bio 178 lecture 25 genetics. reading chapter 13 quiz material questions on p 276-278 chapter 13...
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TRANSCRIPT
Reading
• Chapter 13
Quiz Material
• Questions on P 276-278
• Chapter 13 Quizzes on Text Website (www.mhhe.com/raven7)
Non-Mendelian patterns of Inheritance (cntd.)• Incomplete DominanceWhen 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.
Non-Mendelian patterns of Inheritance (cntd.)• Environmental EffectsWhen 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 >33C - Enzyme inactive (no melanin white)
Temps < 33C - Enzyme active (melanin brown)
Non-Mendelian patterns of Inheritance (cntd.)• Multiple Alleles and CodominanceOften 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 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 - - - +
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)
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• PedigreesFamily trees documenting the inheritance of a trait.
• Example - HemophiliaClotting 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 HemophiliaThe hemophilia mutation occurred either in Queen Victoria or her parents and therefore entered the royal line.
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.)• InheritanceHomozygous recessive.
• CauseMutation in 1 base of the gene encoding -hemoglobin:Mutation: GAG GTGAA: 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.