In humans there are 23 pairs of chromosomes one of each being donated by each parent. The 'arms' or chromatids are joined at the centromere. The position of the centromere and length of the chromatids are used in conjunction with banding techniques to identify individual and pairs of chromosomes. In mammals the gender of an individual is determined by the combination of the sex chromosomes.
Females can only donate an X-chromosome.
If the male also donates an X-chromosome the offspring will be female. In mammals the sex chromosomes are very different sizes.