examples of contrasting traits studies by Mendel.


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