I. Introduction
 

Mendelian inheritance has its physical basis in the behavior of chromosomes.
 

Thomas Hunt Morgan’s experiment on Drosophila melanogaster, fruit fly.
Provided evidence that chromosomes are indeed the location of Mendel’s heritable factor.

To be more specific, fruit fly’s eye color character is linked to its sex chromosome.

Wild type phenotype:  Normal phenotype for a character.

Mutant phenotype:  Traits that are alternatives to the wild type

Sex-linked genes:  Genes located on a sex-chromosome.

Linked genes:  Genes located on the same chromosome tend to be inherited together in genetic
                          crosses, because the chromosome is passed along as a unit.
 
 

A. Sex Chromosomes Versus Autosomes
 

Sex Chromosomes:  XX,  XY.

Autosomes:
 

1. Sex-linked genes have unique patterns of inheritance.
 

In humans, the term sex-linked usually refers to X-linked characters.

Fathers pass X-linked alleles to all their daughters, but not sons.

Mothers can pass sex-linked alleles to both sons and daughters.

Far more males than females have disorders that are inherited as sex-linked recessives.

Female needs to have double dose of recessive, while male only needs a single dose of recessive allele .
 

2. Sex-linked disorders in humans.  X-linked disorders
Color blind:  a mild disorder.
 

Hemophilia:  sex-linked recessive trait, absence of a certain protein, required  for blood clotting, a serious
                    disorder:

X-linked recessive trait got transmitted from Queen Victoria (X+X-):
 
 

II. Sex-limited Traits

Traits expressed in only one sex.  They may be controlled by sex-linked or autosomal loci (locations of certain alleles on the chromosome). Although, the genes are present in both sexes.
 

Ex.  Sex-linked traits:

 Woman:  breast and ovary formation.
 Man:  facial hair distribution and sperm production.

Female bird:  Not brightly colored plumage patterns.
Male bird:     Brightly colored plumage patterns.

Sheep:    Horns found only in males.
 

III. Sex-Influenced Traits (Sex-conditioned traits):

Traits that appear more often in one sex than in another.
Traits are not necessarily transmitted by sex chromosome.

Ex.  Baldness in human beings----- sex- influenced trait.
It is controlled by an allele that appears to be dominant in men, but recessive in woman.  Male hormone, testosterone is required for a full expression of the allele.
For woman, it is usually expressed as a thinning of hair rather than a balding.