Basic Genetics: The sex

Chromosome X, Y

Male

The male has a pair of different chromosomes: XY.

Female

The female has a pair of equal chomosomes: 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 heridity, both the female and the male pass one half of each gene pair to their offsprings. 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 a litter.
Now we split the pair of each parent into its 2 parts:

The male gives either X or Y to his offsprings.
X
Y
The female gives either X or X to her offsprings.
X
X
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).
XX
XY
XX
XY


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 2n combinations possible.


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Last modified on  99-01-16

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