Biology 101: Sex-Linked Genes Handout

 

Sex-linked genes behave differently than ordinary genes.  In their case the chromosome is also important.  There are 46 chromosomes in humans, divided into 23 pairs.  The first 22 pairs are called autosomes.  They regulate everything for being human.  The last pair of chromosomes is called the Sex chromosomes.  They are responsible for the genes that determine gender (maleness or femaleness).  Any gene on a sex chromosome is referred to as sex-linked.

 

In humans, the two sex chromosomes are the X and Y.  In humans, the Y chromosome determines a male and is dominant; the X chromosome determines a female and is recessive.  So if an embryo receives a Y chromosome it is a male.  The Y chromosome does not have any known genes on it that DO NOT determine male sex.  However, only 20% of the X chromosome is sex-determining genes.  What about the rest?  The rest of the X chromosome is filled with genes unrelated to sex determination and are often deleterious.  Because of this most sex-linked genes are also referred to as X-linked.

 

The genotype XY is male.  The genotype XX is female.  (Although the Y chromosome is dominant, the female chromosome is always written first.)

 

Parents have a 50/50 chance of having either a boy or a girl.

 

The X chromosome is much larger physically than the Y (all those extra genes).

 

When performing a cross concerning sex-linked traits, symbols for the genes are not used in the Punnett Square.  Instead the sex chromosomes themselves are used, with symbols for the dominant and recessive alleles written as superscripts on the chromosomes.  Ex. colorblindness:

 

 

XC

Xc

Xc

 

 

Y

 

 

                                                                                      C = normal vision

                                                                                      c = colorblind

 

 

 

 

There are no male carriers.  For sex-linked disorders, typically 2/3 or more of all cases will be male.