Genetics: The Science of Heredity
Transcript Genetics: The Science of Heredity
Genetics: The Science of
Who was Gregory Mendel?
• Gregory Mendel was an
He lived between 1822 to
When he was a young boy, he
was interested in the plants
that grew on the family farm.
Mendel was the gardener of
the monastery where he lived
as a monk. He did
experiments on hundreds of
pea plants there.
He kept careful records and
used mathematics to make
sense of what he observed.
• Heredity is the passing
of traits from parents to
with heredity of certain
traits found in peas.
Mendel studied each
trait separately and
patterns in the way
traits are inherited in
Mendel’s work has
become the basis of
genetics, the study of
Mendel’s Pea Experiments
• Mendel chose pea plants
because their traits were
easy to see and distinguish.
He crossed plants with two
different traits, for example
purple flowers with white
He started his experiments
with purebred plants.
Purebred plants ALWAYS
produce offspring with the
same trait as the parent.
For example, if the parent is
tall, all offspring will be tall.
If the parent is short, all
offspring will be short.
Some Pea Traits that Mendel
• Mendel called the parent plants the P generation.
• He called the offspring from the parents the F1
F is from the Latin word, filial, which means son.
When Mendel crossed pure pea plants with purple
flowers with pure pea plants with white flowers, all
the F1 generation had purple flowers.
• When he crossed the F1 generation peas with one
another, only some of the offspring had purple
flowers. These formed the F2 generation.
Mendel found that in the F2 generation, ¾ of the
plants had purple flowers and ¼ of them had
Lesson 1 Review
• Who was Gregory Mendel?
• Why did he choose peas for his
• What is heredity?
• What is genetics?
• What is a purebred plants?
• What is the P generation?
• What is the F1 generation?
• What is the F2 generation?
Dominant and Recessive Traits
• It seemed to
Mendel, that for
in peas, one trait
was stronger than
He called the
“stronger” one, the
He called the
“hidden” one, the
Genes and Alleles
• The traits of peas (and
yours) are controlled
by factors that
scientists call genes.
• You inherit your genes
from your parents.
• The different forms of
a gene are called
• You inherit a
combination of two
alleles from your
Dominant and Recessive Traits in
• For each of the 7
traits that Mendel
studied in peas, there
is a dominant allele
and a recessive allele.
If a plant inherits
both a dominant allele
and a recessive allele,
the dominant allele
masks the recessive
Some Pea Traits that Mendel
Lesson 2 Review
• What did Mendel find to be the same with all 7
traits of the pea plant that he studied?
What are genes?
What are dominant alleles?
What are recessive alleles?
What happens if a pea plant inherits two
dominant allele of the same gene?
What happens if a pea plant inherits a dominant
allele and a recessive allele of the same gene?
What happens if the pea plant inherits two
recessive alleles of the same gene?
Probability and Genetics
• Probability is the likelihood
that a particular event will
The laws of probability
determine what is likely to
occur, not what does occur.
Mendel was the first
scientist that applied the
principles of probability to
• Punnett square is a table
that shows all the possible
combinations of alleles
that can result when two
• Using Punnett square,
geneticists can predict the
probability of occurrence
of a particular trait.
• The allele that each parent
will pass to its offspring is
based on chance, just like
tossing a coin.
Genotypes and Phenotypes
• Genotype: Indicates the
alleles that the organism
has inherited regarding a
particular trait. THE
• Phenotype: The actual
visible trait of the
organism. THE WORDS:
DETERMINED BY THE
Bb = Brown eyes
bb = Blue eyes
Homozygous and Heterozygous
• Homozygous: An
organism with two
identical alleles for a
trait (a purebred
organism that has
two different alleles
for a trait (a hybrid
• We will practice some monohybrid and
Law of Independent Assortment
• Gregor Mendel came to the conclusion that
alleles for different traits separated
independently when gametes are formed.
Each allele goes its own way as it forms the egg
or sperm cell.
– A mother with brown eyes (Bb) and dimples
(Dd) can produce a gamete that carries Bd, BD,
bD or bd—the separation is independent and
does not “link” with another trait—it is random.
Are all things Mendel?
• Plants and other organisms follow the
same concepts as Mendel, however, as
the organism increases in complexity so
does the process and outcome
• There are some exceptions to strict
• What it probability? How is it related to
What is the Punnett Square? How is it
helpful to geneticist?
What is a genotype?
What is a phenotype?
What is a homozygous organism?
What is a heterozygous organism?
Other Patterns in Genetics
• A cross does not possess a specific
dominant trait it is a blend of both alleles.
– Example: 4 o’clock plants
• A red (RR) plant is crossed with a white (WW)
– Do the Punnett Square in your notes
– What is the resulting genotype? Phenotype?
• You should have gotten 4 RW plants in
• This results in 4 plants that are a perfect
mix of the red and white plants—neither
expresses a dominant trait and all plants
• In codominance, the
alleles are neither
allele is masked by
the other and both
Is both white and red
• Many genes exist in several forms and are
called multiple alleles.
– Having only 2 alleles for a trait is an
exception, not a rule (there are more
multiples out there)
– Blood and rabbit fur are examples
• Traits that involve multiple gene pairs to
determine the genotype (how it looks)
– Example: eye color
• 2 layers of the eye have 4 alleles for each
• The order of the pairings and the layer in
which they are located determines the color
of your eyes
• Yes, the environment can impact the genes.
• Environmental factors, such as the colors during
a season, can influence the expression of a gene
(survival of the fittest)
– Example: a species of the white butterfly has wings
with different color patterns if they hatch in the
summer than they do if they hatch in the spring
– Why does this makes sense?
• It takes a certain amount of heat to fly. The darker
lines in the spring helps the butterfly produce more
• What is incomplete dominance?
• Why does a Roan cow have both red and white
hair in its coat?
What is a multiple allele? Is this an exception or
the most common type of allele pattern seen in
What does the word polygenics mean? What
types of traits are polygenic?
How can the environment impact traits? Can you
give an example that is different from the one in
Chromosomes and Meiosis
How can I remember the difference
between mitosis and meiosis?
• Mitosis happens in “my toes” (somatic
cells) and Meiosis happens in “my
ovaries” (where female sex cells
What are Chromosomes
• Chromosomes are made of
Chromatin is found in the
nucleus of cells and is made of
about 40% DNA and 60%
The DNA of a chromosome is a
very long, double stranded fiber
that extends unbroken through
the entire length of the
The amount of information a
chromosome contains would fill
about 280 printed books of 1000
Chromosomes come in different
The Number of Chromosomes
• Most cells in different organisms
have between 10 to 50
chromosomes in their nucleus.
Humans have 46, which consists
of 23 pairs.
Each chromosome in a pair is
nearly identical to the other
chromosome in the pair.
Humans that have even one
missing or one extra
chromosome usually die before
birth or have serious defects.
Down Syndrome happens when a
person is born with an extra copy
of chromosome number 21.
The Number of Chromosomes in
• Humans: 46
• Dogs: 78
• Silkworms: 56
• Hamster: 44
Chromosomes in Eggs and Sperms
• In 1882, Belgian scientist Pierre-
Joseph van Beneden discovered
that the number of chromosomes in
sex cells (eggs and sperms) are half
the number than in the other cells.
Each of your cells have 46
chromosomes. You inherited 23
from your mother’s egg cell and 23
from your father’s sperm cell.
Diploid Vs. Haploid
• Autosomal cells (somatic cells) are all
diploid (full number of chromosomes)
– 46 for humans or 2n
• Sex cells are haploid
– 23 for humans or 1n
– Why does this make sense?
• 1 egg (23 chromosomes) combines with 1
sperm (23 chromosomes) to produce a
zygote. The zygote has a full set of
chromosomes (46 chromosomes).
• In other words:
– A haploid (1n) cell combines with another
haploid cell (1n) to create a diploid cell
• 23 + 23 = 46
Before a cell can become an egg it has 46
chromosomes. What does it do?
• A reductive-division called Meiosis
– The cell makes a copy, reduces the number of
cells by a division creating 4 egg or sperm
• 1 cell makes 4 egg or 4 sperm
• Meiosis is the process
of cell division in
which sex cells (eggs
and sperms) are
Meiosis results in 4
daughter cells from
each parent cell, each
with half the number
found in the parent
Mitosis and Meiosis
2 divisions: meiosis 1 and meiosis 2
• During meiosis 1 a division that is like
mitosis takes place
– Prophase: replicated chromosomes pair with
homologous chromosomes (the same) and
form a tetrad (4 chromatids)
– Chromatids are laying close together and
pieces switch place. This is called crossingover. This creates new combinations of
2 Mitosis = 1 Meiosis
Results from 1 meiotic division:
• 4 viable sperm cells (males)
• 1 viable egg (females)
– 3 “duds” due to lack of cytoplasm
Comparing Meiosis and Mitosis
have the same
as the parent
Two cell divisions
Four daughter cells
Daughter cells have half the
number of chromosomes as
the parent cells.
Can Genes Ever Link Together?
• Yes, Thomas Hunt Morgan discovered this
while working with fruit flies.
– Genes on the same chromosome tend to link
– Crossing over occurs (2 chromosomes lay
close to each other in meiosis) and pieces are
switched between chromosomes
Lesson 5 Review
• How is the number of chromosomes in sex
cells different than in other cells?
• What is Meiosis?
• During Meiosis, how many daughter cells
are produced from each parent cell?
• During Meiosis, how many cell divisions