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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.
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: 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