Dominance of Alleles, Sex Determination and Sex-Linked Alleles
Incomplete dominance 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). Codominance 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.
Sex determination 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. Sex-linked genes 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 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. XBXB = black female XBY = black male XOXO = orange female XOY = orange male XBXO = tortoiseshell female All females are tortoiseshell. All males are black.
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 /**/