Genetics Basics
A gene can be described in two ways, by:
1. Genotype: genetic makeup; use letters to
represent gene
▫ (allele – different form of a gene; single letter)
- Ex: BB, Bb, bb
2. Phenotype: physical trait; what you see
- Ex Brown hair, blue eyes
Alleles can sometimes be described as Dominant
or Recessive:
1. Dominant: this allele dominates the other
▫
use capital letters
- Ex: Brown Hair (B) dominates blonde hair (b)
2. Recessive: this gene will only be expressed if
2 recessive alleles are present
▫
use lower case letters
- Ex: blonde hair is recessive (bb)
A combinations of alleles can be
Homozygous or Heterozygous:
1. Homozygous: same size letters (alleles)
▫
BB, bb
2. Heterozygous: different size letters (alleles)
▫
Bb

Also called hybrid
Self-Quiz
• What is an allele?
A different form of a gene
• What is the difference between genotype and
phenotype?
Genotype represents genes, phenotype represents physical traits
• When will a recessive gene be expressed?
When both alleles are recessive (lower case)
• Indicate whether the following are heterozygous
or homozygous; Bb, BB, bb
Bb-hetero, BB-homo dominant, bb homo recessive
• Why are there always at least 2 alleles for a gene?
You receive one allele for a gene from each of your parents
Punnett Square Basics
Dominate vs. Recessive Punnett Squares
•
Remember:
▫
▫
▫
BB = homozygous dominant
Bb = heterozygous
bb = homozygous recessive
•
Each parent donates 1 allele to their offspring
•
How can we predict what alleles offspring will inherit?
Punnett Squares!
Dad
Mom
Bb
bb
Offspring
Bb
bb
bb
Bb
Ex 1: What is the predicted offspring of
combining a parent that is Homozygous
recessive with one that is Heterozygous?
B
b
bb x Bb
b
b
Bb
Bb
2/4 = 50% Bb
bb
bb
2/4 = 50% bb
Ex 2: A heterozygous brown bird is crossed with
a black bird. Brown is dominate to black.
What is the predicted offspring of the baby
birds (genotypes and phenotypes)?
B
b
Bb
b
bb
Bb x bb
2/4 = 50% Bb
brown
b
Bb
bb
2/4 = 50% bb
black
Self-Quiz
• One cat carries heterozygous, long-haired traits (Ss), and its mate carries
homozygous short-haired traits (ss). Use a Punnett square to determine the
probability of one of their offspring having long hair.
S
s
s
Ss
ss
s
Ss
ss
• In humans, brown eyes (B) are dominant over blue (b). A brown-eyed man
marries a blue-eyed woman and they have three children, two of whom are
brown-eyed and one of whom is blue-eyed. Draw the Punnett square that
illustrates this marriage. What is the man’s genotype? What are the
genotypes of the children?
b
b
Man’s genotype- Bb
Brown eyed children-Bb, Bb
Blue eyed children-bb
B
Bb
Bb
b
?b
?b
Intermediate Traits
Punnett Squares
Intermediate Traits
• Sometimes, one trait does not dominate another.
Sometimes, both traits will be expressed. There
are two types of intermediate traits
• Incomplete Dominance-a cross between organisms
with two different phenotypes produces offspring with a
third phenotype that is a blending of the parental traits.
INBETWEEN trait
• Codominance-cross between organisms with two
different phenotypes produces offspring with a third
phenotype in which both of the parental traits appear
together. COMBINATION trait
To complete a punnett square for an
intermediate trait, use a combination of
two different letters instead of upper vs.
lower case (because there is no a need to
show dominance)
Ex: a red flower and a white flower
combine to make pink flowered
offspring
• R – red flower
• W – white flower
• RW – pink flower
= RR (or CR CR)
= WW (or CwCw)
= (or CRCw)
A pink flower is crossed with a white
flower. What are the genotypes and
phenotypes of the predicted offspring?
R
W
W
W
RW
WW
RW
WW
RW x WW
2/4
RW= pink = 50%
2/4
WW= white = 50%
Roan Cow
• An example of codominance
• Cattle can be red (RR = all red hairs), white
(WW = all white hairs), or roan (RW = red &
white hairs together). A good example of
codominance.
•
(the EOC loves to use this example, so learn it!)
Multiple Alleles
REVIEW
• BASIC DOMINANCE-When the gene for one trait exists as only
two alleles & the alleles play according to Mendel's Law of
Dominance, there are 3 possible genotypes (combination of
alleles) & 2 possible phenotypes (the dominant one or the
recessive one).
• INTERMEDIATE DOMINANCE-If there are only two alleles
involved in determining the phenotype of a certain trait, but
there are 3 possible phenotypes, then the inheritance of the trait
illustrates either incomplete dominance or codominance.
• MULTIPLE ALLELES-If there are 4 or more possible
phenotypes for a particular trait, then more than 2 alleles for
that trait must exist in the population.
Examples
• Basic-Hitchhiker’s thumb is a dominant trait. The
possible genotypes are HH, Hh, hh. The possible
phenotypes are Hitchhiker’s or no hitchhiker’s thumb.
• Intermediate▫ Codominance- The possible genotypes in cattle are
RR, RW, WW. The possible pheotypes are red, white
or roan.
▫ Incomplete-The possible genotypes in flowers are BB,
BR, RR. The possible phenotypes are blue, blue & red
or red.
Multiple Alleles
• There are multiple alleles in a population
causing 4 or more phenotypes of a trait to
exist
• KEEP IN MIND
▫ There may be multiple alleles within the
population, but individuals have only two of
those alleles
 Why?
…because individuals have only two biological parents. We inherit half of our
genes (alleles) from mommy, & the other half from daddy, so we end up with
two alleles for every trait in our phenotype.
Example
• Human Blood Type:
▫ Blood type exists as four possible phenotypes:
A, B, AB, & O.
▫ There are 3 alleles for the gene that determines
blood type: A, B, O
▫ Alleles A & B are CODOMINANT
 What does that mean?
Combination. You “see” both traits. i.e. AB blood.
▫ Allele O is recessive
 What is the genotype for O blood?
OO
Blood Typing Basics
• Blood types are also represented as: IA, IB, io or
i- (this is how you will most likely see it expressed on your test. )
Phenotype
Genotype
Type O
Type A
Type B
Type AB
OO (io)
AA, AO (IA IA, IB io)
BB, BO (IB IB, IB io)
AB (IA IB)
Example
• A husband is homozygous for blood group B. He
marries a woman with type O blood. What are
the blood types of their predicted offspring?
B
O
O
B
BO
BO
BO
BO
BO x OO
100% BO
100% type B