Introduction to Genetics
Who was Gregor Mendel?
• He was known as the
“FATHER OF GENETICS”
• He discovered how traits were inherited
•GENETICS – study of
heredity
•HEREDITY – the
passing of traits from
parents to offspring
Mendel’s Peas
• Mendel did his study
on pea plants
• Pea plants have
many traits
(tall/short, purple
flowers/white flowers)
• Pea plants can be
self-fertilized or
cross-fertilized
Mendel’s Peas
He studied seven traits.
Mendel's
Procedures
Types of Plants
True-Breeding - these plants always
create plants that look like themselves
when crossed. AKA: Purebred
Tall=Tall x Tall or Short =Short x Short
Hybrids – offspring from crosses of two
true-breeding plants (with different
traits)
Hybrid=Tall x Short
Mendel’s
Experiments
• He experimentally
crosses different
varieties to
develop hybrids.
• He then crossed
the hybrids and
analyzed the
results.
P
F1
F2
Genes and Alleles
Mendel discovered that each trait is
controlled by two factors (alleles)
Genes* – factors that determines traits
So…. Alleles are two forms of the
same gene ex. B vs b
*FYI--Danish botanist Wilhelm Johannsen coined the
word "gene“ in 1909.
Explaining the Cross
When a parent makes sperm or eggs, their
genes (2 factors) separate
(LAW OF SEGREGATION)
The GAMETES (egg or sperm) contain one
allele for each trait. (let T be tall and t be short)
T
t
T
t
Dominance
Some traits were dominant over others.
PRINCIPLE OF DOMINANCE
Tall x Short = all tall offspring (hybrids)
•Tall is the dominant trait
•Short is recessive trait (hidden)
or Tall allele is dominant to the Short allele
Mendel's -First Law
The Principle of Dominance
In a cross of parents that are pure for
contrasting traits, only one form of the trait
will appear in the next generation.
Offspring that are hybrid for a trait will have
only the dominant trait in the phenotype.
“One trait will “hide” the other trait.”
Mendel's Second Law
The Law of Segregation
During the formation of gametes (eggs or
sperm), the two alleles responsible for a trait
separate from each other.
Alleles for a trait are then "recombined" at
fertilization, producing the genotype for the
traits of the offspring.
Mendel studied the appearance of his peas
but did not know about genes.
•PHENOTYPE - what it looks like,
the organisms physical description.
(tall or short)
•GENOTYPE - the genes an
organism has, represented by letters,
(TT, Tt, tt)
Check for understanding
1. A one-eyed purple people eater is crossed with a
two eyed purple people eater. All of their offspring
have two eyes. Which trait is dominant?
2. If you use the letter E for this
gene. What is the genotype of
the offspring?
Are these offspring the F1 or
F2 generation?
4. If you crossed the offspring
with each other? How many ofthe new offspring
would you expect to have two eyes?
Mendel's Third Law
The Law of Independent Assortment
Ma-Ma-Ma-Ma-Mendel's Third Law
The Law of Independent Assortment
Alleles for different traits are distributed to sex cells (&
offspring) independently of one another.
Vocabulary for this section
• Mendel, dominant, recessive, trait, gene,
genetics, heredity, allele, heterozygous,
homozygous, hybrid, true breeding/pure
breeding, test cross, P1, G1, F1, (remember the
1’s are subscripts), Punnett square, phenotype,
genotype (know ratios for phenotype/genotype),
classic Mendelian ratio, gamete, zygote, law of
independent assortment, law of segregation
• This material is covered in ch. 6 – specifically
6.3, 6.4 and 6.5