For any birth we have only two possible outcomes either boy or girl. So the chance of getting boy or girl on birth same which is 1/2.
The probability of getting baby boy or baby girl on any birth is
P(Boy) = P(Girl) = 0.5
If a couple has six children then probability that all are boys is
P( all six are boy) = (0.5)^6
= 0.015
The probability that all six are boy is 0.015