Transcript 14-2 Human Chromosomes

The Human Genome
Chromosomes & Phenotype
Genetic Disorders
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Autosomal vs Sex Linked Traits
• Autosomes
– All the chromosomes that do NOT play a role in
sex determination
• In humans, chromosomes 1 – 22
– Two copies
» One from Mom
» One from Dad
» Same genes
» Same location on chromosome
» BUT may be different alleles (recipes)
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Autosomal vs Sex Linked Traits
• Sex Linked Genes
– Genes on the X or Y chromosome
– X Chromosome
• Large
• Contains many genes, some essential for survival
– Y Chromosome
• Small
• Few genes except those necessary to change fetal
development from female to male
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Autosomal Disorders caused by
Recessive Alleles
• Recessive Alleles cause disorder
– Heterozygotes are Carriers
• Can pass damaged allele to offspring BUT don’t have
the disease/disorder
– Homozygous Recessive individuals have the
disease/disorder
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Autosomal Disorders caused by
Recessive Alleles
Cystic Fibrosis
• Mutation of CFTR gene
on chromosome 7
• Causes thick mucus in
lungs & blockage of
gastrointestinal tract
• Damages the pancreas
• Fatal
• Both parents must be
heterozygous to have a
child with the disease
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Autosomal Disorders caused by
Dominant Alleles
Dominant allele disorders far less common
Huntington’s Disease
• Mutation of HTT gene on chromosome 4
• Appears during adulthood – usually 35-44 yrs. old
– Allow time for person with disease to reproduce
• Nervous system disintegrates slowly over years
• Fatal
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Autosomal Disorders caused by
Dominant Alleles
Neurofibromatosis
Autosomal Dominant
Disorder
Chromosome 22
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Sex-Linked Genes
Sex Linked Genes
Are Genes Located
On The X & Y Chromosome
X – Contains > 100 Genes
Y – Contains Only A Few Genes
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Sex-Linked Genes
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Colorblindness
The X Chromosome
Contains Three (3) Genes
Associated With Color Vision
The Y Chromosome Is Missing Them All
Therefore, Males Are Particularly
Vulnerable To Colorblindness
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Colorblindness
The Images Below are NOT Diagnostic
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Colorblindness
Red-Green Colorblindness
Most Common Form
1:10 Males
1:100 In Females
Why The Difference?
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Colorblindness
Colorblind Males Only Pass
The Recessive Allele On To Their Daughters
It is not on the Y Chromosome They Pass To
Their Sons.
(only daughters will receive the fathers X Chromosome)
Thus - The Gene Appears To Skip A Generation And
Will Only Reappear In His Grandchildren
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Hemophilia
Two Important Blood Clotting Genes Are On
The X Chromosome
Occurrence Rate = 1:10,000 Males
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Hemophilia
Mild
Post-Injection
Bleeding
In A
Patient
With
Hemophilia
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Duchenne Muscular Dystrophy
Sex-Linked
Abnormal Muscle Protein
Patients Rarely Live Beyond
Early Adulthood
Occurrence Rate =
1:3000 Males
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In Mammals
Female X-Chromosome Inactivation
Males
NO
Only Have (1) X Chromosome
Females
Have Two (2) X Chromosomes
Do Female Express
Both X Chromosomes?
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In Mammals
Female X-Chromosome Inactivation
One X Chromosome Is Randomly Switched Off
– That Chromosome Forms A Dense Region In The
Nucleus Known As A Barr Body
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In Mammals
Female X-Chromosome Inactivation
Arrows Indicate Barr Bodies
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In Mammals
Female X-Chromosome Inactivation
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Chromosome Disorders
• Most Common Is Nondisjunction
During Meiosis, Homologous Chromosomes
Fail To Separate
One Gamete Will Have An Extra
Chromosome (Trisomy)
The Other Will Have One Less
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Down Syndrome
• Nondisjunction of Chromosome 21 (trisomy)
• Occurs 1:800 Births
• Mental Retardation May Be Mild To Severe
• Increased Susceptibility To Disease
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Down Syndrome
Why Would 3 Copies Cause Problems
When 2 Copies Don’t?
Don’t Know Yet
Working Hard To Find Out
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Sex Chromosome Disorders
• Turner’s Syndrome
– Females
– Nondisjunction
Results In A
XO Female
– Sterile
– Sex Organs Do Not
Develop During
Puberty
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Sex Chromosome Disorders
• Klinefelter’s Syndrome
– Males
– Nondisjunction Results In A XXY Male
– The Extra X Interferes With Meiosis – Generally
Sterile
– Also Found XXXY & XXXXY
– Exhibit Female Body Patterns In Hips, Partial
Breast Development, Female Pubic Hair Pattern
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Sex Chromosome Disorders
No Babies Are Born Without At Least
One X Chromosome
Indicates The X Chromosome Carries
At Least One Gene Required For Life
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Y Chromosome
• If Present, Individual Is Male
– Even When Multiple X
Chromosomes Are Present
– Contains At Least One Gene
Required For Male Sexual
Development
No “Y” Present –
Embryo Develops As A Female
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