Patterns of Inheritance CH 12

Mendelian Genetics

Study of inheritance patterns established by Gregor Mendel.

Mendel's Experimental Method

1. Produce true-breeding strains for each trait being studied

2. Cross-fertilize true breeding stains having alternate forms of trait (PP x pp)

3. Allowed hybrid (Pp) offspring to self-fertilize for several generations and counted number of offspring showing each trait form

Monohybrid Cross

Cross used to study only two variations of a single trait

• One dominant, the other recessive

F₁ generation

Offspring produced from cross b/w two true-breeding strains

• all offspring resemble/expressed same parent (dominant trait)

• No intermediate characteristics = no blending inheritance

F₂ generation

Produced from self-fertilization of F1 plants

• Dominant masked other expressions in F1, but in F2 recessive trait reappeared

  • Counted in 3:1 ration,

  • really 1 (PP): 2(Pp): 1(pp)

1:2:1 ratio of Monohybrids

F2 plants

• ¾ with dominant form (2 hetero, 1 homo)

• ¼ recessive form (homo)

Mendel’s Conclusion About Traits

• Traits are intact and discrete (don’t mix)

• Each pair of traits has a dominant and recessive form

• Pairs of alternative traits examined were segregated among offspring of particular cross

• F2 generation ratio of ¾ dominant, 1.4 recessive

Principle of segregation

Two alleles from a gene segregate/separate during formation of gametes (parent has Aa→ A and a separate and go to separate gametes)

Then alleles will combine randomly with other alleles from other parent

• Physical basis for allele segregation is movement of chromosomes during meiosis (crossover) (after Mendel’s time)

Punnett Square for Mendel’s Cross

• F1 are all purple heterozygotes (Pp)

• F2 offspring produce

  • PP homozygous dominant (purple)

  • Pp heterozygous (purple)

  • pp homozygous recessive (white)

Dihybrid Cross

Study of two variations of two traits in a single cross

• Mendel produced true-breeding lines each with two traits (RRYY x rryy)

  • F1 (RrYy) all show dominant phenotype

  • F2 9:3:3:1 ratio

9:3:3:1 ratio of Dihybrids

• 9(R_Y_), 3(R_yy), 3(rrY_), 1(rryy)

• R: Round, r: wrinkled, Y: yellow, y: green

Principle of Independent assortment

• Segregation of different allele pairs is independent (seed color is independent of seed shape)

• Independent alignment of different homolog pairs during metaphase = independent segregation of allele pairs

(after crossing over, allele pair randomly in meta I)

Testcross

Used to determine genotypes of unknown phenotype

• Cross unknown with homozygous recessive (pp)

  • Phenotypic ratio among offspring are different depending on genotype of unknown ar different (determine unknown)

Probablity of Monohybrid

• Rule of Addition: probability of two mutually exclusive events is sum of their individual probabilities

  • Pp x Pp; probability of Pp production

  • Pp ¼ + pP ¼ = ½ Pp

• Rule of Multiplication: Probability of two independent events is the product of individual probabilities

  • Pp x Pp; probability of pp

  • Prob from father (1/2) x probe from mother (1/2)

  • ½ x ½ = ¼

Dihybrid probablities

Based on Monohybrid probabilities; dihybrid is equivalent to two independent monohybrid cross

• Probability of 1 gene (rr : ¼ )

• Probability of 2 gene (yy: ¼ )

• Probability of both genes together ( ¼ x ¼ = 1/16)

Dominant Trait

Visible trait in F1, covers up recessive trait when present (AA)

Recessive Trait

Alternative trait in F1, only expressed in F2, must be homozygous to be expressed (aa)

Phenotypic Ratio

Both monohybrid and dihybrid; 3:1 (dominant to recessive)

Genotypic Ratio

Monohybrid; 1:3:1 (Aa, 2Aa, aa)

Dihybrid; 9:3:3:1 (9R_Y_, 3R_yy, 3rrY_, rryy)

Allele

alternative forms of the gene (recessive, dominant, or multiple like ABO)

Homozygous

two of the same allele

Heterozygous

different alleles

Genotype

individual’s complete set of alleles

Phenotype

individual’s physical appearance (based on genotype)

Pleiotropy (extension)

allele that has more than one effect on the phenotype

• difficult to predict b/c a gene that affects one trait often performs other unknow functions

(ex. sickle cell anemia; multiple symptoms track back to one defective allele)

Why where pea plants used by Mendel?

• Could produce hybrids/ many varieties available

• small plants and easy to grow

• Can self-fertilize or cross-fertilize

True-breeding Parent

Produces one type of gamete consistently; homozygous dominant or recessive

Homozygous Dominant

Genotype with two dominant alleles (PP).

Homozygous Recessive

Genotype with two recessive alleles (pp).

Multiple Alleles (extension)

more than two alleles for a gene in a population (usually the case for genes in outbreeding populations)

(ex. ABO blood types in humans)

Incomplete Dominance (extension)

Heterozygote shows intermediate phenotype.

Codominance (extension)

Both alleles expressed equally in phenotype.

Epistasis (extension)

Action of one gene obscures/masks effects of another gene

(ex. color of Labrador retrievers; controlled by two genes [brown and extension], interaction b/w brown and extension result in three coats not four)

ABO Blood Group

Example of multiple alleles and codominance. (A & B are dominate, and can be codominant together, O is recessive)

Environmental Influence (extension)

Environment alters phenotypes from the same genotype.

• Phenotypes plasticity: different phenotypes form same genotypes due to environmental conditions

(ex. siamese cat coats over 30^oC)

Mendel's Assumptions

• Each trait as one dominant and one recessive version

• Pairs of alternative traits are segregated among offspring (seed color, and texture traits stayed independent for each other)

• Alternative traits in F2 generations expressed in ¾ dominant, ¼ recessive

Gamete Types

Formed during meiosis from parent genotypes; one variation of a gene given to offspring from each parent

Sickle Cell Anemia

Example of pleiotropy multiple symptoms from one defective gene