Bio 178 Lecture 25 Genetics

Reading Chapter 13 Quiz Material Questions on P Chapter 13 Quizzes on Text Website (

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

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

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 (I A  galactosamine, I B  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 ABABO A B 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 ABABO A+--+ B O---+

Codominance Example - Roan Horse

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 factorRh - = 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 (X h Y) 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

Single Base Substitution - Sickle Cell Anemia The Disease Defective hemoglobin that is unable to carry O 2 properly. Hemoglobin molecules stick to each other under low O 2 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  E.G. Platzer