Dominance of Alleles, Sex Determination and Sex-Linked Alleles

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Dominance of Alleles, Sex Determination and Sex-Linked Alleles

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.


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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.


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.

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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.

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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. Copyright S-cool

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).

XHXH = normal female

XHXh = carrier female

XHY = normal male

XhY = male sufferer

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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.


XBXB = black female

XBY = black male

XOXO = orange female

XOY = orange male

XBXO = tortoiseshell female

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All females are tortoiseshell.

All males are black.

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Mother can provide XB or XO

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

Since 1 offspring is XO XO, the father must provide 1 of these allele.

Therefore, the father's genotype is XOY

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