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GENETICS AND HEREDITY
Heredity and Genetics
• Heredity is the passing of physical
characteristics from parents to offspring.
• Genetics is the scientific study of heredity.
Mendel
• Gregor Mendel, an
Austrian monk of the
nineteenth century,
made the discoveries
that is the foundation
of our knowledge of
genetics.
Mendel’s Experiments
• He did his
experiments because
he wondered why pea
plants had different
characteristics.
– Tall and short plants
– Green and yellow
seeds
– Round (smooth) and
wrinkled seeds
• Each different form of
a characteristic is
called a trait.
Mendel’s Experiments
• Fertilization is the process where an egg cell
and a sperm cell join together.
• Pollination is the process of the pollen reaching
the pistil of a flower.
• Pea plants are usually self-pollinating, meaning
the pollen of a flower lands on the pistil of the
same flower.
• Mendel developed a method of crosspollination.
– He removed pollen from the flower of one plant and
then brushed the pollen onto a flower on a second
plant.
Crossing Pea Plants
• Mendel decided to cross plants with opposite
traits, for example tall and short plants.
• He began his experiments with purebred plants.
• Purebred organisms are the offspring of many
generations that have the same trait.
– Example: Purebred short plants always come
from short parent plants.
– Purebred individuals are also called “true
breeding” individuals.
The F1 Offspring
• In Mendel’s experiments, the purebred parent
plants are called the parental generation or P
generation.
– Example: Mendel crossed a purebred tall plant with a
purebred short plant.
• The offspring of the P generation are called the
first filial (Latin for daughter or son), or F1
generation.
– Example: In Mendel’s F1 generation, all of the plants
were tall.
– Even though one of the parents was short, that trait
seemed to disappear in the F1 generation.
The F2 Generation
• Mendel let the fully grown F1 plants to selfpollinate.
• The second filial, or F2 generation were a
mix of tall and short plants.
• The short trait reappeared even though
none of the parents were short.
• After counting the F2 plants, Mendel noted
that ¾ of the plants were tall and ¼ of the
plants were short.
Experiments with Other Traits
• Mendel did hundreds of crosses looking at
other traits.
• In all of his crosses, only one form of
the trait appeared in the F1 generation,
but that trait reappeared in the F2
generation in about ¼ of the plants.
Dominant and Recessive Alleles
• Because of his experiments, Mendel concluded
that individual “factors” must control the
inheritance of traits.
• He also reasoned that the factors that control
each trait exists in pairs, one factor from each
parent.
• Based on the results of his experiments, Mendel
concluded that one factor in each pair can mask,
or hide, the other factor.
– Example: The tallness factor masked the shortness
factor.
Genes and Alleles
• Today, scientists call the factors that
control a trait a gene.
• The two different forms of a gene are
called alleles.
• Each pea plant inherits one allele from
each parent.
– A pea plant could inherit 2 tall alleles, 2 short
alleles, or 1 of each.
Genes and Alleles
• An organism’s traits are controlled by the alleles
it inherits from its parents.
• Some alleles are dominant.
– Dominant alleles are those whose trait always shows
up in the organism when that allele is present.
• Other alleles are recessive.
– Recessive alleles are those whose traits are hidden
whenever the dominant allele is present.
• Recessive traits only show up if the organism does not
have the dominant allele. In other words, the organism
has two recessive alleles.
Genes and Alleles
• In Mendel’s crosses, the allele for tall
plants is dominant over the allele for short
plants.
– Only plants that inherit two short alleles will be
short. Plants that receive one or two dominant
alleles will be tall.
Alleles in Mendel’s Crosses
• In Mendel’s experiments, the purebred tall plants
had 2 alleles for being tall, while the purebred
short plants had 2 alleles for being short.
• All of the plants from the F1 generation received
one tall allele and one short allele.
• Organisms that has two different alleles for a
trait is called hybrid.
– All of the hybrid plants were tall because they
received 1 tall and 1 short allele, but the tall is
dominant over the short.
Alleles in Mendel’s Crosses
• When the F1 plants self-pollinated, some of the
F2 plants received two dominant alleles for
tallness.
– These plants were tall.
• Other F2 plants received one dominant and one
recessive allele.
– These plants were tall.
• The rest of the F2 plants received two alleles for
shortness.
– These plants were short.
Symbols for Alleles
• Letters are used to represent alleles.
• Dominant alleles are represented by capital
letters.
– The tall allele would be T.
• Recessive alleles are represented by lowercase
letters.
– The short allele would be t.
• The alleles an organism receives for a trait are
represented by a combination of letters.
– The combination of alleles possible for pea plants are
TT, Tt, and tt.
Homozygous and Heterozygous
• An organism is said to be homozygous for a
trait if both alleles are identical.
– Example: TT and tt are homozygous allele
combinations.
• TT is homozygous dominant.
• tt is homozygous recessive.
• An organisms is said to be heterozygous for a
trait if the organism has both a dominant and
recessive allele.
– Example: Tt is a heterozygous allele combination.
– All hybrids are heterozygous individuals.
Why Mendel was Important?
• Before Mendel, scientists thought that the traits
of an individual were simply a blend of the
parent’s traits.
– Example: If a tall plant and a short plant reproduced,
they would make medium sized plants.
• Because of Mendel’s experiments, traits are
determined by individual, separate alleles
inherited from each parent.
• Mendel’s discovery was not recognized during
his lifetime.
– His work was rediscovered in 1900.
– Mendel is known as the Father of Genetics.
Probability and Genetics
• Mendel carefully counted all of the
offspring from every cross he carried out.
• When he crossed two tall hybrid plants, ¾
of the F2 generation were tall and ¼ were
short.
• Each time he repeated the cross, he
obtained similar results.
• He realized that probability applied to his
work.
Probability and Genetics
• Mendel could say that the probability of
producing a tall plant in the F2 generation
was 3 in 4.
• The probability of producing a short plant
in the F2 generation was 1 in 4.
• Mendel was the first scientist to
recognize that the principles of
probability can be used to predict the
results of genetic crosses.
Punnett Squares
• A Punnett Square is a chart that shows
all the combinations of alleles that can
result from a genetic cross.
• Geneticists use these to show all the
possible outcomes of a genetic cross, and
to determine the probability of a particular
outcome.
How to Make a Punnett Square
• Draw a square and
divide it into 4 smaller
squares.
How to Make a Punnett Square
• Place the alleles from
one parent along the
top of the Punnett
square.
– Make sure that only
one letter is above
each box.
T
t
• Place the alleles from
the other parent along
the left side of the
square.
– Make sure that only
one letter is beside
each box.
t
T
How to Make a Punnett Square
• Copy the alleles from
the top into each box
under them.
T
t
t
T
T
T
T
T
How to Make a Punnett Square
• Now place each letter
on the left of the box
into the boxes to the
right of them.
• When you are finished,
you should have two
letters in each box.
• You always should
write the dominant
allele on the left-hand
side.
T
t
– Tt instead of tT.
t
Tt
Tt
T
Tt
Tt
How to Make a Punnett Square
• The boxes in the
Punnett square
represent all the
possible combinations
of alleles that the
offspring can inherit.
• In this Punnett square,
we see the results of
crossing a purebred
tall plant with a
purebred short plant.
– All of the offspring
are hybrid tall plants.
– From this cross, 4 in
4, or 100% will be
tall.
T
t
t
Tt
Tt
T
Tt
Tt
Using a Punnett Square
•
•
•
T
In a genetic cross, the allele
that each parent will pass on
to its offspring is based on
probability.
In the Punnett square to the
right, there is a 3 in 4
chance, or 75% chance that
the offspring would inherit
the tall trait.
The Punnett square
represents the chances
each time a pair reproduces.
–
–
This does not mean that if
the pair to the right had 4
offspring, 3 would be tall
and 1 would be short.
It says that each time they
reproduce there is a 75%
chance for tall plants and
25% chance for short.
TT
T
t
Tt
t
Tt
tt
Phenotypes and Genotypes
• A pheontype is an organism’s physical
appearance or visible traits.
– Example: tall, short, purple flowers, white
flowers, wrinkled seeds, round seeds, black
fur, white fur
• A genotype is its genetic makeup or allele
combinations. In other words, the
combination of letters.
– Example: TT, Tt, tt, BB, Bb, bb, RR, Rr, rr,
WW, Ww, ww
Codominance
• For all the traits that Mendel studied, one
allele was dominant while the other was
recessive.
• This does not happen 100% of the time.
• In codominance, the alleles are not
dominant nor recessive.
– As a result, both alleles are expressed in the
offspring.
Genetic Laws
The Law of Dominance states that when
an organism has two different alleles for a
trait, the allele that is expressed,
overshadowing the expression of the other
allele, is said to be dominant. The allele
whose expression is overshadowed is said
to be recessive.
Genetics Laws
The Law of Segregation states that the
alleles for a trait separate when gametes
(egg and sperm) are formed. These allele
pairs are then randomly united at
fertilization. Mendel arrived at this
conclusion by performing monohybrid
crosses. These cross-pollination
experiments were with pea plants that
differed in one trait, such as pod color.
Genetics Laws
The Law of Independent Assortment
states that alleles for different traits are
distributed to sex cells and offspring
independently of one another.
– This means that the inheritance of one trait
has nothing to do with the inheritance of
another.
• Example: Just because a pea plant inherits the tall
trait does not mean that they must also inherit the
trait for having wrinkled seeds.
Genetic Disorders and Recessive
Genes
• Many genetic disorders are caused by
recessive genes.
• If an offspring receives two recessive
alleles from the parents, the child inherits
the disease.
• If a person is heterozygous, he/she will not
show the symptoms.
– These people are known as carriers.
Cystic Fibrosis
• This is a genetic disorder in which the
body produces abnormally thick mucus in
the lungs and intestines.
– The mucus fills the lungs and makes it hard to
breathe.
• It is caused by a recessive allele on one
chromosome.
– It is the result of a mutation in which three
bases are removed from DNA.
Sickle Cell Disease
• Sickle cell anemia results from a substitution
mutation of the DNA in the sex cells. This has
resulted in a recessive trait.
• Sickle cell commonly affects people of African,
Indian, and Mediterranean descent.
• It causes the red blood cells to become sickleshaped.
– This prevents the blood from passing normally
through the capillaries, resulting in oxygen not being
passed on to the tissues.
Hemophilia
• This is a genetic disorder in which a person’s
blood clots very slowly or not at all.
• They do not produce one of the proteins needed
for normal blood clotting.
– Have a high risk of internal bleeding from small
bumps and bruises.
• Caused by a recessive allele on the X
chromosome, making it a sex-linked disorder.
– Occurs more often in males than females.
Heredity and Meiosis
• Sometimes mistakes happen during
meiosis, the production of egg and sperm
cells.
• This can result in individuals having more
or fewer chromosomes than normal.
– Individuals with Down’s Syndrome have an
extra copy of chromosome 21.
• This results in a variety of physical and/or mental
conditions.
Sex Chromosomes
• The sex chromosomes carry genes that
determine whether a person is male or
female. They also carry genes that
determine other traits.
• The sex chromosomes are the only pair
that do not always match.
– In females, the chromosomes match. The
female genotype is XX.
– In males, the chromosomes do not match.
The male genotype is XY.
Sex-Linked Genes
• Genes on the X and Y chromosomes are called
sex-linked genes because their alleles are
passed from parent to child on a sex
chromosome.
• Traits controlled by sex-linked genes are called
sex-linked traits.
• One sex-linked trait is red-green colorblindness.
– A person with this trait cannot distinguish between the
colors red and green.
Pedigrees
• A pedigree is a chart or “family tree” that tracks
which members of a family have a particular
trait.
• Pedigrees include two or more generations.
• Females are represented by circles, while males
are represented by squares.
• Those with a trait are shaded, while those that
do not have a trait are left clear.
• If the organism is a carrier of a trait, but does not
show the trait, their symbol is only shaded
halfway.
A Pedigree for Albinism
(A condition where the skin, hair,
and eyes lack normal coloring)