gen bio - lecture 12: intro to genetics

gene

unit of hereditary info

allele

variant of a gene

character

observable, heritable feature

ex. eye color

trait/character state

detectable variant of a character

ex. blue, black, green

genotype

genetic makeup; what specific alleles are present

phenotype

observable physical traits

connect the following into two equations: gene, allele, trait, character, hair color, red hair

gene → character → hair color

allele → trait → red hair

gregor mendel - personal background

• austrian monk

• first to determine the basic rules of inheritance in eukaryotes

gregor mendel - experimental organism (what was it + advantages)

• graden pea

• advantages:

  • inexpensive + easy to obtain

  • identifiable traits

  • easy to grow w/ short generation time

  • easy to control pollination

  • many varieties

true-breeding

• always expresses the same phenotype after self-fertilization

• mendel developed these true-breeding lines for his experiment

blending inheritance hypothesis

gametes contain a sampling of fluids from parents

• these fluids fuse during reproduction → offspring has an intermediate phenotype

define experimental crosses, P generation, F1 generation, and F2 generation

• experimental crosses: mating two organisms to see the offspring’s phenotype

• P generation: parental generation

• F1 generation: 1st filial generation

• F2 generation: 2nd filial generation

gregor mendel - experiment that tested blending inheritance (claim, prediction, experiment, observations, conclusion)

• claim: crossed true-breeding plants (P generation) w/ contrasting traits — one true breeding white and one true breeding purple

• prediction: F1 should be intermediate between the two P phenotypes

• experiment: start w/two true breeding P generations with different phenotypes + create F1 generation

  • mate F1 with each other

• observations:

  • F1 ALWAYS RESEMBLES just one parent (no intermediate, other phenotype is absent)

  • F2 is a mix of both P2 phenotypes

traits absent in F1 reappear in F2 in a consistent 3:1 phenotypic ratio

• conclusion:

  • no intermediate phenotypes appeared

  • lost phenotypes reappear after skipping a generation

  • BLENDING HYPOTHESIS REJECTED!

mendel’s model

• model of particulate inheritance

  • characters are determined by heritable factors (genes)

  • each character is controlled by 2 factors — one from each parent

4 components of mendel’s model (just list them)

1. alleles

2. two factors for each character

3. dominance

4. mendel’s laws/principles of heredity (law of independent assortment + law of segregation)

mendel’s model - alleles

alternative versions of a gene (purple petals vs. white petals)

mendel’s model - two factors for each character

• diploid individuals inherit 2 copies of each gene → 1 from each parent

• may be identical (true-breeding lines) or may be different

mendel’s model - dominance

if two alleles differ…

• dominant allele determines phenotype

• recessive allele has NO noticeable affect on phenotype

mendel’s model - law of independent assortment + importance (include genetic recombination)

• genes on different chromosomes assort independently during gamete formation

  • due to RANDOM ORIENTATION OF TETRADS on metaphase plate during metaphase 1 of meioisis

• importance:

  • results in genetic recombination: a new combination of alleles in offspring

independent assortment + crossing over (prophase 1 of meiosis) = new combos of genes and LOTS of genetic variation

mendel’s model - law of segregation (define hetero and homozygous)

• the two alleles for a character separate /segregate during gamete formation

  • each gamete only gets one allele

  • occurs in ANAPHASE 1 of meiosis

• homozygous: two of the same allele at a locus

• heterozygous: two different alleles at a locus

punnett square + monohybrid cross

• punnett square: tool used to determine possible offspring and their frequencies in a genetic cross

ex. for a monohybrid Yy x Yy cross

• genotype ratio: 1:2:1 (1 YY, 2 Yy, 1 yy)

• phenotype ratio: 3:1 (3 express Y, 1 expresses y)

multiplication rule

used for independent events (occurrence of one event does NOT affect the probability that the other will occur)

• multiply the separate probabilities

example: Aa and Aa looking for probability that 1st child will be homozygous recessive

• P(recessive allele in egg) = 0.5

• P(recessive allele in sperm) = 0.5

• P(homozygous recessive offspring) = 0.5 × 0.5 = 0.25

addition rule

used for mutually exclusive events (events that cannot occur simultaneously)

• add combined probability

example: Ea and Ea looking for probability of heterozygous

• E egg (0.5) and e sperm (0.5) = 0.25

• e egg (0.5) and E sperm (0.5) = 0.25

• 0.25 + 0.25 = 0.5 probability