Heredity and Genetics
Why do we look the way we do?
Heredity = the passing on of traits from one generation to the next.
Trait = a specific characteristic that is unique.
 Traits affect the way we look
 Traits affect how our bodies function
 Traits are inherited
Pedigree = a diagram that shows the history of a trait as it is passed from one generation
to the next.
 Pedigrees indicated 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:
 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
Diagram of a simple pedigree:
Trait:
Parents
Youngest Offspring to Oldest
What kinds of traits are there?
How do we look the way we do?
Dominant trait = a trait that is always expressed, or shown.
Examples are brown hair, brown eyes, right handed
Recessive trait = a trait that is covered up or seems to disappear.
Examples are blonde hair, blue eyes, left handed
Factors that make up the individual must come from both parents. These are genes.
Gene = a specific location on a chromosome that controls a certain trait.
Chromosome = 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 dominant trait determines what letter is used to represent the gene.
Use the capital letter for the dominant trait
Use the small letter for the recessive trait
Example: Right-handedness is the dominant trait so use R
Left-handedness is the recessive trait so use r
Example 2: Tall is the dominant trait so we use T
Short is the recessive trait so we use t
Where do genes come from?
An individual needs 2 genes for each trait – one gene from each parent.
One gene comes from the sperm cell (from the Father)
One gene comes from the egg cell (from the Mother)
How are genes expressed?
Pure Dominant: the individual only has genes for the dominant trait.
Example: TT = a pure tall individual
Pure Recessive: the individual only has genes for the recessive trait.
Example: tt = a pure short individual
Heterozygote: 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
How do we predict offspring?
Punnett Square: is a way to show the possible combinations of genes that offspring of parents
could have.
Example:
FATHER’S GENES
MOTHER’S GENES
Trait:
Phenotype: is the way that we look or appear.
Example: brown eyes, blonde hair, tall
Genotype: is the genetic make up for a trait.
Example: Homozygous brown; BB = pure brown: (both genes are the same)
Homozygous blue; bb = pure blue: (both genes are the same)
Heterozygous brown; Bb = hybrid (mixed) brown: (both genes are different)
Examples of Punnett squares to predict possible traits:
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
Trait: Height
Tall Parents (Tt)x(Tt)
T
t
T
TT
Tt
t
Tt
tt
75% Tall
25% Short
25% TT, homozygous tall
50% Tt, heterozygous tall
25% tt, homozygous short
Trait: Eyecolor
Blue-eyed Parents (bb)x(bb)
b
bb
bb
b
bb
bb
b
b
Results:
Phenotypes = 100% Blue-eyed
Trait: Handedness
Pure Right and Pure Left handed
Parents (RR)x(rr)
R
R
r
Rr
Rr
r
Rr
Rr
100% Right-handed
Genotypes = 100% bb, homozygous blue
100% Rr, heterozygous right-handed
How do genes control traits?
What is DNA? (Deoxyriboneucleic Acid)
DNA = 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.
INSERT PICTURE OF DNA
There are 4 types of bases in DNA: Adenine, Guanine, Cytosine and Thymine.
These bases fit together like puzzle pieces
Adenine
Cytosine
Thymine
e
Guanine
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 is what
accounts for all the differences between life forms on earth.
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.
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
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.
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.
Advances in genetics
We have an increased understanding of certain disease that are inherited.
 We have increased knowledge of many health conditions (treatment, prevention,
and cure).
Example: Down Syndrome is caused by an extra chromosome.