Going Beyond Mendel. Chapter 11

Introduction• Sometimes one gene simply controls a one

trait (another reason why Mendel was lucky—genotypes matched phenotypes)

• However, sometimes a genetic trait can be controlled by:

– a number of genes that interact with each other with some being dominant over other and…

– the “conditions” the gene is under to finally produce the phenotype.

Dominance Relationships and Gene Interactions

• There are various ways in which two alleles of a gene can affect the phenotype. – Often they are simply dominant or recessive– Other times, there are situations where the

recessive allele may function a little or interact with the dominant allele.

1. Incomplete dominance (partial dominance) • phenotype of a heterozygote is between the two

phenotypes of the two homozygotes • Example

– cR cR = red flowers – cW cW = white flowers– cR cW = pink flowers

2. Codominance

• the equal expression of both alleles resulting in a mixed phenotype.

• examples– Cattle –– Blood type/group –

3. Multiple alleles

• Himalayan Rabbit example

• Blood type/group example

4. Lethals

• alleles that cause the death of the organism• recessive lethal alleles

– Corn: – Huntington’s disease

5. Epistasis

• one gene interferes with the expression of another gene. Fig 11.8 in text

• ExampleB = black mouseb = brown mouseC = makes the pigment melanin necessary for either black or brownc = can’t make pigment necessary for either color

Conditional Phenotypes

• How genes are expressed depends on the conditions under which they exist

• The same genotype can result in different phenotypes in:– separate individuals– different tissue of the same individual

Environment

Development • As an organism grows from fertilized egg to a

mature organism, certain nerves and tissues may or may not be “completed”– incomplete penetrance -– variable expressivity –

• Ex.) Polydactyly: tendency to have extra fingers or toes

Sex

• a sex-limited or linked trait shows up in only one sex or the other– Ex.) uteran cancer.

• a sex-influenced trait can affect both sexes but the effect is different. – Ex.) middle-aged male go bald but women’s

hair only thins.

Age

• Some genetic traits can have variable ages of onset– Ex.) muscular dystrophy, Huntington’s

disease, and baldness.

Pleiotropy

• Example– Phenylketonuria (PKU)

• Victims can’t metabolize a.a. phenylalanine which then builds up and leads to brain damage.

• Victims also have light hair and skin color.

Polygenetic inheritance

– Examples• skin color: • height:• intelligence• birth weight