Chromosome X, Y

Male
The male has a pair of different chromosomes: XY. 
Female
The female has a pair of equal chromosomes: XX. 
Possible combinations

male 

X 
Y 
female 
X 
XX
female 
XY
male 
X 
XX
female 
XY
male 

How are these combinations of genes done ?
Let's stick to the above table of possible combinations.
Each gene is represented in a pair. The gene pair of the male was XY, that of the female
XX.
Each parent gives 1 half of its gene pair to its litter, both the female and the male.
Often you may hear that the male is more important at the mating, that's not true when it
comes to heredity, both the female and the male pass one half of each gene pair to their
offspring. But the male may play an important role when you look at a cat population
where the male is used as stud male, he can sire more females than one female may have litter.
Now we split the pair of each parent into its 2 parts:
The male gives either X or Y to his
offspring. 

The female gives either X or X to her
offspring. 

Out of these parts of each parent we build all possible combinations.
That reading is easier, we always put the gene pairs in alphabetical order. 
 X (first half from the male) plus X (first half from the female)
 Y (second half from the male) plus X (first half from the female)
 X (first half from the male) plus X (second half from the female)
 Y (second half from the male) plus X (second half from the female).


If we have more than one gene pair we start with the last pair varying and build all
combinations. Then we vary the second last pair and build all combinations, and so on.
Let's show this on an example: a cat might have Bb Dd.
Split now each pair into its 2 parts:
1st pair into B and b, 2nd pair into D and d.
We
keep the 1st part of the 1st pair fixed and vary the parts of the 2nd pair:
the cat can
pass BD or Bd.
Now we keep the 2nd part of the 1st pair fixed and vary the parts of the 2nd pair:
the cat
can pass bD or bd.
Therefore the cat can pass the following combinations: BD, Bd, bD, bd.
For those who are interested in calculating the amount of possible combinations:
If n is the amount of gene pairs, then there are 2^{n} combinations possible.
