Sex-Linked Inheritance

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

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In 50 words or less, summarize the key

concepts from today's lesson.

PROPERTY OF PIMA COUNTY JTED, 2010 15

50 Word Summary