Mendelian Genetics

The term ‘Mendelian genetics’ typically relates to the outcomes of simple dominant and recessive gene pairings

Shows specific ratios or patterns of inheritance within a lineage of offspring generations (e.g., F1 and F2 generations)

Early ideas of heredity

1) Constancy of species – heredity occurs within the boundary of the species; not so prior to the Middle ages

(Ex: giraffe and minotaur)

View held thru time of Darwin

• Direct transmission of traits – child is formed after hereditary material from all parts of parent’s body come together – blending occurs

Gregor Mendel (1822 – 1884)• Studied garden peas• 1st to use mathematics to examine

outcomes of crosses• Large # of pea varieties with at least

7 easily distinguished traits• Peas are small, easy to grow, short

generation time• Peas can self-fertilize; bisexual

Some definitions for tracking traits via Mendelian inheritance

• Genotype/Phenotype

• Gene/allele

• Dominant/Recessive alleles

• Homozygous/Heterozygous

• P/F1/F2 generations

• Genotypic ratio/Phenotypic ratio

• Monohybrid cross/Dihybrid cross

Mendel conducted studies in 3 stages

1. Self-crossed flowers to make sure white/purple flowered plants were true-true-breedingbreeding

2. Crossed true-breeding plants (white X purple)

(X means “crossed with”)

3. Crossed F1 plants to see traits in future generation (F2 generation)

Mendel came to understand….

• Plant progeny (offspring) did not show blending of traits

• For each pair of alternative traits, 1 was not expressed in F1 generation, but re-appeared in F2 generation

• Traits segregate among the progeny

• Alt, traits are expressed in 3:1 ratio in F2

Punnett squares allow analysis using symbols for gametes and genotypes

Outcome of crossing true breeding purple-flowered and

white-flowered pea plants

F1 progeny: All purple flowered

F2 progeny: 3 purple to 1 white

Self cross each of the F2’s

The Mendelian ratio

• Phenotypic ratio of 3:1

yet,• Genotypic ratio of

1:2:1

When crossing heterozygous individuals of trait controlled by simple dominant/recessive alleles

Mendel proposed a simple model of heredity – 5 parts:

1. Parents transmit “factors’ to offspring2. Each individual receives 2 factors which code

for the same trait3. Not all factors are identical – alternative gene

forms are called alleles4. Alleles do not influence each other as alleles

separate independently into gametes5. The presence of an allele does not insure that its

trait will be expressed

Monohybrid Crosses

genotype: total set of alleles of an individual

PP = homozygous dominant

Pp = heterozygous

pp = homozygous recessive

phenotype: outward appearance of an individual

Monohybrid Crosses

Principle of Segregation – Mendel’s first Law of Heredity

Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization.

Dihybrid Crosses

Dihybrid cross: examination of 2 separate traits in a single cross

-for example: RR YY x rryy

The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait.

Dihybrid cross between two

heterozygous parents

Instead of 4 possible outcomes, there are now 16!!

Dihybrid Crosses

Principle of Independent Assortment: Mendel’s 2nd Law.

In a dihybrid cross, the alleles of each gene assort independently.

Probability – Predicting Results

Rule of addition: the probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities.

When crossing Pp x Pp, the probability of producing Pp offspring is probability of obtaining Pp (1/4), PLUSprobability of obtaining pP (1/4)¼ + ¼ = ½

Probability – Predicting Results

Rule of multiplication: the probability of 2 independent events occurring simultaneously is the PRODUCT of their individual probabilities.

When crossing Rr Yy x RrYy, the probability of obtaining rr yy offspring is:probability of obtaiing rr = ¼probability of obtaining yy = ¼probability of rr yy = ¼ x ¼ = 1/16

Testcross

Testcross: a cross used to determine the genotype of an individual with dominant phenotype

-cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp)

-the phenotypic ratios among offspring are different, depending on the genotype of the unknown parent