SEX-LINKED TRAITS  = traits which result from genes located on a sex chromosome

Nearly all sex-linked traits result from _______________________ genes located on the _____ chromosome; thus they are called ________________ _________.

X-linked traits are more common in  ____________, because it takes only ______ gene for a male to be affected, but it takes _______ genes before a female will be affected.

Examples of X-linked traits include:

• _____________________________________- caused by defective cones in the retina of the eye (approx. 8 % of all men). Your textbook’s description of how this trait is inherited (pg. 118) is wrong – ignore it.

• _____________________________________ - poor vision in dim light that is not due to vitamin deficiency

• ________________________ (bleeder’s disease) - defective blood clotting mechanism results in prolonged bleeding

• _____________________________________ – gradual wasting away of the skeletal muscles (1/7000 male births). Affected individuals rarely live longer than 20-30 years.
• _____________________________________ - the second leading cause of mental retardation, characterized by an abnormally long, fragile arm on the X chromosome (1/2500 births worldwide).
• _____________________________________ - body lacks appropriate androgen receptors, so doesn’t recognize testosterone. One cause of male pseudohermaphroditism. (1/20,000 births).

• _____________________________________- a severe immune system deficiency prevents manufacture of antibodies. Individual must be confined to a protective, sterile environment or may die from a simple bacterial infection.

Inheritance of X-linked traits: example of colorblindness

1. Assign one letter per gene:
Dominant gene (normal vision)   =  _____
Recessive gene (colorblind)         =  _____

2. The phenotypic expression of a trait (it’s physical manifestation or appearance) depends on the underlying genotype (combination of inherited genes):

3. Perform _________________________________ analysis to determine all of the potential phenotypes (and genotypes) of offspring

Sample crosses (See Figure 1):

#1: normal male (X^NY ) + colorblind female (XⁿXⁿ⁾ =

#2: normal female (X^NX^N) + colorblind male (XⁿY) =

Try these two examples at home:

#3: Normal male + carrier female =

#4: Colorblind male + carrier female =

X-influenced traits:

• Differ from X-linked traits because the action of the genes is strongly influenced by _____________________.
• An example is __________________. Males with only one baldness gene (X^BY) will eventually go bald as they age due to the synergistic effect caused by high levels of ________________. Females with one baldness gene (X^BX^b) will not go bald because they have less testosterone circulating in their blood. Aging-related baldness in females requires two baldness genes (X^BX^B). However, even in this case, the degree of baldness is much less severe than in men due to the low levels of circulating testosterone.

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