Recall that the sex of a person is controlled by one pair of chromosomes XX in a female and XY in a male.
Describe the determination of sex offspring at fertilisation using a genetic diagram.
In females they carry a pair of chromosomes X,X (at the same length) and in males they carry XY (X is longer than Y). So when they mate - X x Xy they produce two possibilities XX (female) or XY (male) which is at the ratio of 1:1. So it's the male who determines the sex of their offspring.