BI SC 002 LECTURE 2—GENETICS
Draft: October 26, 2010
Darwin knew that traits were passed from generation to generation . . . this was the basis of
However, he didn’t know how.
At the same time as Darwin (but unknown to him or
anyone else in the world), Gregor Mendel, a Czech monk, discovered the principles of genetics.
Mendel studied pea plants for generations and combined botany with mathematics to determine
ratios of offspring.
He was published in 1866 in an obscure local journal, but his research wasn’t
really found until the early 1900s.
Mendel determined that traits are spread from parent to offspring in discrete units.
called these units “factors”; we now refer to them as
The genes are passed through the
gametes (sperm and egg).
Through Mendel’s research, he found that each person has 2 copies of
the same gene—one from mom and one from dad.
Mendel also found that only one copy of each
gene is passed on to offspring.
For one particular gene, there can be several different variations.
For example, we have genes
for eye color, yet people have different versions (brown, green, blue).
Different forms of the
same genes are called
Let’s say for instance that for the gene for eye color that a person has an allele for brown eyes
and an allele for blue eyes.
What color eyes do they have?
They could have brown eyes, they
could have blue eyes, they could have a blending of brown and blue, they could have one brown
eye and one blue eye . . . the possibilities are endless.
Which allele(s) get(s) expressed?
Alleles come in two types.
are alleles that are fully expressed; that is, having
one dominant allele present in the set of two makes a person express the dominant trait.
doing genetics problems, we represent dominant alleles with a capital letter.
are alleles that, when paired with a dominant, are hidden.
In other words, to express the
recessive trait, a person must have 2 recessive alleles (no dominant blocking it out).
represent recessive alleles with a lower case letter in genetics problems.
Because individuals have 2 copies of each gene, individuals will have 2 alleles for the same
Thus, there can be 3 possible
(actual allelic makeup) for an individual.
brown eyes are dominant to blue eyes, we can represent the brown allele as B and the blue allele
as b (always keeping the same letter for each gene—if in the trait of hair color red is dominant to
blonde, we would call red R and blonde r).
There are three possible combinations of genotypes:
BB—this individual has two brown (dominant) alleles.
This would give the individual a
(physical trait manifested by the genotype) of brown eyes
bb—this individual has two blue (recessive) alleles.
There is no dominant masking the recessive,
so this individual would have blue eyes
Both the BB and bb genotypes have two of the same alleles.
When an individual has two of the
same alleles for a gene, we call it