UNIT 7:
GENETICS & HEREDITY
What is Heredity?
• Heredity is the passing of traits from parent to
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offspring.
Genes on chromosomes control the traits that
show up in an organism.
Genes are separated during meiosis into the
egg and sperm.
The different forms of a trait that a gene may
have are alleles.
The study of how traits are inherited is
genetics.
Genetic Traits Examples
Who was Gregor Mendel?
• Austrian monk, known as
“The Father of Genetics”
• 1856 - Studied pea plants
• Examined the color, shape
and other physical
characteristics of plants
• Discovered that plants
received genetic information
from its parents passed down
through generations.
Mendel’s Findings
What are Dominant & Recessive
Alleles?
• Dominant alleles will cover up, or dominate a
recessive allele.
• Represented by a capital letter (B)
• Only need to inherit one dominant allele for trait
to appear
• Recessive alleles can be masked by a dominant
allele.
• Represented by a lowercase letter (b)
• Need to inherit two recessive alleles for the trait
to appear
How do Alleles Determine Traits?
• Organisms inherit an allele from each
parent to make an observable trait.
• Homozygous: Two of the same allele
(example: BB, bb)
• BB would be “homozygous dominant”
• bb would be “homozygous recessive”
• Heterozygous: Two different alleles
(example: Bb)
What are Genotype & Phenotype?
• Genotype: the genetic make-up of an organism
(represented by letters, Aa, Dd, Rr, RR, DD, etc.)
• Phenotype: The way an organism looks or
behaves as a result of its genotype (example: red
hair, brown eyes, etc.)
What is a Punnett Square?
• A Punnett square is a tool used to predict
results of genetic crosses.
How do I Solve a Punnett Square Problem?
1. Determine the genotypes of the parent organisms
2. Write down your "cross" (mating)
3. Draw a Punnett square
4. Split the letters of the genotype for each parent &
put them outside of the Punnett square
5. Determine the possible genotypes of the offspring
by filling in the Punnett square
6. Answer what the problem is asking for based on
the results of your Punnett square
Punnett Square Example 1
Mom is homozygous for having freckles (F) and dad
(f) has none. Having freckles is dominant over not
having freckles. What percentage of their children
will have freckles?
The parents’ homozygous genotypes are:
Mom =
FF
Dad =
ff
The parents’
homozygous
genotypes are:
Mom = F F
Dad =
ff
SPLIT the parents’
genotypes across the
top and the sides of
the Punnett square.
F
f
f
F
Fill in the Punnett
square by dragging
down and across the
parent genotypes.
Final Answer:
Since all of the
children are Ff, 100%
will have freckles.
F
F
f
Ff
Ff
f
Ff
Ff
Punnett Square Example 2
Both mom and dad are heterozygous (Ff) and
have freckles. What percentage of their
offspring will NOT have freckles?
Their heterozygous genotypes are:
Mom = Ff
Dad =
Ff
The parents’
heterozygous
genotypes are:
Mom = F f
Dad =
Ff
F
F
f
f
Final Answer:
There is a 25% (1
out of 4) chance
of their child
NOT having
freckles.
F
f
F
FF
Ff
f
Ff
ff
Do Mendel’s Observations Apply
to all Genes?
• No… some genes are neither dominant nor
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recessive.
Incomplete dominance: the inherited alleles from
the parents are blended together in the offspring.
Codominance: both alleles are expressed in the
offspring.
Multiple alleles: when a trait is controlled by more
than two alleles
Polygenic inheritance: when a group of gene pairs
act together to produce a trait.
What is Incomplete Dominance?
• When two parents are homozygous for different forms of
trait, the offspring shows an intermediate phenotype.
• Use two different capital letters to represent alleles.
• Example: four-o’clock plants
A red 4-o’clock flower is crossed with a white,
resulting in offspring with all pink flowers.
What would be the results of a Pink and White
flower?
RW x WW
Offspring:
RW: pink 50%
WW: white
50%
R
W
W
RW
WW
W
RW
WW
What is Codominance?
• Prefix “Co” means together.
• When two parents are homozygous for different forms of a
trait, the offspring will express BOTH inherited alleles
together.
• Use two different capital letters to represent alleles.
• Examples:
In chickens, black and white feather color are
codominant. What would be the resulting
genotypes and phenotypes of a cross between
two black and white feathered chickens?
BW x BW
Offspring:
BB: black 25%
BW: black and
white 50%
WW: white
25%
B
W
B
BB
BW
W
BW
WW
What are Multiple Alleles?
• You can only inherit 2 alleles, but more than that may
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exist in the population.
Example: Human Blood Type
3 alleles exist for blood type: A, B, and O.
A and B are both codominant
O is recessive
What is Polygenic Inheritance?
• Polygenic inheritance occurs when a group of gene
pairs acts together to produce a trait
• Produces a wide variety of phenotypes
• Examples in humans include height, hair, skin and
eye color
What is a Genetic Disorder?
• Caused by DNA
mutations
• Caused by mistakes in
meiosis (abnormal
chromosome number)
• Caused by inheriting a
dominant or recessive
allele for the disorder
• Caused by inheriting a
dominant or recessive
allele for the disorder on
the X-Chromosome (sexlinked)
Chromosomal Genetic Disorders
• Happens when the incorrect number of chromosomes
is inherited from parents.
• Examples: Down Syndrome, Turner’s Syndrome,
Klinefelter’s Syndrome
Dominant & Recessive Genetic
Disorders
• Offspring must inherit two copies of recessive allele
for recessive genetic disorder to appear.
• Carriers carry recessive allele but do not show
symptoms
• Examples: Cystic Fibrosis, Albinism, Tay-Sachs
Disease
• Offspring only need to inherit one dominant allele for
a dominant genetic disorder to appear.
• No carriers
• Examples: Huntington’s Disease, Marfan
Syndrome
How is Gender Determined?
• Chromosomes that determine sex in humans are XX in
females and XY in males.
• Females produce eggs with only one X chromosome.
• Males produce sperm with either an X or a Y
chromosome.
• Father’s sperm determines sex of the offspring
Sex-Linked Genetic Disorders
• Some genes are inherited on the X or Y chromosome,
called sex-linked genes.
• Color-blindness is a sex-linked disorder caused by a
recessive allele on the X chromosome.
• Sex-linked genetic disorders are more common in
males.
What is a Pedigree?
• A pedigree is a diagram that follows a trait
through generations of a family. Like a “family
tree”
Pedigree Example
What is Genetic Engineering?
• Changing the arrangement of DNA that makes up a
gene
• Using a bacterium or a virus, abnormal DNA can be
replaced with normal DNA. Can be used to treat
genetic disorders.
• Genetically Modified Organisms (GMOs) are plants
and other food organisms where the DNA has been
altered.