chapter 12

Question 1

heredity (pre 20th century)

Answer 1

• heredity occurs within species
• traits are transmitted directly from parent to offspring

*thought traits were borne through fluid (like blood) and blended in offspring

Question 2

Josef Kolreuter (1760)

Answer 2

• crossed tobacco strains to produce hybrids
• hybrid offspring differed from both parents
• more variation in F2 (second gen of offspring) contradicted direct transmission theory

Question 3

T.A. Knight (1823)

Answer 3

• 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

Question 4

Gregor Mendel

Answer 4

quantified results of Knight’s experiments using pea plants

Question 5

why were pea plants optimal

Answer 5

• 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

Question 6

Mendel’s experimental method

Answer 6

1. produce true-breeding strains for each trait that he was studying
2. cross-fertilize true-breeding strains having alternate forms of a trait
   *also do reciprocal crosses to ensure source of pollen is from both types
3. allow hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait

Question 7

how mendel conducted his experiments:

Answer 7

1. the anthers are cut away on the purple flower
2. pollen is obtained from the white flower
3. pollen is transferred to the purple flower
4. all progeny result in purple flowers

Question 8

monohybrid crosses

Answer 8

used to study only two variations of a single trait

Question 9

Mendel’s monohybrid cross

Answer 9

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

Question 10

F1 generation

Answer 10

• first filial generation
• offspring produced by crossing two true-breeding strains

Question 11

Mendel’s F1 generation

Answer 11

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)

Question 12

Mendel’s F2 generation

Answer 12

• 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

Question 13

3:1 ratio

Answer 13

• 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

Question 14

Mendel’s conclusion

Answer 14

• 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

Question 15

Mendel’s 5 element model

Answer 15

1. parents must transmit discrete factors (genes)
2. each individual receives one copy of a gene from each parent
3. not all copies of a gene are identical
4. alleles remain discrete - no blending
5. presence of allele does not guarantee expression (dominant allele = expressed; recessive allele = hidden by dominant allele)

Question 16

allele

Answer 16

alternative form of a gene

Question 17

homozygous

Answer 17

two of the same allele

Question 18

heterozygous

Answer 18

different alleles

Question 19

genotype

Answer 19

an individual’s complete set of alleles

Question 20

phenotype

Answer 20

an individual’s physical appearance

Question 21

Principle of Segregation

Answer 21

• 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

Question 22

dihybrid crosses (Mendel)

Answer 22

• 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)

Question 23

dihybrid ratio

Answer 23

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)

Question 24

principle of independent assortment

Answer 24

• 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

Question 25

testcross

Answer 25

- 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

Question 26

rule of addition

Answer 26

- the probability of either of two mutually exclusive events occurring is the sum of their individual probabilities

Question 27

dihybrid probabilities

Answer 27

- based on monohybrid probabilities - principle of independent assortment means one dihybrid cross is equivalent to two independent monohybrid crosses

Question 28

extensions to Mendel

Answer 28

- 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

Question 29

phenotypic plasticity

Answer 29

different phenotypes from the same genotype due to environmental conditions

Question 30

environmental influence

Answer 30

environment does effect phenotype

Question 31

continuous variation

Answer 31

- 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

Question 32

quantitative traits

Answer 32

- continuous variation traits, - more common than traits due to a single gene

Question 33

broad sense heritability

Answer 33

the fraction of phenotypic variation due to underlying genetic variation

Question 34

narrow sense heritability

Answer 34

the fraction of phenotypic variation due to additive genetic variance (the genetic variance for a specific trait)

Question 35

pleiotropy

Answer 35

- 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

Question 36

effect of pleiotropy

Answer 36

can be super difficult to predict