Mendel’s TheorySection 8-2

Ag Biology

Before Mendel’s Experiments People thought that

offspring were a blend of the parents characteristics.

ExampleCross a short plant and a

tall plant to produce a medium plant in height.

Mendel’s Hypotheses

Mendel developed four hypotheses from his experiments that later became the Mendelian theory of heredity.

Hypothesis #1

For each inherited trait, an individual has two copies of the gene- one from each parent.

Hypothesis #2

There are alternative versions of genes. Different versions of a gene are called

alleles.

Hypothesis #3

When two different alleles occur together, one of them is expressed, while the other may have no observable effect on the organism’s appearance.

Expressed form of the trait is dominant. Dominant alleles are represented by capital letters.

Not expressed traits when a dominant form is present are called recessive. Recessive alleles are represented by lower case letters.

Hypothesis #4 When gametes are formed,

the alleles for each gene in an individual separate independently of one another.

Gametes only carry one allele for each inherited trait.

When gametes unite during fertilization, each gamete contributes one allele.

Terminology

Homozygous- when an individual has the two alleles of a particular gene that are the same

RR rr Heterozygous- when an individual has

different alleles of a particular gene Rr

Terminology

Genotype- the set of alleles an individual has

Phenotype- the physical appearance of a trait; what is physically expressed

Laws of Heredity

During meiosis (gamete production), Alleles for a trait separate (Law of segregation)The inheritance of one trait does not influence

the inheritance of different trait. (Law of Independent Assortment)