MODERN GENETICS IV

 

Multiple alleles

 

   In a population

                    there may be more than two varieties of a gene

                   an individual may possess no more than two alleles

 

 This is known as multiple alleles; i.e. one gene, two or more alleles

 

 Eye colour seems to fit here, but eye colour in its entirety is probably

determined by more than one gene - though you'd never guess that from textbooks!

 

   A better example is ABO blood type groups in humans

    There are three alleles governing carbohydrates found on the surface of red blood cells:

   I^A, I^B, and I  

   Don't confuse the alleles with the phenotypes. 

  both I ^A and I ^B areominant over i

  I ^A and I ^B are codominant when together

With three alleles and codominance thrown in,

there are four phenotypes and, therefore, four blood groups.

 

   I ^A I ^A     and      I ^Ai  have   blood type A. 

 

   I ^B  I ^B      and      I ^Bi  have blood type B.

 

   I ^AI ^B     has blood type AB and is recognized as the universal recipient.

 

   ii    has blood type O  and is recognized  as the universal donor

 

   NOTE!!  Blood type O is a known genotype.

 A person with type O blood receives one I  allele from each parent.

 

   Another blood type group shows codominance but does not have multiple alleles.  The M,

   N, MN groups also have to do with specific molecules (glycoproteins) on the RBC  surfaces.

 

Important points about dominant/recessive traits

 

    1. They may range from complete dominance, through varying degrees of incomplete dominance,

    to codominance. They may be sex-linked or under control of another genes. They may show

    pleiotropic effects or show varying degrees of penetrance.

 

    2. They reflect genetic mechanisms of expression of proteins, NOT the ability of one allele to

    mask the other at the DNA level.

 

    3. They do not determine the relative frequency of alleles in a population. (Anyone with six

    fingers?)

 

 

X-inactivation

 

    Female mammals possess two X chromosomes in each nucleated cell, but one of those Xs (the

    paternal or maternal) becomes inactivated in the early embryo. These are seen as Barr bodies (a

    London connection). The inactivation is entirely random and independent of other embryonic

    cells. After inactivation, all mitotic descendents will have the same inactive X. Calico cats are

    female, and a woman who is heterozygous for an X-linked recessive disorder may have parts of

    her body expressing the normal X product and other parts expressing the mutant X.

 

    Take home: Females are mosaics!