Heredity and Mendelian Genetics
Genetics is the study of heredity, or the passing of traits from parents to
offspring.
A trait is an inherited characteristic, such as eye or hair colour.
Experiments in genetics have been happening for thousands of years (farming,
livestock, etc.). Animals, like dogs, have a history of selective breeding, where
specific traits are selected for over time.
The study of genetics began with the work of an Austrian monk, Gregor
Mendel. He studied how 7 different traits were passed from one generation to
the next in pea plants.
Pea plants normally self-fertilize (male and female gamete from the same
plant), but could be cross-pollinated by hand. Mendel kept careful track of the
traits in the plants he bred, and formed laws of how genetic information was
passed.
Mendel got the same results for each trait he tested. One trait disappeared in
the 1st generation and reappeared in the 2nd. To explain these results, Mendel
proposed that:
- each plant had 2 “factors” that act as sets of instructions for each
characteristic
- each parent donates one of these factors to the offspring
- one factor or trait may dominate the other if it is present
**We can link these ideas to what we now know about chromosomes, meiosis,
and fertilization.
Homologous chromosomes carry the genes for traits in the same relative
positions. The different versions of the same gene are called alleles.
Alleles can be dominant or recessive. The presence of the allele will determine
if that characteristic is expressed. (Mendel’s Law of Dominance)
Dominant – if at least 1 dominant allele is present, its characteristics will be
visible. *Represented by an upper case letter.
Recessive – characteristic will only be visible if both recessive alleles are
present. *Represented by the corresponding lower case letter.
Example: Trait: Flower colour
Alleles: Purple – ___ white – __
Each parent/cell starts with 2 alleles for a trait. There are several possible
combinations:
 allele combinations are called a genotype.
 appearance of the trait is its phenotype
If the combinations have the SAME alleles, it is called homozygous.
If the combinations have DIFFERENT alleles, it is called heterozygous.
During meiosis, these alleles are separated into the gametes they can pass on.
This is called the law of segregation.
Example: Parent cell
Gametes formed after meiosis:
Genetic crosses (‘x’) involve deliberate breeding between a male and female.
A monohybrid cross involves a difference in only 1 trait being studied.
We can use a Punnett square to show and predict the inheritance of a trait in
offspring (*see steps on page 137). The frequencies of each possible offspring
combination are expressed as genotypic and phenotypic ratios ( #, a/b or % ).