Bio 178 Lecture 25Genetics

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 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.

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 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)

Himalayan Rabbit

http://www.bio.miami.edu/dana/250/himalayanbun.jpg

Arctic Fox

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 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• 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.

Royal Hemophiliahttp://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.)• 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.

Sickle Cell Anemia

Plasmodium falciparun

E.G. Platzerhttp://www.sb-roscoff.fr/CyCell/Page45.htm