Bio 178 Lecture 25

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Bio 178 Lecture 25
Genetics

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Reading

• Chapter 13

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Quiz Material

• Questions on P 276-278
• Chapter 13 Quizzes on Text Website
  (www.mhhe.com/raven7)

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Outline

• Genetics
• Non-Mendelian Patterns of Inheritance
• Human Genetics

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Non-Mendelian patterns of Inheritance (cntd.)

• Incomplete Dominance

When 2 alleles produce heterozygotes that are
phenotypically different from either parent. The
heterozygotes have a phenotype that is
intermediate between the phenotypes of the
parents.
Example - Japanese four oclocks
Explanation
The unmasked Mendelian genotypic ratio is a
result of the heterozygotes having less red
pigment than the red homozygotes.
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Incomplete Dominance
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Japanese Four Oclocks
http//www.bio.miami.edu/dana/250/mirabilis.jpg
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Incomplete Dominance - Snapdragon
?J. Elson-Riggins
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Non-Mendelian patterns of Inheritance (cntd.)

• Environmental Effects

When the degree of expression of allele is
affected by the environment.
Example - Temperature Sensitive Alleles
The ch allele in Himalayan rabbits and Siamese
cats encodes an enzyme that controls melanin
production.
Effect of Temperature
Temps 33?C - Enzyme inactive (no melanin ?
white)
Temps
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Himalayan Rabbit
http//www.bio.miami.edu/dana/250/himalayanbun.jpg
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Arctic Fox
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Non-Mendelian patterns of Inheritance (cntd.)

• Multiple Alleles and Codominance

Often there are multiple alleles for a trait.
When no single allele is dominant, the alleles
are codominant.
Example - ABO Blood group
The gene I codes for an enzyme that puts sugars
on red blood cells (rbcs).
The sugars are important for self-recognition by
the immune system.
Alleles
I Sugar on (IA ? galactosamine, IB ?
galactose) i Gene non-functional
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ABO Blood Group Genotypes Phenotypes
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ABO Blood Groups (Cntd.)
Immune Response
The immune system will reject rbcs coated with a
sugar it does not recognize as self, as it has
antibodies to non-self.
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ABO Blood Groups (Cntd.)
Immune Response
The immune system will reject rbcs coated with a
sugar it does not recognize as self as it has
antibodies to non-self.
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Codominance Example - Roan Horse
http//www.bio.miami.edu/dana/250/roanhorse.html
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Non-Mendelian patterns of Inheritance (cntd.)

• Multiple Alleles (Cntd.)

Example - Rhesus Blood group
Usually treated as a 2 allele system, but there
are 8 alleles for this trait (Rh factor).
Rh codes for a surface protein on rbcs.
Rh Has Rhesus factor Rh- No Rhesus factor
Rh is dominant to Rh-
In contrast to ABO, Rh- individuals do not have
antibodies to Rh unless they have been previously
exposed.
Eg. Erythroblastosis fetalis (hemolytic disease
of newborns)
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Human Genetics
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Human Genetic Disorders

• Hemophilia
• Sickle Cell Anemia
• Huntingtons Disease

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Genetic Disorders

• Frequency

Although genetic disorders are rare
(a) Their frequencies may be higher in
particular populations.
(b) Natural selection does not entirely remove
them from a population.

• Dominance

Mostly, but not all, are recessive.
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Studying Patterns of Inheritance

• Pedigrees

Family trees documenting the inheritance of a
trait.

• Example - Hemophilia

Clotting disorders caused by a mutation resulting
in a loss of function in any one of the 12
clotting factors (proteins) involved in the
clotting cascade.
Sex-Linked hemophilia
A recessively inherited hemophilia that is
carried on the X chromosome.
? Any male with the recessive allele (XhY) will
have the disease. Any heteozygous female will be
a carrier.
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Royal Hemophilia
The hemophilia mutation occurred either in Queen
Victoria or her parents and therefore entered the
royal line.
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Royal Hemophilia
http//www.people.virginia.edu/rjh9u/roylhema.htm
l
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Single Base Substitution - Sickle Cell Anemia

• The Disease

Defective hemoglobin that is unable to carry O2
properly.
Hemoglobin molecules stick to each other under
low O2 conditions ? sickle cell crisis.
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Sickle Cell Anemia (Cntd.)

• Inheritance

Homozygous recessive.

• Cause

Mutation in 1 base of the gene encoding
?-hemoglobin
Mutation GAG ? GTG AA Glutamic acid
(polar) ? Valine (non-polar)
Results in a sticky patch (hydrophobic
interaction).

• Why does this deleterious allele persist?

Common in blacks.
Confers resistance to falciparum malaria -
heterozygotes have a survival advantage in areas
where Plasmodium falciparum is endemic ? NATURAL
SELECTION.
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Sickle Cell Anemia
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Plasmodium falciparun
http//www.sb-roscoff.fr/CyCell/Page45.htm
?E.G. Platzer