Transcript ENVI 30 Environmental Issues

I.
Human Genetic Disorders
A.
Autosomal Recessive
3.
Sickle-cell disease
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Single amino acid substitution (valine for glutamate) in
hemoglobin
Causes some erythrocytes to form sickle shape
Abnormal erythrocytes slow blood flow and may block
capillaries
I.
Human Genetic Disorders
A.
Autosomal Recessive
3.
Sickle-cell disease
•
•
•
•
•
Single amino acid substitution (valine for glutamate) in
hemoglobin
Causes some erythrocytes to form sickle shape
Abnormal erythrocytes slow blood flow and may block
capillaries
Most common in people of African descent (1 in 10
African Americans is heterozygous – “sickle cell trait”)
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Why so common?
May be advantageous in areas where malaria is a
problem
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Heterozygous people more resistant to malaria
than homozygous dominant people
Fig. 23.17
Malaria
Sickle Cell Allele Frequency
http://upload.wikimedia.org/wikipedia/commons/1/10/Malaria_versus_sickle-cell_trait_distributions.png
http://www.cdc.gov/malaria/about/biology/sickle_cell.html
I.
Human Genetic Disorders
A.
Autosomal Recessive
4.
Tay-Sachs disease
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Absence in brain of enzyme that helps to break down
membrane lipids and prevents their accumulation
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Accumulation causes brain damage
Especially common in people of Ashkenazi Jewish
(Eastern European) descent
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Possibly due to population bottleneck during
persecution & restriction to ghettos in Middle Ages
I.
Human Genetic Disorders
B.
Autosomal Dominant
1.
Achondroplasia
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Abnormal gene on chromosome 4  skeletal growth
disorder  dwarfism (relatively normal torso, short
arms and legs)
Most common growth-related disorder
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Results from inheritance in <20% of cases
Huntington’s Disease
2.
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Defective allele  proteins with long glutamine strands
Affects nervous system  severe mental and physical
deterioration  death
Typically appears later in life – Almost always before
age 50 but almost never before age 20
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Usually after reproductive age
II.
Chromosomal Theory of Inheritance
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Proposed in early 1900s
Unified understanding of mitosis and
meiosis with Mendel’s work on
inheritance
Fig. 15.2
III. Linkage and Recombination
A.
Linkage
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Alleles don’t always assort independently
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Linkage first studied in Drosophila by
Thomas Morgan (early 1900s)
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Two genes on same homologous chromosome
Worked with wild type and mutant fruit flies
Studied inheritance with two-point test cross
between heterozygous individual and
homozygous recessive individual
Fig. 15.9
III. Linkage and Recombination
B.
Recombination
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Occurs during crossing over in meiosis
Drosophila example
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F1 parent produced some recombinant gametes
Fig.
15.10
III. Linkage and Recombination
B.
Recombination
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Greater distance between genes 
Greater probability of recombination
Distance between two genes expressed in
map units
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1 map unit = 1% recombination frequency
Fig. 15.11