Heredity and Genetics
“Why We Look the Way We Do”
What is Heredity?
• Heredity = the passing on of
traits from one generation to
the next.
What is a Trait?
A trait is a specific characteristic
that is unique.
Traits affect the way we look
Traits affect how our bodies
function
Traits are inherited
• Examples are hair color, eye color,
handedness, etc.
How Do We Keep Track of
Traits?
• We use a pedigree chart to keep
track of how traits are passed
on from generation to
generation.
What is a Pedigree?
A pedigree is a diagram that shows
the history of a trait as it is passed
from one generation to the next.
Pedigrees indicate patterns
Pedigrees identify carriers of genetic
disorders
Pedigrees are useful for genetic
counseling
• Example: a pedigree is like a family tree for
one trait.
Rules For Making A Pedigree
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Females are represented by circles
Males are represented by squares
Mother/Father couples are connected by a line
Offspring are shown oldest on the left to
youngest on the right
Half-shaded circle represents a female carrier
for the trait
Half-shaded square represents a male carrier
for the trait
Full-shaded circle represents a female with the
trait
Full-shaded square represents a male with the
trait
A Pedigree
Trait:
Parents
Offspring
Youngest to Oldest
What is Mendelian
Genetics?
• Gregor Mendel was an
Augustinian monk who lived in
the late 19th century and,
through studying peas,
developed the basis for the
science of Genetics still used
today.
What Kinds of Traits Are There?
• Traits are either Dominant or
Recessive.
Dominant Trait
A dominant trait is a trait that is
always expressed, or shown.
– Examples are brown hair, brown
eyes, right handed
Recessive Trait
A recessive trait is a trait that is
covered up or seems to
disappear.
– Examples are blonde hair, blue
eyes, left handed
Where Do Traits Come
From?
Factors that make up an individual
come from both parents.
The trait information is passed on
from generation to generation in
the form of genes.
What are Genes?
A gene is a specific location
on a chromosome that
controls a certain trait.
Where Do Genes Come
From?
• An individual needs 2 genes for
each trait – one gene from each
parent.
• This gene pair is called an allele.
– One gene comes from the sperm cell
(from the Father)
– One gene comes from the egg cell
(from the Mother)
How Do Genes Get Their
Information?
A chromosome is a structure in
the cell that contains the
genetic information.
– This information is passed on from
one generation to the next
generation.
Gene Representation
How Do We Write Our Genetics?
• The name of the dominant trait determines
what letter is used to represent the gene.
• Use a capital of the first letter of the dominant
trait to represent the dominant gene.
• Use a small version of the first letter of the
dominant trait for the recessive gene.
Example: Right-handedness is the dominant trait
so use R for the dominant gene and use r for
the recessive gene for Left-handedness.
Example 2: Tall is the dominant trait so we use
T and we use t for the recessive gene for Short.
How Are Genes Expressed?
Traits are how our genes show and
since traits are formed from two
genes, they are described by the
combination of genes that make
the pair.
Traits are described as either:
– homogeneous, (pure)
– heterogeneous, (mixed)
Homozygous = Pure
(Homogeneous)
• Pure Dominant: the individual only has
genes for the dominant trait.
– Example: TT = a pure tall individual has only
tall (T) genes.
• Pure Recessive: the individual only has
genes for the recessive trait.
– Example: tt = a pure short individual has
only short (t) genes.
Heterozygous = Mixed
(Heterogeneous)
A heterozygous individual has
one dominant gene and one
recessive gene for a trait. The
result is the dominant gene is
the one expressed, or shown.
– Example: Tt = a heterozygote tall
individual has both tall (T) and
short (t) genes but looks tall.
How Do We Predict
Offspring?
A Punnett Square is a way to
show the possible combinations
of genes that offspring of
parents could have.
Punnett Square
What is Phenotype?
Phenotype is the way that we
look or appear.
– Example: brown eyes, blonde hair,
tall
What is Genotype?
Genotype: is the genetic make up for
a trait.
Example: Homozygous brown; BB =
pure brown: (both genes are the same)
Trait: Handedness
Right-handed Parents
(RR)x(Rr)
R
R
R
RR
RR
r
Rr
Rr
Results:
Phenotypes: 100% Right handed
Genotypes: 50% RR, homozygous right
50% Rr, heterozygous right
0% rr, homozygous left
short
Trait: Height
Tall Parents
(Tt)x(Tt)
T
t
T
TT
Tt
t
tT
tt
75%
25%
25%
50%
25%
Tall
Short
TT, homozygous tall
Tt, heterozygous tall
tt, homozygous
Trait: Eyecolor
Blue-eyed
Parents (bb)x(bb)
b
b
b
b
bb
bb
bb
bb
Results:
Phenotypes: 100% Blue-eyed
Genotypes: 100% bb, homozygous blue
Trait: Handedness
Pure Right and
Pure Left handed
Parents (RR)x(rr)
R
R
r
r
Rr
Rr
Rr
Rr
100% Right-handed
100% Rr, heterozygous
right-handed
What Is Incomplete
Dominance?
• Incomplete dominance is when
neither gene that determines a
trait dominates, so the recessive
trait is not fully hidden.
• The result is a “mixed” genetic
trait that is neither dominant nor
recessive.
– Example: Red (RR) x White (WW)
flowers yield Pink (RW)
How Do Genes Control Traits?
Deoxyriboneucleic Acid
DNA is the “instruction code” that
the genes use to form traits.
– DNA is long threads of material found
in all cells.
– DNA contains the “master code” that
instructs all cells in their daily jobs.
Genes are short pieces of DNA that
make up our chromosomes.
Each piece of DNA that is related to a
gene makes up one trait.
Structure of DNA
DNA looks like a twisted
“ladder” made of chemical
compounds called bases.
Models of
DNA
How Bases Make Up DNA
• There are 4 types of bases in DNA:
Adenine, Guanine, Cytosine and
Thymine.
– These bases fit together like
puzzle pieces
•Adenine with Thymine
•Cytosine with Guanine
But What ARE Genes?
• Genes are pieces of DNA that make up a trait
• Different genes consist of different
arrangements of the Adenine, Thymine,
Cytosine, Guanine bases.
• These bases can be arranged to form different
proteins (chemical messages)
• These messages control different traits (some
determine how we look, some determine how
we feel and function).
• There are many millions of possible
combinations of these 4 bases – this accounts
for the differences, and similarities, between
life forms on earth.
Traits from Proteins
• All characteristics are affected
by the DNA in the cells of the
individual organism.
• These characteristics are
called traits. Traits depend on
the types of proteins that the 4
bases (A,C,G,T) make up.
How Do We Look The Way We Do?
• Parents pass on copies of their DNA to
their offspring.
• The DNA from each parent combines to
form the DNA of the offspring.
• How the offspring develops depends on
the instructions coded in the DNA
donated by both parents.
• Offspring are similar to parents, but
different due to the many possible
combinations of the 4 bases.
• Every individual is unique.
How Can We Use
Genetics?
• To predict looks of offspring
• To predict risks of diseases/defects
•…
Genetics Help Predict
Diseases
• Sickle-cell Anemia is a genetically
inherited disease where red blood
cells are misshaped.
– Red blood cell shape is an inherited
trait.
– Sickle-cell shape vs Normal cell shape
• Sickle-cell anemia is cause by
incomplete dominance.
Advances in Genetics
• We have an increased
understanding of certain diseases
that are inherited.
• We have increased knowledge of
many health conditions (treatment,
prevention, and cure).
– Example: Down Syndrome is caused
by an extra chromosome.