Lecture 17: Beyond Mendel

Question 1

Incomplete Dominance

Answer 1

In between (red and white = pink)

Question 2

Incomplete Dominance in Human Traits

Answer 2

Sickle-cell disease
•Homozygote recessive has sickle-cell disease
•Heterozygote has milder sickle-cell trait

Familial hypercholesterolemia
•Homozygote has severe form of disease
•Heterozygote has mild form of disease

Tay-Sachs disease
•Homozygote has serious symptoms
•Heterozygote has no symptoms but has detectable biochemical effects

Question 3

Sickle Cell Anaemia

Answer 3

OMIM 603903
•A mutation in the gene coding for Beta haemoglobin reduces the ability of the red blood cells to carry oxygen.
•People with sickle cell anaemia are homozygous for the recessive Hb-S allele
•Hb-A/Hb-S individuals (heterozygotes) have sickle cell trait

Question 4

Co-dominance

Answer 4

Different alleles of gene have equal effects in heterozygotes
•Both alleles expressed
•Human M, MN, and N blood types
•LMLM = M glycoprotein present; blood type M
•LNLN = N glycoprotein present; blood type N
•LMLN = both glycoproteins present; blood type MN
•Similar inheritance to incomplete dominance

Question 5

Multiple Alleles

Answer 5

Three or more alleles for a gene
•Found among all individuals in a population
•Diploid individuals only have two of the alleles
•Phenotype depends on relationship between alleles
•Still follows Mendel’s principles

Question 6

Multiple Alleles (and Co-dominance)

Answer 6

A and B Antigens
•IA allele produces A antigen (dominant)
•IB allele produces B antigen (dominant)
•i allele produces neither A nor B (recessive)

ABO Blood types (phenotypes)
•IAIA or IAi = type A blood
•IBIB or IBi = type B blood
•ii = type O blood
•IAIB = type AB blood

Question 7

Epistasis in Labrador Retrievers

Answer 7

Genes interact
•Allele of one locus inhibits or masks effects of allele at a different locus
•Some expected phenotypes do not appear among offspring

i.e.Labrador Retrievers
•Melanin pigment gene
•B allele: black fur colour (dominant)
•b allele: brown fur colour (recessive)
•Pigment deposition gene
•E allele: pigment deposition normal (dominant)
•e allele: pigment deposition blocked (recessive)
•Phenotypes
•Black fur: BB EE, BB Ee, Bb EE, Bb Ee
•Brown fur: bb EE, bb Ee
•Yellow fur: BB ee, Bb ee, bb ee

Question 8

Epistasis in Humans

Answer 8

Bombay phenotype

Question 9

Pleiotropy

Answer 9

the condition in which single genes affect more than one character of an organism

Question 10

Multifactorial inheritance

Answer 10

• 1 gene  multiple phenotypes = pleiotropy
• 1 phenotype  multiple genes/factors
• Multifactoral =
• Multiple genes (and alleles)
• Environment – internal (other genes)
• • external (physiological, etc)

Question 11

Polygenic inheritance

Answer 11

Several genes at different loci interact to control the same character
•Produces continuous variation
•Phenotypic distribution: Bell-shaped curve

Question 12

Multifactoral inheritance – Effect of environment

Answer 12

• C gene = determines pigment expression
• C = permits colour expression
• c = prevents colour expression
• c/c = epistatic to other colour genes; albino
• ch = Himalayan allele; ts allele i.e. pigment is only made at colder body extremities

Question 13

Variable Expressivity

Answer 13

Coat colour in Siamese cat