Sex-linked inheritance
Most sex-linked traits are considered to be recessive; however, this adequately describes conditions in females, but not males. A woman who inherits only one copy of a defective gene on an X chromosome will not be affected; but a man inheriting a defective copy has no counterpart on his Y chromosome to mask the effects. In fact, the term for this condition is "hemizygous" - only one copy of the gene is present in a diploid organism.
We can establish a Magic Six for sex-linked recessive disorders using the following legend
X = normal Xa = affected allele Y = normal
There are three genotypes possible for the female: normal, carrier, and affected
There are two genotypes for the male: normal and affected.
Some common sex-linked disorders are colour-blindness, hemophilia, and Duchenne muscular dystrophy.
Try these.
Multiple alleles
In a population, there may be more than two varities of a genes, although an individual can 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. In the population, there are three major alleles governing carbohydrates found on the surface of red blood cells: IA, IB, and i. Don't confuse the alleles with the phenotypes. While both IA and IB are dominant over i, IA and IB are codominant when together - the RBC surfaces show both the A and the B substance. The recessive condition of ii shows neither substance on the RBC surface. With three alleles and codominance thrown in, there are four phenotypes and, therefore, four blood groups.
IA IA and IA i have blood type A.
IB IB and IB i have blood type B.
IA IB 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.
Pleiotropy
Pleiotropic effects describe the multiple effects caused by a single allelic disorder. Think of pleiotropy as having a single cause but showing multiple symptoms. Sickle-cell anemia is a good example of a pleiotropic gene.
Genetic imprinting
Who said a mother's allele was equivalent to a father's allele? For many years, this was the standard belief, now it is regarded as a major obstacle to cloning. Imprinting means that the mother and father pass on a "signature" with their alleles that designates the allele as maternal or paternal in origin.
A dramatic example of this is the first discovery of this phenomenon. Praer-Willi syndrome is characterized by mental retardation, obesity, short stature, and unusually small hands and feet.
Angelmann syndrome is characterized by spontaneous (uncontrollable laughter), jerky movements, and other motor and mental defects. Both are caused by the same DNA deletion on chromosome 15. But if the deletion is on the paternal chromosome, the child will have Prader-Willi syndrome; if the deletion is on the maternal chromosome, the child will have Angelmann syndrome.