Transcript Biol 178 Lecture 25

Bio 178 Lecture 25
Genetics
Reading
•
Chapter 13
Quiz Material
•
Questions on P 276-278
•
Chapter 13 Quizzes on Text Website
(www.mhhe.com/raven7)
Outline
• Genetics
 Non-Mendelian Patterns of Inheritance
 Human Genetics
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 o’clocks
Explanation:
The unmasked Mendelian genotypic ratio is a result of the
heterozygotes having less red pigment than the red
homozygotes.
Incomplete Dominance
Japanese Four O’clocks
http://www.bio.miami.edu/dana/250/mirabilis.jpg
Incomplete Dominance - Snapdragon
J. Elson-Riggins
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 < 33C - Enzyme active (melanin  brown)
Himalayan Rabbit
http://www.bio.miami.edu/dana/250/himalayanbun.jpg
Arctic Fox
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
ABO Blood Group Genotypes & Phenotypes
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.
Recipient/
Donor
A
B
AB
O
A
B
AB
O
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.
Recipient/
Donor
A
B
AB
O
A
+
-
-
+
B
-
+
-
+
AB
+
+
+
+
O
-
-
-
+
Codominance Example - Roan Horse
http://www.bio.miami.edu/dana/250/roanhorse.html
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)
Human Genetic Disorders
• Hemophilia
• Sickle Cell Anemia
• Huntington’s Disease
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.
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.
Royal Hemophilia
The hemophilia mutation occurred either in Queen
Victoria or her parents and therefore entered the royal
line.
Royal Hemophilia
http://www.people.virginia.edu/~rjh9u/roylhema.html
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.
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.
Sickle Cell Anemia
Plasmodium falciparun
http://www.sb-roscoff.fr/CyCell/Page45.htm
E.G. Platzer