Quantitative Genetics

Question 1

Qualitative Traits

Answer 1

• phenotypic differences from alternative genotypes at a single locus
• divide individuals into distinct types or classes
• discontinuous variation

(ex: round vs. wrinkled, flower color)

Question 2

Quantitative Traits

Answer 2

• phenotypic differences from genetic and environmental factors
• phenotypes determined by counting/ measuring
• continous variation (multiple genes)
• ability for significant change
• additive alleles
• individual genes cannot be identified by their segregation
• mendelian ratios not produced

ex: grain yield, skin color, blood pressure

Question 3

additive alleles

Answer 3

basis of continuous variation
two or more pairs of genes
additive effect on the inherited phenotype
each gene locus may contain an additive allele (contributes a set amount to the phenotype)
additive effects of all alleles are ~equal
various allele combos produce substantial phenotypic variation

Question 4

nonadditive allele

Answer 4

nonfunctional

doesn’t contribute quantitatively to phenotype

Question 5

distribution

Answer 5

a description of the population in terms of the proportion of individuals that have each possible phenotype
usually presented as a frequency distribution (bell shaped curve)
2 major features= mean and variance

phenotypes at extremes= low frequency
phenotypes at middle= high frequency

Question 6

mean (average)

Answer 6

center of distribution curve

Question 7

variation

Answer 7

measure of the spread of the distribution
divergence from the mean
large value= distribution is spread out
small value= distribution is clustered near the mean
estimated in terms of squared deviation of each observation from the mean (S^2)
value relates to genetic variation

F2 segregating population resulting from 2 pure lines:
- big variance= lots of genetic variation (F2)
Pure strains, inbred strains, and F1 hybrids
- low variance = no genetic variation (homozygous parents and F1)

Question 8

phenotypic variation (VP)

Answer 8

VP= VA + VD + VI + VE + VGxE

sources:
VG= genotypic variation: due to the differences in the genotypes among individuals
VE= environmental variation: due to the differences in the environment (if there is no genetic variation then any phenotypic variation is due to environment)
VGxE= variation due to genotype by environment interaction: different genotypes respond to an environment differently

Question 9

heritability

Answer 9

the proportion of a populations phenotypic variation that is due to genetic variation
ranges from 0 to 100%
higher values= variation due to genetic effects
- potential to improve upon trait being studied because the trait is heritable

Question 10

broad sense heritability (H^2)

Answer 10

ratio of total phenotypic variation due to genetic variation
H^2= VG/VP * 100 = VG/(VG+VE) * 100
-defines if the variation observed is due to environmental of genetic effects

high H^2 = difference due to genetic effects

Question 11

narrow sense heritability (h^2)

Answer 11

ratio of the additive genetic variance to phenotypic variances
h^2 = VA/VP * 100 = VA/ (VA+VD+VI+VE) *100
genetic variance composed of 3 components:
- additive genetic variance VA
- dominant genetic variance VD
- epistasis VI
(VI and VD of little value to predicting heritability)
defines if improvement is possible through selection for a given trait (selection relates to the accumulation of additive traits)

high h^2 = improvement possible through selection; selection is more effective

Question 12

Monozygotic (MZ)

Answer 12

identical; differences due to environmental effect

Question 13

Dizygotic (DZ)

Answer 13

fraternal; no more genetically alike than normal siblings

Question 14

Concordance

Answer 14

occurs if both twin express/neither express trait

high MZ, low DZ = strong genetic component
high MZ, high DZ = strong environmental component