Why must a gamete have only one half the number of chromosomes found in body cells?

We are diploid organisms thus require two sets of chromosomes. In order to ensure the zygote is a diploid (has 2 sets of chromosomes) the gametes must be haploids (has 1 set of chromosomes) as two gametes join to make a zygote. 

Having half the set of chromosomes also ensures that the offspring receives half of its chromosomes from its father and half from its mother. This becomes very important as to ensure there is genetic diversity in the offspring, both the fathers and mothers genes are needed.  

For cyclohexane compounds drawn in a chair conformation:   -axial bonds are vertical  -equatorial bonds are parallel to two bonds within the ring.  thus, as i assume the chair conformation is drawn:   -the x group is in an axial position, as are the five other vertical bonds.  -three axial bonds are on the top face of the ring  -the other three axial bonds are on the bottom face  the position of the second substituent on each ring carbon is equatorial.  gauche groups must be on an adjacent carbon such as carbon 6 and carbon 2, and also gauche groups are rotated by 60 degrees relative to each other.  drawing the newman projections for c1–c2 and c1–c6 aids in visualizing the groups that are gauche to the y group on c1.  click this link and watch some academic videos that may you more understand the topic