Introduction to Genetics

The Work of Gregor Mendel

• Genetics is the study of heredity.• Heredity is the inheritance of traits by

young from their parents.• Gregor Mendel was a monk, born in 1822 • He spent 14 years as a teacher & was in

charge of the garden at the Monastery.• Mendel observed certain traits were

passed generation to generation in pea plants (Pisum sativum).

The Pea Plants

• Pea plants are normally self-pollinating.• This creates clones of the parent plant.• These pea plants are called true-breeding.• Mendel observed seven traits and kept detailed

records.• A trait is specific characteristic.• He would physically cross-pollinate plants to

produce Hybrids.• Hybrids are created by crossing parents of

different traits.

Genes and Dominance• The original set of parents is the P generation.

• The off-spring is the F1 or first Filial generation.

• There is no blending, one trait disappeared only to return in the F2 generation.

• Mendel concluded that factors (genes) are passed from parent to off-spring.

• Each “factor” had two different forms which we now call alleles.

• Some alleles are dominate & some are recessive

• Dominant alleles make a working protein.

• Recessive alleles do not make or make a nonfunctioning protein.

Seed Shape

Flower Position

Seed CoatColor

Seed Color

Pod Color

Plant Height

PodShape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth Green Axial Tall

The F2 generation will show the recessive alleles are only hidden not removed.

Probability in Genetics

• Mendel kept careful records and was able to predict results of crosses using math.

• Probability is the likelihood that and event will occur.

• Probability is the number of events divided by the number of possible outcomes.

• Coin flip is 1 event (heads) 2 outcomes.

• Heads three times is ½ x ½ x ½ = 1/8

Punnett Squares

• Punnett Squares are used to predict all the possible outcomes of a cross.

• The alleles from one parent are placed on top and segregated. The alleles from the other parent are placed on the side.

• Letters are assigned to represent the trait.

• Capital letters are dominant

• Lower case are recessive.

Punnett Vocabulary

• The Phenotype of an organism is its physical appearance for a trait. (Tall)

• The Genotype is the alleles an organism has in its genome. (Tt)

• Homozygous organisms have identical alleles. (TT) or (tt)

• Heterozygous organisms have different alleles in a pair. (Tt)

Punnett Squares

Mendelian Genetics

• Independent Assortment of alleles means the segregation of one pair does not affect the segregation of other alleles.

• Independent Assortment is illustrated in a two factor cross.

• The F1 plants were all heterozygous dominant.

• The F2 plants have a 9:3:3:1 phenotypic ratio.

Summary of Mendel’s Principles

• The inheritance of characteristic is determined by individual units known as genes. Genes are passed from parents to offspring.

• In cases where two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.

• In most sexually reproducing organisms, each adult has two copies of each gene- one form each parent. These genes are segregated from each other when gametes are formed.

• The alleles for different genes usually segregated independently of one another.

Other Allele Relationships

• Incomplete Dominance- occurs when a single dominant does not make enough protein to mask the recessive.

• Codominance- occurs when two alleles both make their protein.

• Multiple Alleles- occurs when there are more than two alleles. (Human Blood type)

• Polygenetic Traits- occurs when a trait has more than one location