Genetics: The Science of Heredity
Table of Contents
Mendel’s Work
Probability and Heredity
The Cell and Inheritance
The DNA Connection
Genetics: The Science of Heredity
Mendel’s Work pg 76
His observations revolutionized the study of
HEREDITY
He asked why his pea plants had different
characteristics
Genetics: The Science of Heredity
BEGIN NOTES
Mendel’s Laws of Heredity
1. Why Mendel Succeeded
a. Gregor Mendel-Father of Genetics
b. 1st studies of heredity-the passing of
characteristics (traits) to offspring such as bone
structure, and eye color.
c. Genetics-Study of Heredity
Genetics: The Science of Heredity
Mendel’s Experiment
2. Mendel’s Experiment Crossing Pea Plants
A. crossed plants with contrasting traits (Tall and
short).
B. Controlled the fertilization process of this peas
plants. He removed pollen from a flower on one
plant and transferred it to another
C. Started experiment with Purebred plants (Tall
from Tall plants and short from short plants)
D. Purebred-organisms are the offspring of many
generations that have the same trait.
Genetics: The Science of Heredity - Mendel’s Work
Crossing Pea Plants
Gregor Mendel crossed pea plants that had different traits.
The illustrations show how he did this.
Genetics: The Science of Heredity
P Generation (purebred) trait
Purebred Tall
TT
Purebred short
tt
cross fertilization
Capital Letter
T
Lower Case Letter t
= Dominate Trait
= recessive trait
Genetics: The Science of Heredity
3. The 1st Generation
A. Mendel crossed two plants- 1 tall and 1
short. (purebred Tall and short) called the
parental generation (P Generation)
B. The off spring from the P Generation were
all called the 1st filial generation(F1
Generation)
C. All the offspring were tall (the short plants
were totally excluded)
WHY???????????????????????????
Genetics: The Science of Heredity
F1 Generation
TT
tt
cross fertilization
Created the F1 Generation
Tt, Tt, Tt, Tt
All the plants are _________________?
Capital Letter
T
Lower Case Letter t
= Dominate Trait
= recessive trait
Genetics: The Science of Heredity
4. The 2nd Generation
A. 1. Next, Mendel crossed two plants from the F1
Generation
B. 2. The offspring from this cross are called the 2nd filial
generation (F2 Generation)
C. Mendel found that ¾ of the offspring were Tall and ¼
were short (the short plants reappeared).
Genetics: The Science of Heredity
F2 Generation
Tt
Tt
cross fertilization
Created the F2 Generation
Tt, TT, TT, tt
3 out of 4 plants are _________________?
1 out of 4 plants are _________________?
Capital Letter
T
Lower Case Letter t
= Dominate Trait
= recessive trait
Genetics: The Science of Heredity - Mendel’s Work
Mendel’s Experiments
In all of Mendel’s crosses, only one form of the trait
appeared in the F1 generation. However, in the F2
generation, the “lost” form of the trait always reappeared in
about one fourth of the plants. (DRAW PICTURE BELOW)
Genetics: The Science of Heredity
Brain Pop & Quiz
Brain Pop Heredity
Genetics: The Science of Heredity
Alleles
~Alternate forms of a gene are called alleles.
~Alleles are represented by a one or two letter symbol.
(e.g. T for tall and t for short)
~These two alleles are now known to be found on copies of
chromosomes-one from each parent.
Genetics: The Science of Heredity - Mendel’s Work
Dominant and Recessive Alleles
Mendel studied several traits in pea plants.
Genetics: The Science of Heredity
Dominant and Recessive Alleles
Dominant: A gene that is always express and hides others
It is present and the trait always shows up and is always
represented by a capital letter.
Recessive: A gene that is only expressed when a dominant
gene is not present. A recessive trait will only be
expressed if both alleles are recessive. They are
represented by a lower case letter.
Genetics: The Science of Heredity
Genetics
Dominant and Recessive Genes
A dominant gene will always
mask a recessive gene.
A “widows peak” is dominant,
not having a widows peak is recessive.
If one parent contributes a
gene for a widows peak, and the
other parent doesn’t, the offspring will have a widows peak.
Widows Peak
Genetics: The Science of Heredity
Genetics
~Punnet Square - A tool we use for predicting the traits
of an offspring
~Letters are used as symbols to designate genes
~Capital letters are used for dominant genes
~Lower case letters are used for
recessive genes
~Genes always exist in pairs
Genetics: The Science of Heredity
Genetics
All organisms have two copies of each gene, one
contributed by the father, the other contributed
by the mother.
Homozygous - Two copies of the same gene
Heterozygous - Two different genes
Genetics: The Science of Heredity
Genetics
For the widows peak:
WW - has a widows peak
dominant
Ww - has a widows peak
ww - no widows peak
recessive
Homozygous
Heterozygous
Homozygous
Genetics: The Science of Heredity - Probability and Heredity
A Punnett Square
The diagrams show how to make a Punnett square. In this
cross, both parents are heterozygous for the trait of seed
shape. R represents the dominant round allele, and r
represents the recessive wrinkled allele.
Genetics: The Science of Heredity - Probability and Heredity
Phenotypes and Genotypes
An organism’s phenotype is its physical appearance, or
visible traits, and an organism’s genotype is its genetic
makeup, or allele combinations.
Genetics: The Science of Heredity - The Cell and Inheritance
Meiosis
During meiosis, the chromosome pairs separate and are
distributed to two different cells. The resulting sex cells have
only half as many chromosomes as the other cells in the
organism.
Genetics: The Science of Heredity - The Cell and Inheritance
Punnett Square
A Punnett square is actually a way to show the events that
occur at meiosis.
Genetics: The Science of Heredity - The Cell and Inheritance
A Lineup of Genes
Chromosomes are made up of many
genes joined together like beads on a
string. The chromosomes in a pair may
have different alleles for some genes
and the same allele for others.
Genetics: The Science of Heredity - The DNA Connection
Mutations
Mutations can cause a cell to produce an incorrect protein
during protein synthesis. As a result, the organism’s trait, or
phenotype, may be different from what it normally would
have been.
Genetics: The Science of Heredity
End of Section:
Graphic Organizer