chapter 12 Flashcards
heredity (pre 20th century)
- heredity occurs within species
- traits are transmitted directly from parent to offspring
*thought traits were borne through fluid (like blood) and blended in offspring
Josef Kolreuter (1760)
- crossed tobacco strains to produce hybrids
- hybrid offspring differed from both parents
- more variation in F2 (second gen of offspring) contradicted direct transmission theory
T.A. Knight (1823)
- crossed two varieties of garden pea
- crosses two true-breeding strains (self-fertilization produces 1 type)
- 1st gen resembled 1 parent strain -> 2nd gen resembled both
Gregor Mendel
quantified results of Knight’s experiments using pea plants
why were pea plants optimal
- other research showed that pea hybrids could be produced
- many pea varieties were available
- peas are small plants/easy to grow
- peas can self-fertilize/be cross fertilized
Mendel’s experimental method
- produce true-breeding strains for each trait that he was studying
- cross-fertilize true-breeding strains having alternate forms of a trait
*also do reciprocal crosses to ensure source of pollen is from both types - allow hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait
how mendel conducted his experiments:
- the anthers are cut away on the purple flower
- pollen is obtained from the white flower
- pollen is transferred to the purple flower
- all progeny result in purple flowers
monohybrid crosses
used to study only two variations of a single trait
Mendel’s monohybrid cross
produced true-breeding pea strains for 7 different traits
(flower color, seed color, seed texture, pod color, pod shape, flower position, plant height) - each trait had two variants
F1 generation
- first filial generation
- offspring produced by crossing two true-breeding strains
Mendel’s F1 generation
all F1 plants resembled the same parent for every trait that he studied (visible trait = dominant, alternative trait = recessive (white flower))
- no plants with intermediate characteristics were produced (no blending inheritance)
Mendel’s F2 generation
- second filial generation
- produced from self-fertilization of F1 plants
- although masked in F1 generation, recessive trait reappeared among some F2 individuals
*proportions of traits always found to be 3:1 ratio
3:1 ratio
- actually means 1:2:1
- F2 plants: - 3/4 plants in dominant form, 1/4 plants in recessive form
- 1 true-breeding dominant, 2 non true-breeding dominant, 1 true-breeding recessive
Mendel’s conclusion
- Mendel’s plants did not show intermediate traits (each trait was intact, discrete)
- for each pair, one trait was dominant and the other recessive
- alternative traits were expressed in F2 generation in ratio of 3/4 dominant to 1/4 recessive
Mendel’s 5 element model
- parents must 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 (dominant allele = expressed; recessive allele = hidden by dominant allele)
allele
alternative form of a gene
homozygous
two of the same allele
heterozygous
different alleles
genotype
an individual’s complete set of alleles
phenotype
an individual’s physical appearance
Principle of Segregation
- 2 alleles for a gene segregate during gamete formation (one from each parent) and are rejoined randomly during fertilization
- physical basis for allele segregation is the movement of chromosomes during meiosis
*Mendel didn’t know about meiosis and chromosomes so he deduced this principle based on trait ratioss
dihybrid crosses (Mendel)
- used to study 2 variations of 2 traits in a single cross
- Mendel produced 2 true-breeding lines (each with two traits)
RRYY x rryy (round yellow x wrinkled green)
-F1 generation of a dihybrid cross only shows dominant phenotypes (allows F1 to self fertilize to produce F2)
dihybrid ratio
9:3:3:1 ratio
- F1 self fertilizes to produce F2 (RrYy x RrYy)
- F2 generation shows all 4 possible phenotypes
(round yellow):(round green):(wrinkled yellow):(wrinkled green)
principle of independent assortment
- in a dihybrid cross, the alleles of each gene assort independently
- the segregation of different allele pairs is independent (ex: seed shape is independent of seed color)
- independent alignment of different homologous chromosome pairs during metaphase I leads to the independent segregation of different allele pairs
testcross
- used to determine the genotype of an individual with unknown phenotype
- cross the unknown’s genotype with a homozygous recessive
- ratios will indicate what the genotype of the unknown individual was
rule of addition
- the probability of either of two mutually exclusive events occurring is the sum of their individual probabilities
dihybrid probabilities
- based on monohybrid probabilities
- principle of independent assortment means one dihybrid cross is equivalent to two independent monohybrid crosses
extensions to Mendel
- mendel’s model of inheritance assumes that each trait is controlled by a single gene, each gene has only two alleles, there is a clear dominant-recessive relationship between alleles
*most genes do not meet these criteria
phenotypic plasticity
different phenotypes from the same genotype due to environmental conditions
environmental influence
environment does effect phenotype
continuous variation
- a range of possible phenotypes across genotypes
- the phenotype is often the result of an accumulation of contributions by multiple genes
- these traits show continuous variation and are referred to as quantitative traits
ex: human height
quantitative traits
- continuous variation traits,
- more common than traits due to a single gene
broad sense heritability
the fraction of phenotypic variation due to underlying genetic variation
narrow sense heritability
the fraction of phenotypic variation due to additive genetic variance (the genetic variance for a specific trait)
pleiotropy
- an allele which has >1 effect on the phenotype
- can be seen in human diseases like cystic fibrosis and sickle cell anemia
- multiple symptoms can be traced back to one defective allele
effect of pleiotropy
can be super difficult to predict
