Rules of Inheritance

• The rules of inheritance were unknown when Darwin developed the theory of natural selection

• The ‘hip’ idea at the time was the ‘blending inheritance hypothesis’

Rules of Inheritance

• This theory simply states that as a result of sexual reproduction, offspring should be a genetically intermediate between parents

• This forced Darwin to postulate an extremely high mutation rate to account for the high genetic variability observed in actual individuals

Rules of Inheritance

• Mendel performed careful breeding experiments with different strains of peas

• Using ‘true’ Y & G peas, he cross bred them, all F1 were Y; F2 had about ¼ G peas

Rules of Inheritance

• Cytological events

Rules of Inheritance

• Terms with which to be familiar:– 1) allele (character controlling a particular trait)– 2) its position on a chromosome is its locus– 3) a single character is the haploid condition (n)– 4) may be many alleles at a single locus (gene)*– 5) purebred diploids with identical alleles:

homozygous– 6) individuals with mixed alleles: heterozygous

Rules of Inheritance

• Terms with which to be familiar:– 7) an allele that masks the expression of the other is

termed dominant– 8) unlinked alleles separate, or segregate, from

each other in the formation of gametes– 9) new combinations of alleles form through

reassortment– 10) observable traits: phenotype– 11) whether an organism breeds true or not is

determined by its genotype

Rules of Inheritance

• There are cases of incomplete dominance where the phenotype accurately reflects the genotype

• If intermediates are not selected against, they may persist (although relatively rare)

• Basic concept: alleles that infer an advantage usually become dominant (and can be virtually impossible to get rid of deleterious ones: why?)

Rules of Inheritance

• Numerous different loci and allelic systems occur on each chromosome

• Different traits are controlled by different alleles, but may not be completely independent from one another (linkage)

• If loci are close, may ‘travel’ together• Groups may function as ‘coadapted alleles’

Rules of Inheritance

• Natural selection operates on phenotypes of individuals (its immediate fitness), the effectiveness of selection in changing the composition of the population (and hence species) depends on the heritability of phenotypic characteristics

Rules of Inheritance

• Traits that are under strong selection usually display low heritability because the genetic component of phenotypic variability has been reduced by selection

• Different genotypes may actually have similar phenotypes (if selection is strong)

• Selection may favor genotypes that ‘mix’ relatively well with others…why?

Rules of Inheritance

• Consider humans and chimps; 99% similarity of identical amino acid sequences of presumably structural genes (code for a specific cell)

• A few changes in major regulatory genes (act to control the expression of other genes) apparently have profound phenotypic effects

Nature vs. Nurture

• It is important to remember that most phenotypic trails are not attributable to either genetic or environmental factors, they are a blend

• However, it is also important to remember that even traits that are apparently very plastic have a genetic basis and therefore subject to natural selection

Nature vs. Nurture

• E.g. Texas grasshoppers when fed dry grass become brown; however when fed moist grass…

Nature vs. Nurture

• Under what conditions could you envision selection for developmental plasticity? What about the lack thereof?

Nature vs. Nurture

• The complete set of different phenotypes that can be produced by a given genotyped across a range of environments is called its reaction norm

Selfish Genes

• Certain alleles do not obey the Mendelian lottery of meiosis and recombination and instead obtain a disproportionate representation (via meiotic drive)

• E.g. a sex-linked trait in Drosophila shows up 95% compared to the expected 50%

Selfish Genes

• Genes themselves are perfect replicators, although sexual reproduction is imperfect

• They also do not exist in isolation, but rather come as a ‘package deal’; consequently, they can only be selected for if they ‘get along’ with other genes and the entire package works as a whole (selection on the individual)

Population Genetics

• The genetic variability in the population exists in the gene pool

• The gene frequency is an alleles’s frequency in the haploid gene pool

• Equilibrium frequencies occur in a given gene pool

Population Genetics

• The frequencies in a 2 allele system are:– p2, 2pq, q2

Rules of Inheritance