Chapter 8 Mendel & Heredity Guided Notes
Section 2: Mendel’s Theory
Objectives
 Describe the four major hypotheses Mendel developed.
 Define the terms homozygous, heterozygous, genotype, and phenotype.
 Compare Mendel’s two laws of heredity.
A Theory of Heredity

Each pea has two separate “heritable factors” for each trait- one from each parent.

When gametes form, each receives only one out of the two factors for each trait.

When gametes fuse, the offspring has two factors (now called genes for each trait.
Mendel’s Hypotheses – Mendelian Theory of Heredity:
1. For each inherited trait, an individual has two copies of the gene— one from each
parent.
2. There are alternative versions of genes. Different versions are called its 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
of the trait is dominant. The trait that was not expressed when the dominant form of the trait was
present is 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 trait.
Mendel’s Findings in Modern Terms

Dominant alleles are: indicated by writing the first letter of the trait as a capital letter.

Recessive alleles are: indicated by writing the first letter of the dominant trait, but the letter is
lowercase.

If the two alleles are the same, the individual is said to be: homozygous.

If the alleles are different, the individual is: heterozygous.

The set of alleles that an individual has: genotype

The physical appearance of a trait: phenotype
The Laws of Heredity

The Law of Segregation (describes the behavior of chromosomes during meiosis).
o the two alleles for a trait segregate (separate) when gametes are formed

The Law of Independent Assortment
o
the alleles of different genes separate independently of one another during gamete formation