Multiple allele & Example (Albinism)

Multiple allele.

Multiple allele is a situation where by more than two alleles are controlling a certain characteristic. For example alleles A, B and O control the ABO blood group system in man.

Other conditions in Man transmitted in mendellian fashion. (Monohybrid inheritance in man)

1. 1.       Albinism

Is a condition which results when the pigment for normal skin colour fails to form and this due to a recessive gene a.

Characteristics of albinism

White skin, Pink eyes and Golden hair.

To obtain an albino the child must receive recessive genes from both parents. This implies that an albino is homozygous recessive.

Gene for normal skin pigment: A and gene for Albino: a

Homozygous dominant: (AA) normal skin colour

Heterozygous:                  (Aa) Normal skin colour

Homozygous recessive: (aa) Failure of formation of normal skin pigment (albino)

To get a child who is an albino:

I)        Both parents must be carriers

Parents           Mother (carrier)           X                father (carrier)

Genotype                 A a                                             A a

Gametes

Random fertilization

AA      aa                        Aa                          Aa

AA: normal skin colour

Aa: Normal skin colour but carrier

aa: Albino

II)      One parent is a carrier and the other is an albino

Parents           Mother (albino)           X               father (carrier)

Genotype                 a  a                                            A  a

Gametes

Random fertilization

Aa       aa                    Aa                       aa

Aa: normal skin colour but carrier

aa: albino

From the above two marriages, the mode of transmission of genes is similar to the mendellian fashion.