Intro. to Mendelian Genetics
Notes: Genetics!

Before DNA and chromosomes were discovered, heredity was one of the greatest
mysteries of science.
o Heredity is the passing of characteristics from parents to offspring.

Genetics is the study of heredity.

The term character refers to inherited characteristics.

The term trait refers to a single form of a character (having purple flowers is a trait).

Gregor Johann Mendel was an Austrian monk who carried out experiments in which
he bred different varieties of the garden pea.
o Through these tests, Mendel became the first to accurately predict patterns
of heredity.

Why the garden pea??
o Flower petals are either purple or white. No intermediate forms.
o The male and female reproductive parts are enclosed within the same flower.
Mating can be controlled by allowing self-fertilization or by transferring the
pollen of one flower to a flower on another plant (cross-pollination).
o The garden pea is small, grows easily, matures quickly, and produces many
offspring. Results can be obtained quickly and there are plenty of subjects to
count.

Mendel began his experiments by doing monohybrid crosses.
o A monohybrid cross is a cross that involves one pair of contrasting traits.

Mendel’s first monohybrid cross experiment overview:

(see foldable for more details)
o Mendel produced a true-breeding P (parental) generation (purple and
white).
o The P generation was cross-pollinated, producing the F1 generation.

All offspring in the F1 generation were purple.
o The F1 generation was allowed to self-pollinate, producing the F2 generation.

Three of the offspring in the F2 generation were purple and only one
was white. This is referred to as a 3:1 ratio.

All of the traits eventually tested by Mendel showed this 3:1
ratio in the F2 generation.

Based on the results of his experiments, Mendel made four hypotheses which now
make up the Mendelian theory of heredity:
1. For each inherited character, an individual has two copies of the gene—one
from each parent.
2. There are alternative versions of genes.

Today, these different version of a gene are called alleles.
3. When two different alleles occur together, one of them may be completely
expressed, while the other may have no observable effect on the organism’s
appearance.

The expressed form is called dominant.

The form not expressed is called recessive.
4. When gametes are formed, the alleles for each gene in an individual separate
independently of one another. Thus, gametes carry only one allele for each
inherited character. When gametes unite during fertilization, each gamete
contributes one allele.

Letters are used to represent the different alleles.
o Dominant alleles are represented by capital letters.
o Recessive alleles are represented by lowercase letters.
o If the two alleles for a particular gene are the same, they are called homozygous.
o If the two alleles for a gene are different, they are called heterozygous.

The set of alleles an individual has for a character is its genotype.

The physical appearance of a character is its phenotype.

The Law of Segregation states that two alleles for a character segregate (separate)
when gametes are formed.

The Law of Independent Assortment states that the alleles of different genes
separate independently of one another during gamete formation.