Heritability

Heritability

The proportion of the total phenotypic variation that is due to genetic difference

• heritability of reproductive traits is usually lower

How to Calculate Heritability

Components of phenotypic variance - Vp = VG + VE + VGE

• genetic variance: VG

• Environmental variance: VE

• Genetic-environmental interaction: VGE

Components of genetic variance - VG = VA + VD + VI

• additive genetic variance: VA

• Dominance genetic variance: VD

• Genie interaction variance: VI

Broad sense (H2 = VG/VP)

Narrow sense (h2 = VA/VP) - only traits with high narrow sense heritability can be efficiently passed on from generation to generation

Example: five measurements of height

• Measurements: 160, 161, 167, 164, & 165

• Mean: the average

  • Add all measurements and divide by the amount of measurements

  • (160+161+167+164+165)/5 = 163.4

• Variance: the variability of a group of measurements

  • Ex. (160-163.4)²+(161-163.4)²+etc/5-1

Genetic Environmental Interaction VGE

Genetic environmental int eraction variance is obtained when the effect of a gene depends on the specific environment in which it is found

How to Estimate Environment Variance

• Plants of the same genotype showing different phenotypes (height) under normal or deficient environments indicates that the variation is caused by difference in the environment

• Therefore, environment variance can be estimated using materials with same genotypes such as F1s from homozygous parents or genetic clones of a particular genotype

How to Estimate Narrow-Sense Heritability

Calculating narrow sense heritability by comparing the phenotypes of parents and offspring:

• the mean phenotype of the parents is plotted against the mean phenotype of the offspring

• The line represents the best fit of all of the points of the graph

• A complex mathematical proof demonstrates that the slope of the line equals the narrow sense heritability

• The reason only traits with high narrow sense can be efficiently passed on from generation to generation

Limitations of heritability

1. An individual does not have heritability

2. There is no universal heritability for a characteristic

3. Heritability does not indicate the degree to which a characteristic is genetically determined

Natural Selection

Selection that arise through the differential reproduction of individuals with different genotypes

AKA organisms with heritable traits better suited to their environment ten to survive and reproduce more successfully passing the traits to the next generation

Artificial Selection

Selection by promoting the reproduction of organisms with traits perceived as desirable

AKA humans select for desirable traits in agricultural producers or animals rather than leaving species to evolve without interference

Genetically Variable Traits Change in Response to Selection

• Predicting the Response to Selection

  • The extent to which a characteristic subject to selection changes in one generation

• Factors influencing response to selection

  • Selection differential

• Calculation of response to selection

  • R = h2 x S

• Estimating heritability from response to selection

  • H2 = R/S; realized (narrow sense) heritability

• A quantitative trait may be improved by artificial selection but response to selection may level off after many generation

  • If the response levels off h2 is becoming small, reducing response to selection

Concept Check

The narrow sense heritability for a trait is 0.4 and the selection differential is 0.5, What is the predicted response to selection?

• R = h² x S - 0.4 × 0.5 = 0.2

NEED TO KNOW

• QTL mapping

  • Steps and cloned genes

• Response to selection

  • How to calculate response to selection