3. NON ADDITIVE GENE ACTION

Question 1

Define trait

Answer 1

Observable/ measurable characteristic

Question 2

Define phenotype

Answer 2

Measurable level of performance for a trait

Question 3

Describe a quantitave trait

Answer 3

• has continues variation
• has no distinct classes
• E plays an important role
• Difficult to ID superior genotypes

Question 4

Describe qualitative traits

Answer 4

• discontinuous variation
• distinct classes
• E plays a small role
• Easy to ID genotypes

Question 5

Describe additive gene action

Answer 5

• alleles have equal ability to be expressed
• has many genes ( 100-300) influence the trait
• responsible for quantitative traits
• includes growth, carcass traits and milk yield
• these are the most economically important

Question 6

Describe non additive gene action

Answer 6

• alleles are not equally expressed
• only 1 or a few genes
• responsible for qualitative traits
• includes horns and coat color
• includes gene interactions, eg. dominance

Question 7

Define dominance

Answer 7

Expression of one allele(B) at locus is dependant on the other allele present(b). The one allele has a greater effect than the other.

Question 8

Describe with an example complete dominance

Answer 8

The heterozygous animal is phenotypically identical to the animal which is homozygous dominant. Ex. coat color in cattle, black B is completely dominant over red b.

Question 9

Describe with an example partial dominance.

Answer 9

• Heterozygous phenotype is intermediate between those of the 2 homozygotes
• Hyperkalemic periodic paralysis(HYPP) in horses, which is muscle tremors in horses. Homozygous recessive hh is where muscle tremours are so bad the animal collapses, homozygous dominant HH is where there are almost no tremors and a heterozygous animals have some tremours.

Question 10

Describe no dominance

Answer 10

Expression of the heterozygote is exactly midway between the 2 homozygotes.

In homozygous dominant animals DD enzyme activity is at 100%, in homozygous recessive animals the enzyme activity is at 0 % and in heterozygous animals the enzyme activity is at 50%. ( aromatase enzymes in chicken

Question 11

Explain over dominance

Answer 11

• Expression of heterozygote is outside the range defined by the homozygotes.
• callipyge muscular hypertrophy, homozygous animals do not have double muscleling but the heterozygous animal will have double musceling if the C allele is inherited from the sire.

Question 12

Describe epistasis

Answer 12

• Genes of diffirent loci interact. Expression of genes at one locus is dependent on alleles present at other loci.
• Coat color in labradors, B_E_=black, bbE_= chocolate, __ee= yellow

Question 13

Explain color inheritance of charolais cattle

Answer 13

They posses a D locus which is the dilution locus, DD and Dd reduce color intensity of black and red.

• For charolais specifically:
  DcDc = white
  Dcdc = cream, yellow, ashy

Question 14

Explain sex linked

Answer 14

• genes are on the sex chromosomes
• Ex. tortoishell coulour in cats
• Locus for orange is on X, which means females can have 2 copies and males only 1.

Females:
OO = orange
Oo = tortoishell
oo = not orange

Males:
O = orange
o = not orange

Question 15

Explain sex limited

Answer 15

• The phenotypic expression is limited to one sex
• Genes are not on the sex chromosomes and expression is mostly hormonally conditioned
• ex. milk production and henny feathering in roosters

Question 16

Explain henny feathering in roosters

Answer 16

Male has too much aromatase enzyme, results in estrogen production, so testosterone goes down and feathering then looks like hens. Henny feathering in females is normal, but it is a disorder in roosters

Question 17

Describe sex influenced.

Answer 17

• Modes of gene expression differs between males and females
• Dominant in one sex and recessive in the other
• Ex are scurs, Sc is dominant in males, but not females

Females:
SnSn = normal
ScSn = normal
ScSc = scurs

Males:
SnSn = normal
ScSn = scurs
ScSc = scurs

Question 18

What are threshold traits

Answer 18

• Poligenic traits (quantitative) but express as categorical (qualitative)
• This means phenotypically it is expressed as distinct classes, but genotypically it is continuesly distributed
• ex. fertility
• difficult to select for

Question 19

Why is it difficult to select for threshold traits

Answer 19

they are determined by both the Environment and genotype factors, so the genetic variation is distributed normally but the but the environment determines what the threshold is. This threshold shifts between years and seasons. This environmental factors along with the fact that there is a very limited amount of phenotypes to help with selection. This makes it very difficult to determine the animals genotype

Question 20

What is incomplete penetrance

Answer 20

less than 100%od individuals with genotype exhibit the associated phenotype

Question 21

What type of melanin is present in black and brownd colors?

yellow, red, tan?

Answer 21

eumelanin

phaemelanin