The sex of an infant is determined by the type of sex chromoscme contributed by the male gamete. A maile produces two types of sperms-one type bears 22+X composition and the Other, 22+ Y Therefore, a male have 50% sperms with X-chromosomes and other 50% with Y-chromosome.
Any one of the two types of sperms can fertilise the egg I a Y-bearing sperm fertilises the egg. the zygote will be XY (male) and when an X-bearing sperm fertilises the egg, the resulting zygote will be XX (fermale). Since the ratio of X-chromosome and Y -chromosomes in male gamete is 50: 50 The statistical probability of male or a female infant is also 50:50.
