This is when neither allele is dominant.

Both alleles are expressed and contribute equally to the phenotype.

A heterozygote has an intermediate phenotype as there is partial influence from both alleles.

Example:.

Snapdragons can be red (alleles = RR), white (alleles = WW) or pink (alleles = RW).

In this case, both alleles are dominant.

They are independent, so there is no 'blending' as in the snapdragons, instead the phenotype is a result of the full expression of both alleles.

Example:.

Blood group AB.

Gender is determined by sex chromosome s in many animals. The 3 most common systems are:

1. The XY System (e.g, in humans, Drosophila)

Female are XX, males are XY

Both sexes have 2 chromosomes but the females' chromosomes are the same, the males are different.

2.The XO System (e.g, grasshoppers, bugs)

Females are XX, males are XO

The male has only 1 sex chromosome whereas the female has 2.

3. The WZ System (e.g, birds, butterflies, some fish)

Females are ZW, males are ZZ

Both sexes have 2 chromosomes but the females' chromosomes are different, the males are the same.

Some genes are part of the sex chromosomes and so are inherited with them. Usually it is the X chromosome that is considered in which case the female will have two alleles, the male will only have one.

The genetic condition of haemophilia is carried on the X chromosome.

The normal allele is dominant (H), the allele for haemophilia is recessive (h).

X^HX^H = normal female

X^HX^h = carrier female

X^HY = normal male

X^hY = male sufferer

The ratio of males to females = 1 : 1

Of the males, there is a ratio of 1 : 1 normal : sufferer

Therefore there is a 25% probability that any offspring will be a sufferer. There is a 50% probability that a boy is affected.

In cats (which are also XX if they are female and XY if they are male) the allele for coat colour is carried on the X chromosome.

The alleles are black and orange but they are codominant.

Example.

X^BX^B = black female

X^BY = black male

X^OX^O = orange female

X^OY = orange male

X^BX^O = tortoiseshell female

All females are tortoiseshell.

All males are black.

Mother can provide X^B or X^O

Father can provide Y (accounting for orange & black males) plus 1 other allele.

Since 1 offspring is X^O X^O, the father must provide 1 of these allele.

Therefore, the father's genotype is X^OY