Exam 2 - Heredity

1/2 ch4 on exam 2

blending hypothesis

hypothesis that genetic material from two parents mixes

• blue + yellow = green

particulate hypothesis

hypothesis that parents pass on discrete heritable units

• retain separate identities in offspring

Generations

P generation - true-breeding parents - produce progeny with all the same trait when selfed, are parents of this generation

• results in hybridization

F₁ generation - showed a lack of blending and disappearance of one of the phenotypes

• monohybrid cross - self or cross-pollinate makes the F₂

F₂ generation - one of the disappeared phenotypes reappeared

results inconsistent with blending hypothesis

Mendel’s F₁ cross

The phenotype observed in the F₁ generation indicated which trait is dominant trait

The phenotypes observed in the F₂ generation all had dominant:recessive traits in a 3:1 ratio

Mendel’s first law

principle of segregation

Four concepts of Mendel’s model

1. alternative versions of heritable factors (genes) account for variations in inherited traits

2. an organism has two copies of a heritable factor controlling each trait

3. the dominant trait will determine the organism’s phenotype, and the recessive trait will have no noticeable effect

4. during gamete formation, heritable factors segregate such that each gamete receives only one

homozygous

two chromosomes contain identical alleles 

• observed in the P generation parents

heterozygous

two chromosomes contain different alleles

• observed in the F₁ hybrids

Principle of separation

(Mendel’s first law) Each gamete contains one allele for each heritable factor.

• alleles of a single heritable factor segregate

• gametes receive only one pair of homologous chromosomes

• studied using a monohybrid cross

monohybrid

heterozygous for a single gene controlling a single trait being followed

Ex. Yy

• created by crossing two true-breeding parents

what is the phenotypic ratio for the progeny of a monohybrid cross?

3:1

what is the genotypic ratio for the progeny of a monohybrid cross?

1:2:1

Dominant and recessive

the dominance and recessiveness of an allele is always relative to another allele

• dominant allele does not mean it is the most common

testcross

Breed a mystery individual with a homozygous recessive individual (the tester) to determine the genotype of the mystery individual

Dihybrid

heterozygous for TWO characters

ex. YyRr

what is the phenotypic ratio for the progeny of a dihybrid cross?

9:3:3:1

9/16, 3/16, 3/16, 1/16

what is the genotypic ratio for the progeny of a dihybrid cross?

dependent assortment

if YR and yr from the P generation do NOT assort independently, then only two types of gametes will arise from the dihybrid parents.

• proven incorrect

Principle of Independent assortment

(Mendel’s second law) alleles for different heritable factors (genes) are transmitted to gametes independently of one another.

multiplication rule

the probabiliity that two or more independent events will occur together is the product of their individual probabilites

“what is the probability that X AND Y happens?”

addition rule

The probability of one or more mutually exclusive events will occur is the sum of their individual probabilities

“what is the probability that X OR Y happens?

What is the probability that a monohybrid cross of purple and white happens in the first two offspring, in any order?

(3/4 × 1/4) + (1/4 × 3/4) = 6/16

[probability of purple then white] + [probability of white then purple]