Example 1

As first example we take the spangle gene Sp. We know there exist two different alleles (forms) of this gene:

the not mutated or wild type allele: Sp⁺
the mutated allele: Sp

For simplicity we only focus on spangle gene and the chromosome of the spangle gene.

Since genes come in pairs there are three possible combinations:

Sp⁺ ---: normal Sp⁺

Sp⁺ Sp --- = ---: single factor spangle Sp Sp⁺

Sp ---: double factor spangle Sp

We have two homozygous genotypes (normal and double factor spangle) and one heterozygous genotype (single factor spangle).

Pairing 1.1

We immediately start with the most complicated pairing:

Pairing 1.1: single factor spangle cock x single factor spangle hen

Genetic formula of the cock:
Sp⁺/Sp
So, keeping in mind meiosis, the possible gametes are:
Sp⁺ and Sp
Genetic formula of the hen:
Sp⁺/Sp
So, the possible gametes are:
Sp⁺ and Sp
Punnet square:

Sp⁺ Sp

Sp⁺ Sp⁺/Sp⁺ Sp/Sp⁺

Sp Sp⁺/Sp Sp/Sp

The solution: there are 3 different possibilities for the offspring:

`Sp⁺ ---: normal Sp⁺`	(1/4 = 25%)
`Sp⁺ Sp --- = ---: single factor spangle Sp Sp⁺`	(2/4 = 50%)
`Sp ---: double factor spangle Sp`	(1/4 = 25%)

If you understand pairing 1.1, then the next two pairings should be no problem. If you've not yet understood it, hopefully these next pairings will help.

Pairing 1.2: normal cock x single factor spangle hen

Genetic formula of the cock:
Sp⁺/Sp⁺
So, the only possible gamete is:
Sp⁺

Genetic formula of the hen:
Sp⁺/Sp
So, the possible gametes are:
Sp⁺ and Sp

Punnet square:

Sp⁺

Sp⁺ Sp⁺/Sp⁺

Sp Sp⁺/Sp

The solution: there are 2 different possibilities for the offspring:

Sp⁺ ---: normal Sp⁺ (1/2 = 50%)

Sp⁺ Sp --- = ---: single factor spangle Sp Sp⁺ (1/2 = 50%)

The pairing single factor spangle cock x normal hen gives the same result
(you can try this yourself).

Pairing 1.3

Pairing 1.3: double factor spangle cock x single factor spangle hen

Genetic formula of the cock:
Sp/Sp
So, the only possible gamete is:
Sp

Genetic formula of the hen:
Sp⁺/Sp
So, the possible gametes are:
Sp⁺ and Sp

Punnet square:

Sp

Sp⁺ Sp⁺/Sp

Sp Sp/Sp

The solution: there are 2 possibilities for the offspring:

Sp⁺ Sp --- = ---: single factor spangle Sp Sp⁺ (1/2 = 50%)

Sp ---: double factor spangle Sp (1/2 = 50%)

The pairing single factor spangle cock x double factor hen gives the same result
(you can try this yourself).

When the parent is homozygous, only one gamete is possible. A pairing between two homozygous parents is the simplest to calculate, since only one combination of gametes is possible. When these homozygous parents have the same alleles, the offspring will have the same genotype as the parents:
normal x normal gives only normal
double factor spangle x double factor spangle gives only double factor spangle

Pairing 1.4

Apart from these two trivial pairings, the other pairing between homozygous is:
Pairing 1.4: normal cock x double factor spangle hen

Genetic formula of the cock:
Sp⁺/Sp⁺
So, the only possible gamete is:
Sp⁺

Genetic formula of the hen:
Sp/Sp
So, the only possible gamete is:
Sp

Punnet square:

Sp⁺

Sp Sp⁺/Sp

The solution: there is only 1 possibility for the offspring:

Sp⁺ Sp --- = ---: single factor spangle Sp Sp⁺ (1/1 = 100%)

The pairing double factor spangle cock x normal hen gives the same result
(you can try this yourself).