Mendel and Heredity
I. The Origin of Genetics
A. Mendel’s Studies of Traits
1. Gregor Johann Mendel-(1822-1884) Austrian
Augustinian priest and scientist often called the
“father of modern genetics” for his study of the
inheritance of traits in pea plants.
2. He bred varieties of the garden pea; Pisum sativum
in an attempt to understand heredity.
3. Useful Features in Peas – The garden pea is a good
subject for studying heredity because:
a. it has several traits (7) that exist in two clearly
different forms.
b. self-fertilization and cross-fertilization are
possible.
c. the garden pea is easy to cultivate and produces
many offspring.
B. Traits Expressed as Simple Ratios
1. Monohybrid Crosses – crosses that involve one
pair of contrasting traits ( Hh x Hh)
2. Mendel’s Results
a. Mendel observed that contrasting traits appear
in offspring according to simple ratios.
b. In Mendel’s experiments, only one of the two
contrasting forms of a trait was expressed in the F1
generation. (purebred x purebred)
c. The other form reappeared in the F2 generation
in a 3:1 ratio.
Purebred
smooth
Purebred
wrinkled
II. Mendel’s Theory
A. A Theory of Heredity
1. Mendel’s Hypothesis
a. Different versions of a gene are called alleles.
b. An individual usually has 2 alleles for a gene,
each inherited from a different parent
2. Mendel’s Findings in Modern Times
a. Individuals with the same two alleles for a gene
are homozygous
b. those with two different alleles for a gene are
heterozygous
3. Genotype and Phenotype
a. The set of alleles that an individual has is called
its genotype.
b. The physical appearance of a trait or outward
expression of the genotype is called the phenotype.
B. The Laws of Heredity
1. The Law of Segregation - the two alleles for a trait
separate when gametes are formed (meiosis)
2. The Law of Independent Assortment – two or more
pairs of alleles separate independently of one
another during gamete formation. (meiosis)
Each allele (S or s, Y or y)
can be combined for 4
possibilities. (SY, Sy, sY, sy)
when combined with another
dihybrid, there are 16
possibilities
III. Studying Heredity
A. Punnett Squares
1. Function of Punnett Squares – The results of
genetic crosses can be predicted with the use of
Punnett squares.
2. One pair of Contrasting Traits – Punnett squares
can be used to predict the outcome of a
monohybrid cross with one pair of contrasting traits.
3. Determining Unknown Genotypes – A test cross
can be used to determine whether an individual
expressing a dominant trait is heterozygous or
homozygous.
Punnett Square
B. Outcomes of Crosses
1. Probability of a Specific Allele in a Gamete
a. The probability of a specific allele in a gamete can
be predicted with the use of probabilities.
b. For a gene with two alleles, the chance of
contributing one allele or the other to the gamete is ½.
2. Probability of the Outcome of a Cross
a. The results of a genetic cross can be predicted
with the use of probabilities.
b. To find the probability that a combination of two
independent events will occur, multiply the separate
probabilities of the two events.
Probability of a Coin Toss
Probability of heads
Probability of tails
1 of 2 or ½
1 of 2 or ½
Probability of heads twice in a row
½x½=¼
Probability of heads four times in a row
½ x ½ x ½ x ½ = 1/16
C. Inheritance of Traits
1. Pedigrees – A trait’s pattern of inheritance within a
family can be determined by analyzing a pedigree.
2. Patterns of Inheritance – Scientists use pedigrees to
determine whether a trait is autosomal or sexlinked, dominant or recessive, and heterozygous
or homozygous.
The End