Intro to Genetics- Exam 2 (Monohybrid and Dihybrid Crosses)

Gregor Mendel

• 1822-1884

• Foundation of genetics

• Studied segregation of traits in garden peas beginning in 1854

• Published theory of inheritance in 1865

• “Experiments in plant hybridization”

  • “Why do we see similar phenotypes across generations”

Mendel’s Model Organism

The garden pea

What properties suggest that a model organism is suited to study inheritance?

• Easy to work with

• Produces large number of offspring

Notes about Mendel’s experiments

• Examined discontinuous rather than continuous traits

• Each trait was represented by 2 contrasting forms

• Examined one or a few pairs of traits in each experiment

• Looked for patterns and analyzed ration phenotypes seen in the offspring

Discontinuous Traits

Only get a few possibilities (either/or traits)

Continuous Traits

Multiple possible phenotypes in offspring

Monohybrid Crosses

• True-breeding P generation

• Analyze a single pair of contrasting traits

• A cross between individuals differing in a single character

P Generation

Original parents

F1 Generation

1st filial generation, progeny of the P generation

• Resembles only 1 of the parents

F2 generation

2nd filial generation, progeny of the F1 generation

• Offspring of F1 generation crossed (self-fertilizing: “selfing”)

Mendel’s F2 Progeny

• Showed BOTH phenotypes, not 1

  • (BOTH wrinkled and smooth)

• Self-fertilized F1

• Traits that disappeared in the F1 reappeared in the F2

• Quantified mathematically and analyzed the results

Factors

Mendel’s reasoning that the information to create the trait was present in the F1 in this form

• Now called genes

Mendel’s 3 Postulates

• Unit factors exist in pairs

  • Genetic characters controlled by unit factors (genes)

• Dominance/Recessiveness

  • In a pair of unit factors, one unit is dominant, the other recessive (now we know this does not apply to all traits)

• Segregation

  • Paired unit factors segregate independently during gamete formation

Punnett Squares

• Visualize the segregation and assortment of genes during meiosis and fertilization

• Used to predict possible outcomes in offsprings

Mendel’s First Law

The Principle of Segregation: Genes are in pairs that separate in formation of sex cells

• During the formation of gametes, the pair of alleles segregates randomly such that each gamete will receive one or the other allele with equal likelihood

• During fertilization, the allelic pair is reestablished

Principle of Segregation is explained by:

The segregation of homologous chromosomes in meiosis

Test the rules of segregation

• Perform self-crosses for multiple generations

  • Both traits appear in the offspring

Testcross

Tell us something about an organism that displays a dominant phenotype, but their genotype is unknown. The unknown genotype is either SS or Ss, results of this test tell us which is correct

Allele

Variant form of a gene; has its own DNA sequence

A diploid organism has __ alleles

2

A gamete produced by a diploid organism has __ alleles

1

Locus

Address for an allele on a chromosome

• chromosome number, p or q arm, distance from centromere

Phenotype

Trait (characteristic) observed

Allele

Variant form of a gene. Alleles have different DNA sequences

Genotype

Your combination of alleles. Written in pairs (DD, Dd, or dd)

Homozygous (homozygote)

Both alleles are the same (AA/aa)

Heterozygous/heterozygote

Alleles are different (Aa)

AaxAa ratio

• 3:1 phenotypic ratio

• 1:2:1 genotypic ratio

Aaxaa ratio

1:1 genotypic and phenotypic ratio

Dihybrid Cross

Description of the inheritance of 2 traits

Dihybrid F2 ratio

9:3:3:1 phenotypic ratio

Principle of Independent Assortment is explained by:

The independent separation of homologous chromosomes in Meiosis I

Independent Assortment leads to:

Extensive genetic variation

Expected from crosses of heterozygotes with n independently assorting genes

• Number of segregating gene pairs - n

• Number of different gametes from F1- 2n

• Number of phenotype classes in F2- 2n

• Number of genotypes in F2- 3n

• Frequency of homozygous recessive in F2- (1/4)n

Mendel’s Principal of Uniformity in F1

F1 offspring of a monohybrid cross of true-breeding strains resemble only one of the parents

• Why: Dominance in traits

Mendel’s Principle of Segregation

Recessive characters masked in the F1 progeny of two true-breeding strains, reappear in a specific proportion of the F2 progeny

• Why: During the formation of the gametes, the two factors (genes) for a specific trait segregate, so that each of the gametes receives one of the two factors for each trait

Mendel’s Principle of Independent Assortment

During gamete formation, segregating pairs of alleles for different traits assort independently of each other

• Why: Genes on different chromosomes behave independently in gamete production

Mendel’s Theory of Inheritance

Traits are inherited as particles, offspring receive a particle from each parent (alleles)