Download Notes - Sex-Linked Inheritance

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NOTES: SEX-LINKED DISORDERS
Review of Chromosomes
 First 22 pairs of chromosomes are
called autosomes
 The 23rd pair of chromosomes are
called sex chromosomes.
o Males are XY
o Females are XX
 Controlled by genes located on sex
chromosomes, usually on the X.
 Males show the trait with only one
copy of the allele. (one X)
 Females must have two copies of
the allele to show the trait.(two X’s)
 The trait always shows up when it
is present in males.
Human Sex-Linked Genetic Disorders
Colorblindness
• Colorblindness is an example of a sex-linked incomplete dominance disorder in humans.
• It is a recessive trait.
• The colorblind trait is carried on the X chromosome.
• Colorblindness in Females
• XC= normal
• Xc=defective
• Normal Female = XC XC
• Colorblind Female = Xc Xc
• Carrier Female = XC Xc
• Female must have two genes for the trait in order to be colorblind.
• Colorblindness in Males
• Normal Male = Xc Y
• Colorblind Male = Xc Y
Some Types of Colorblindness
• Achromatopsia – complete color blindness – same as Monochromacy
• Monochromacy - complete inability to distinguish any colors and perceive only variations
in brightness.
• Dichromacy – These individuals normally know they have a color vision problem and it
can affect their lives on a daily basis.
– Protanopia - unable to distinguish between colors in the green-yellow-red section
of the spectrum
– Deuteranopia - unable to distinguish between colors in the green-yellow-red
section of the spectrum
– Tritanopia - unable to distinguish between the colors in the blue-yellow section of
the spectrum. This form of color blindness is not sex-linked.
Trichomacy – usually do not know they have a problem unless they have to take a colorblindness test.
– Protanomaly – (Red- Weak) are less able to discriminate colors, and they do not
see mixed lights as having the same colors as normal observers.
– Deteranomaly – (Green-Weak) reduction in sensitivity to the green area of the
spectrum
– Tritanomaly – see a shift towards the green area of the spectrum. It is the rarest
form of colorblindness. gene for this is carried on chromosome 7
What causes color blindness?
• Color blindness is usually passed on at birth, but can come about through illnesses or
accidents such as:
– retinal damage from accidents / infections / etc.
– Retinal damage due to UV light (from lack of protection - usually during childhood)
• This is the leading cause of blindness worldwide
– brain damage
Other Sex-Linked Disorders
• Hemophilia: Individuals with the recessive allele on X chromosome are unable to clot
blood normally. Affects roughly 1 male in 10,000
• Duchenne Muscular Dystrophy: The recessive allele produces a defective protein that
causes the muscles to weaken and break down. Affects close to 1 in 3000 males in the
US.
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