• Goal of genetics - analysis of the genotype of organisms
• Identified through its phenotypic effects
• Phenotypes of their carries are different
• DNA sequences - read off the genotype directly from phenotypes

• Find a uniquely distinguishable phenotype for each genotype
• Find a simple genotype for each phenotype

• Allelic substitutions that cause qualitative differences in phenotype usually studied
• Actual variation between organisms is usually quantitative, not qualitative.

• Varies more or less continuously over a range
• Generally does not assort in a simple way in crosses
• Mendelian mechanisms still apply

• Genotype does not have a single phenotypic expression but a norm of reaction that covers a wide phenotypic range
• Many segregating loci have alleles that make a difference to the phenotype being observed

• Impossible to separate environmental from genetic effects in an organism that cannot be experimented
• Difficult to answer even the question of the presence of genetic influence on behaviour in humans

• distribution
• mode
• mean
• variance
• correlation
• covariance
• regression

population - entire group of objects or people about which information is wanted
units - individual members of the population
sample - group of units examined in order to gather information.
 

Genotypes and Phenotypic Distribution

Quantitative character - average phenotypic differences between genotypes are small compared to variation between individuals within genotypes

                                                                                                                        Griffiths et al., 1997.

• Population consists of three genotypes affecting behaviour (aa:Aa:AA)
• Environmental variation contributes to variation within each phenotype
• Ratio of genotype (aa:Aa:AA) = 1:2:3

Result:  Cannot carry out any simple Mendelian analysis to determine the genotype of an individual organism
 

• Quantitative characters can be converted into qualitative characters (Griffiths et al., 1997 Figure 27-10)
• Units raised in identical environments = enhanced trait differences
• Population separated into three non-overlapping phenotypic distributions

Polygenes - hypothetical factors with small-but-equal effect as opposed to the genes of simple Mendelian analysis.
 

• One-gene/two-allele system can result in continuous character variation when the difference between genotypic means is small compared with environmental variance

Norm of Reaction and Phenotypic Distribution  

                                                             Griffiths et al., 1997

• Environment will result in expression of phenotype for a given genotype
• Distribution of environments reflected biologically as a distribution of phenotypes

• Population consisting of two genotypes with different norms of reaction will result in a variety of phenotypic expression if raised in different environmental conditions

Heritability

Heritability - role that gene differences play in the phenotypic differences between individuals or groups
 

• In principle, easy to determine whether genetic variation influences the phenotypic variation among organisms for a particular trait
• Involvement of genes - biological relatives should resemble one another more than unrelated individuals do

Two general methods for establishing the heritability of a trait
    1.  Phenotypic similarity
    2.  Marker-gene segregation

Phenotypic similarity

• Easy in experimental organisms - Offspring from two extreme lines can be raised in the same environment (Griffiths et al., 1997 Figure 27-13)
• Difficult in natural populations- Family members share genes and environment.  Adoption studies using identical twins raised apart

Marker-gene segregation

• Depends upon showing that genotypes carrying different alleles of marker genes also differ in their average phenotype for the quantitative character (will be discussed in detail later)

• Variation between phenotypes in a population arises from two sources

    1.  Average differences between the genotypes
    2.  Genotype exhibits phenotypic variance because of environmental variation

Phenotypic variance (s_p²) in the population can be broken into two portions
    1.  Genetic variance (s_g²) - variance between genotypic means
    2.  Environmental  variance (s_e²) - remaining phenotypic variance
 

• possible to quantify the degree of heritability
• degree of heritability defined as the portion of the total variance that is due to genetic variance

Broad heritability (H²)

• measure of "genetic influence"
• tells what portion of the population's variation in phenotype assigned to variation in genotype
• does not tell what portions of an individual's phenotype ascribable to heredity and environment.
• framed in terms of the proportion of variance ascribable to genotype (application of ANOVA technique)

• estimate s_e² (inappropriate in human studies)
•     make a number of homozygous lines
•     cross them in pairs - reconstituting individual heterozygotes
•     measure phenotypic variance within heterozygous genotype
• consider the genetic similarities between relatives
•     Mendelian principles - differences in sibling phenotype
•     1/2 the genes of full-siblings will on average be identical
•     1/4 the genes of half-siblings will on average be identical

The Meaning of H²

• used as a measure of how "important" genes are in influencing a trait - appropriate?
• H² has a special and limited
•     non-zero heritability - genetic differences matter to the trait
•     limited prediction of the effect of environmental modification under particular         circumstances.
 
• separation of variance into genetic and environmental components s_g² and s_e² does not really separate the genetic and environmental causes of variation (Griffiths et al, 1997 Figure 27-14).
• high heritability does not mean that a trait is unaffected by its environment.

Summary of Key Concepts

• In natural populations, variation is in most characters takes the form of a continuous phenotypic range rather than discrete phenotypic classes.  In other words, the variation is quantitative, not qualitative.
• Mendelian genetic analysis is extremely difficult to apply to such continuous phenotypic distributions, so statistical techniques are employed instead.
• A major task of quantitative genetics is to determine the ways in which genes interact with the environment to contribute to the formation of a given quantitative trait distribution.
• The genetic variation underlying a continuous character distribution can be the result of segregation at a single genetic locus or at numerous interacting loci that produce cumulative effects on the phenotype.
• The estimated ratio of genetic to environmental variation is not a measure of the relative contribution of genes and environment to phenotype.
• Estimates of genetic and environmental variance are specific to the single population and the particular set of environments in which the estimates are made.