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Genetically, what determines whether a fetus
is a boy or girl?
Who determines gender, Mom or Dad?
Explain.
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Explain how gender is determined
Explain why sex-linked traits are more
common in males than in females
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Autosome
Sex-linked chromosome
Locus/Loci
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Humans have 23 pairs of chromosomes
1-22 chromosome pairs
◦ are autosomal chromosomes (autosomes)
◦ chromosomes not related to gender
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23rd chromosome pair are the sex chromosomes
◦ XX = female
◦ XY = male
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Most X-linked genes
◦ have no homologous loci (positions) on Y
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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
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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
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Males have only one X chromosome
◦ If they have one disorder allele they have the disorder
◦ more common to have sex-linked disorders
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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?
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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
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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
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Duchenne Muscular Dystrophy
◦ disease that results in the progressive weakening of
skeletal muscle
◦ X-linked recessive disorder
50 Word Summary
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In 50 words or less,
summarize the key concepts
from today's lesson.
PROPERTY OF PIMA COUNTY JTED, 2010
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