Unit 4 - Ch 11 - Genetic Interactions

Question 1

Incomplete Dominance

Answer 1

Appears when the heterozygote has a phenotype intermediate between those of its two parents. In these cases, the genotypic and phenotypic ratios are identical.

Examples:
Snapdragons RR=red, Rr=pink, rr=white
Coat color in cows RR=red, Rr=roan, rr=white

Question 2

Complete dominance

Answer 2

One trait is completely dominant over the recessive trait, versus being incomplete. The terms dominant and recessive can be used in instances of complete dominance.

Example: pea traits have complete dominance

Question 3

Codominance

Answer 3

Refers to instances where the heterozygote simultaneously expresses the phenotypes of both types of homozygotes.

Example: blood types in humans (A and B alleles)

Question 4

Multiple alleles

Answer 4

Exist if there are three or more alleles for a given locus within a population.
Example: ABO blood types

Zygote receives only two alleles

Question 5

Modifier genes

Answer 5

A gene locus alters the expression of another locus

Example - Holsteins (amount of color in spotted pattern)

- one locus determines color
- one locus determines pattern

Question 6

Epistasis

Answer 6

The presence of certain alleles of one locus can prevent or mask the expression of alleles of a different locus and express their own phenotype instead.
The term epistasis means “standing on.”

Example

• Human Albinism (aa = albino)
• aa locus is epistatic / hides other genes: hair color, eye color, skin color

Question 7

Polygenic Inheritance

Answer 7

Several loci that influence a single trait
- alleles operate in a cumulative manner (more dominance, more expression)

Example - Ear Length in Corn

• aabbcc (short) x AABBCC (maximum long) = AaBbCc (medium length)
• F2 generation will be mostly medium lenght with a few short and a few long

Question 8

Pleitropy

Answer 8

A single gene locus that influences several different phenotypes. The ability of a single gene to have multiple effects.

Examples

1. Marfan’s Syndrome - connective tissue disorder caused by a dominant allele (M)
   - Phenotypes => tall, long limbs, long digits, scoliosis, dislocated lenses in eyes, weak aortas. Abraham Lincoln is thought to have had Marfan’s
2. PKU - lack the enzyme to break down phenylalanine
   - phenotypes => large heads, light hair color

Question 9

Lethal Genes

Answer 9

Lethal at some point in the life cycle

Examples

1. Amputated Calves
   - recessive trait in cattle
   - born without legs, typically stillborn
2. Huntington Chorea
   - in humans - dominant allele - neuromotor disease - usually lethal mid life.
3. Tay-Sachs - in humans - recessive allele - lack enzymes to break down brain lipids - usually lethal first year of life

Question 10

Incomplete Penetrance

Answer 10

A percentage of a genotype is not showing in the phenotype of a population.

Example - polydactylism - a dominant trait

Question 11

Complete Penetrance

Answer 11

Genotype is expressed in the phenotype

Question 12

Variable Expressivity

Answer 12

Variability in the phenotype of an individual

Example - polydactylism, eye color, hair color, foot size