Human Genetics- Sex Linked traits



Genetically, what determines whether a fetus
is a boy or girl?
Who determines gender, Mom or Dad?
Explain.


Explain how gender is determined
Explain why sex-linked traits are more
common in males than in females



Autosome
Sex-linked chromosome
Locus/Loci


Humans have 23 pairs of chromosomes
1-22 chromosome pairs
◦ are autosomal chromosomes (autosomes)
◦ chromosomes not related to gender

23rd chromosome pair are the sex chromosomes
◦ XX = female
◦ XY = male

Most X-linked genes
◦ have no homologous loci (positions) on Y

Very few genes found on the Y chromosome
◦ most genes on the Y chromosome associated with
maleness (e.g. testes determining factor)

More than 100 have now been mapped
 Father determines gender
 Donates Y chromosome to sons
 Donates X chromosome to daughters
 Fathers cannot pass sex-linked traits to son
because
 He doesn’t give them an X chromosome
 Males receive X chromosome from mother
 So the mother passes sex-linked traits to son

Pass sex-linked alleles (X) to both daughters
and sons

Females have fewer sex-linked disorders
because
◦ It only shows up if homozygous recessive
◦ Females have two X chromosomes
◦ more often carriers of recessive allele

Males have only one X chromosome
◦ If they have one disorder allele they have the disorder
◦ more common to have sex-linked disorders





Red-green colorblindness is a sex-linked
trait.
The dominant allele, C, produces normal
color vision
The recessive allele, c, produces red-green
color blindness.’
If a man with normal color vision marries a
color-blind woman, what is the chance of
their having a color-blind son?
A color-blind daughter?

Color blindness
◦ most commonly red-green color blindness
◦ X-linked recessive disorder
◦ Individuals with this color blindness see fewer than
25 colors
◦ Results from some or all of the receptors that
respond to red and green being weakened or
absent

Hemophilia A
◦ Causes lack of blood clotting
◦ X-linked recessive disorder
◦ Symptoms
 tend to bruise easily
 internal bleeding can cause problems with muscles
and joints

Duchenne Muscular Dystrophy
◦ disease that results in the progressive weakening of
skeletal muscle
◦ X-linked recessive disorder
50 Word Summary
21
48
37
13
14
18
23
In 50 words or less,
summarize the key concepts
from today's lesson.
PROPERTY OF PIMA COUNTY JTED, 2010
15