biol 1406 chapter 12

two main ideas about heredity before Mendel

1. heredity occurs within cells

2. direct transmission of traits parent to offspring

why pea plants were optimal

pea hybrids could be produced, many distinct varieties were available, peas grow quickly and are easy to cultivate, peas can self-fertilize which allows for precise control over parentage

Mendel’s experimental method

produce true-breeding strains for each traits he was studying, crossed two true-breeding plants with different traits, self-fertilization of hybrids

true-breeding plant

a plant that consistently produces offspring with the same trait when self-fertilized

F1 generation

offspring of two true-breeding parents, only the dominant trait was visible

F2 generation

offspring from self-fertilized F1 plants, 3:1 phenotypic ratio, 1:2:1 genotypic ratio

monohybrid crosses

used to study only two variations of a single trait

Mendel’s conclusions

plants did not show intermediate traits (no blending)

Mendel’s five-element model

parents transmit discrete factors (genes), each individual receives one copy of a gene from each parent, not all copies of a gene are identical, alleles remain discrete (no blending), presence of allele does not guarantee expression

allele

alternative form of a gene (homozygous AA/aa, heterozygous Aa, dominant is expressed, recessive is hidden)

genotype

an individual’s complete set of alleles

phenotype

an individual’s physical appearance

principle of segregation

two alleles for a gene segregate during gamete formation and are rejoined at random during fertilization

what explains allele segregation

the movement of chromosomes during meiosis

dihybrid crosses

used to study two variations of two traits in a single cross

testcross

determines the unknown genotype of an organism showing a dominant phenotype by crossing it with a homozygous recessive

F2 phenotypic ratio of dihybrid cross

9:3:3:1

principle of independent assortment

genes for different traits segregate independently during meiosis

phenotypic plasticity

different phenotypes from same genotype due to environmental conditions

continuous variations

refers to a range of possible phenotypes across genotypes

quantitative traits

traits showing continuous variation influenced by multiple genes

pleiotropy

refers to an allele which has more than one effect on the phenotype

multiple alleles

more than two alleles exist for a gene (like blood types)

codominance

both alleles are expressed (AB blood)

recessive allele

heterozygote shows an intermediate phenotype (red x white = pink)

epistasis

when the action of one gene obscures the effects of another gene