Genetics & Modern Evolutionary Synthesis

BIO1131 - Delcourt

Darwin’s old theory, ___________ was a version of:

pangenesis, blending inheritance

Why is blending inheritance flawed?

variation would quickly be lost, and natural selection would be ineffective

Germ Plasm Theory (for multicellular)

• heritable info is transmitted by the germ cells in exclusively the gonads (reproductive cells)

• The soma (new somatic cells) are produced by germ cells and they separate early

• The soma is disposable, germ plasm is potentially immortal

Germ Plasm Theory for plants, corals, sponges

• germ and somatic cells are not separated

• both can be produced by stem cells

• mutations in these cells can affect the germ cells they produce

Particulate Inheritance

Mendel’s model that heredity is controlled by “factors”, which individuals inherit from their parents and whose distinct forms account for variation in phenotype

Zygote

unicellular phase of a multicellular organism

⇨ first cell after fertilization

Gametes are created thru ___________ and consist of either:

meiosis, egg or sperm cells

Gregor Mendel is known as the ___________ of __________ genetics

father, modern

Mendel got lucky when he chose peas. Why? (5)

• easy to artificially cross and grow

• short generation time

• produce many descendants

• discrete rather than quantitative traits

• worked with true-breeding strains (homozygous)

P vs F1 vs F2 generations

P: Parental generation, contrasting and true-breeding

F1: The offspring of the P generation, called the First Filial generation

F2: The result of F1 individuals self or cross-pollinating with other F1s

in the F2 generation, Mendel observed a consistent ratio of _____ for the two phenotypes

3:1

Interestingly, a trait that was missing in the ___ reappeared in the ___, which indicated that the “factor”:

F1, F2, was masked rather than diluted/lost

Genes

Mendel’s “factors” — a specific sequence of DNA that encodes synthesis of a gene product (protein or RNA)

Locus (Loci)

a specific location/sequence of DNA on a chromosome

A locus might or might not contain an actual _____

gene

All genes are _______, but not all _______ are _____.

loci, but not all loci are genes

Allele

unique variants of a gene that differ in nucleotide sequence at a given locus

very few _________ are purely Mendelian. Instead, most are influenced by __________ genes with varying degrees of ___________

phenotypes, multiple, dominance

Even though genetics is more complex than Mendel thought, he was still correct in his fundamental idea:

that inheritence occurs via discrete factors that are passed intact from generation to generation according to probability

Mendel’s law of dominance

if 2 alleles at a locus differ, then the more dominant one determines the phenotype and the recessive one has no detectable effect

Dominance and Recessivity are ________!

relative

→ a “dominant” allele may be dominant over one allele but recessive to another

mendel’s law of segregation

alleles segregate independently during meiosis, and the result is that a heterozygote produces 50% gametes w one allele and 50% carrying the other allele

mendels law of segregation predicts that:

if you cross two heterozygotes you should see a 3:1 of dominant to recessive phenotypes in the offspring

mendel’s law of independent assortment

when non-homologous chromosome pairs align at the cell’s midpoint during metaphase 1 they have equal chance of adopting two different alignments

monohybrids vs dihybrids

individuals that are heterozygous for 1 trait vs those that are heterozygous for 2 traits

Test crosses are done between a ___________ ___________ individual and one whose genotype at 1+ ___ may be ___________

homozygous recessive, loci, unknown

Mendelian inheritance is governed by ___________, as reflected in his laws of:

probability, segregation + independent assortment

when one outcome has no influence on the outcome of another, these events are _____________. Ex:

independant

ex: result of one coin flip has no effect on the result of another flip

Product Law

the probability of two or more independent events occuring simultaneously

Sum Law

the probability of obtaining any single outcome, where that outcome can be achieved by one or more events

incomplete/partial dominance vs co-dominance

incomplete: phenotype shows an intermediate of possible phenotypes

co-dominance: contribution of both alleles are distinctly visible in the phenotype

meiotic drive

when a locus manipulates meiosis to favour the transmission of one allele over another (breaks law of segregation being 50/50)

→ more common in certain groups than others

ex: maize in the 1940s where a Rr x rr produced 70% rr

Pleiotropy

when one gene affects multiple phenotypes

→ extremely common

Linkage

tendency for alleles at diff loci to be inherited together as a unit

Recombination

intra-chromosomal recombination thru crossing over during prophase 1, this produces chromosomes

☆ contribute to much of the genetic variation within a population!

Epistasis

interaction between alleles at different loci thata determines a phenotype — occurs when specific alleles at one locus alter the effect on the phenotype of another locus

Quantitative traits

traits that vary in a continuous manner — phenotypes such as height, hair colour, etc that don’t fall into discrete categories

Quantitative variation arises because: (2)

1. multiple loci affect this trait (polygenic)

2. environment affects expression

For quantitative genetics, what does P = G + E mean?

an organism's observable phenotype (P) is the sum of its genotype (G) and its environment (E)

polygenic trait determination

as # of loci increases, phenotypic variation becomes very finely graded — ie closer to becoming truly continuous

Mendelian principles help explain how __________ is ___________, allowing evolution thru natural selection

variation, maintained

Gene pools

consists of all copies of each allele at a given locus

The Hardy-Weinberg Equilibrium describes the mathematical relationship between ________ and ___________ within an ______ population

alleles, genotypes, IDEAL

The ideal population described by "Hardy-Weinberg Law”

• infinitely large

• random mating ONLY

• no mutation

• no drift

• no migration

• no selection