Genetics: The
Science of
Heredity
Heredity: The
passing of
physical
characteristics
from parents
to offspring
•Every organism is a
collection of traits
all inherited from its
parents.
What have you
inherited?
Eye color, hair color,
nose shape, and
many other physical
features are types
of traits that you
inherit
from your parents.
Traits are controlled by genes
• Genes are made up
of DNA.
• Genes are located on
the chromosomes.
• Our genes determine
our traits (hair color,
eye color, etc.)
Our genes come from our parents
Alleles: Different forms of a gene
• Alleles are different
forms of genes.
• We inherit two alleles
for each trait
(example hair color)
• One allele comes
from our mother and
the other from our
father
Alleles control traits
• Some alleles are
dominant, while others
are recessive.
• A dominant allele is one
whose trait always shows
up in the organism when
it is present.
• A recessive allele is
hidden whenever the
dominant allele is
present.
Dominant traits control recessive
traits
• In this diagram the
trait for black fur
dominates the trait for
white fur.
• Even though one of
the parents has white
fur none of the
offspring will have
white fur.
Dominant and Recessive Alleles
Continued.
• Dominant traits are
represented by a CAPITAL
letter.
• Example from diagram: the
CAPITAL letter B represents
the dominant trait for black fur.
• Recessive traits are
represented by a lower case
letter.
• Example from diagram: the
lower case letter b represents
the recessive trait for white fur.
TWO Alleles for each trait
• In the diagram each animals
alleles are represented by
TWO letters.
• One of the letters came from
that animals mother and one
from its father.
• Notice how the offspring are a
COMBINATION of the parents
alleles.
• The parents are BB for the
black fur and bb for the white,
but the offspring are Bb a
combination of the parents
alleles.
Genotype
• Genotype: The genetic
makeup of an organism
or allele combinations
• Example from diagram
The genotype of the
animal with black fur is
BB and the one with
white fur is bb.
• Genotype is represented
by either a capital or
lowercase letter.
Phenotype
• Phenotype is your
physical appearance or
visible traits.
• Example from diagram.
Black fur and white fur
are the phenotypes of the
animals.
• The offspring in this
example have a genotype
of _____ and a
phenotype of ______ for
their fur color.
Homozygous and Heterozygous
• Homozygous: Two alleles for a
trait that are exactly the
same.
•
- Genotype: BB or bb
• Also called PUREBRED
• The parents in this example
are homozygous purebreds.
• Heterozygous: Two different
alleles for a trait.
•
- Genotype: Bb
•
- Also called HYBRIDS
• The offspring in this example
are Heterozygous Hybrids.
Punnett Squares
• A punnett
square is a
tool to predict
results in
Mendelian
genetics.
Step one: Punnett Squares
• Step one: Add the
Genotypes of each
parent on the top and
left side of the punnett
square.
Step Two: Punnett Squares
• Step two: Copy the
genotype letters of
the parent on the top
down into the boxes
below each letter
Step Three: Punnett Squares
• Step Three: Copy the
genotype letters of
the parent on the left
side into the boxes
next to the letters.
Punnett Square Practice
• Show a cross between a
Heterozygous (Bb)
animal with a
homozygous purebred
(bb).
• B = Black Fur
• b = White Fur
• What are the genotypes
of the offspring?
• What are the phenotypes
of the offspring.
Punnett Squares Predict Probability
• In a genetic cross, the allele that each
parent will pass on to its offspring is based
on probability.
• You can determine what percentage
chance you have of passing on certain
alleles.
• Since there are four squares each square
represents 25% (4 x 25 = 100)
Probability Example
• What percentage of
the offspring are
Heterozygous?
• What percentage of
the offspring are
homozygous?
• What would the ratio
of the offspring be?
• 2:2 or 1:1
Probability Examples