Review
• Genes contain the information to make proteins.
• Proteins cause traits based on what that protein
does.
• Different versions of a gene (alleles) can cause
different versions of a trait.
• The overall appearance of a trait depends on
what genotype (combination of genes) an
organism inherits.
• The overall dependence of a trait depends on
what proteins are being made by the inherited
genes.
Gene Combinations
• The description of what alleles an organism
carries for a given gene is its genotype.
• Heterozygous indicates the organism carries two
different alleles.
• Homozygous means the organism carries two
identical alleles.
– Homozygous Dominant indicates the alleles are both
dominant.
– Homozygous Dominant indicates the alleles are both
recessive.
Dominant
• Traits that are more common in a population and
that rarely disappear from generation to
generation tend to be visible even if an organism
is heterozygous and is only carrying one allele
that causes that trait.
• Example: An organism inherits genes that make a protein that
cuts the ear lobe free from the face. The allele it inherits from
one parent makes a protein that doesn’t work. The allele it
inherits from the other parent makes a protein that does work.
Different alleles mean it is heterozygous. The working allele
makes a functioning protein that cuts the ear lobe free. Its ear
lobe is unattached to its face. This will be true also if it inherits
two homozygous alleles that both make a working protein.
Recessive
• Traits that are less common in a population and
that can disappear from generation to generation
are typically recessive, and only are visible if an
organism is homozygous for that allele.
– Example: An organism inherits genes that make a
protein that cuts the ear lobe free from the face. The
allele it inherits from one parent makes a protein that
doesn’t work. The allele it inherits from the other
parent makes a protein that does not work. Identical
alleles mean it is homozygous. Its ear lobe remains
attached to its face.
Punnett Squares
• Punnett Squares make it possible to predict
likely offspring based on parent phenotype
and genotype.
• Punnett Squares make it possible to figure out
parent Genotype based on the genotype and
phenotype of the offspring.
Setup
• You have been given a diploid organism. (2 copies of each
chromosome, so 2 copies of each gene.)
• The gene in question affects the color trait.
• Using what you know of genetics, you must design an
experiment to determine:
– the probable genotype of your organism. (Hetero/homo
dominant/recessive.
– What alleles for the gene are present in the population.
– What the effect is of each protein made by an allele. (What
does that allele’s protein DO?)
– If the trait is binary.
• To find the results of a cross, you must consult with Mr.
Stevenson with the crossed organisms’ numbers and colors.
Sample Hypothesis
• If my organism’s genotype is heterozygous,
then when I cross it with a homozygous
recessive, the offspring should have a
genotype ratio of 2 homozygous recessive:2
heterozygous, and a phenotype ratio of 2
dominant trait:2 recessive trait because:
Hh x hh
H
h
h
Hh
hh
h
Hh
hh
Test Cross Ratios
• Certain crosses have predictable patterns.
• A self-cross:
– Will have a 4/4 genotype/phenotype if
homozygous.
– Will Have a 1/2/1 genotype and at least two
different phenotypes if heterozygous.
• Other crosses also have predictable
genotypes.
Sample cross patterns: Homo. Dom
HH x hh
Homo. Dom x Homo
Rec.
H
H
h
Hh
Dominant Phenotype
Hh
Dominant Phenotype
h
Hh
Dominant Phenotype
Hh
Dominant Phenotype
HH x Hh
Homo Dom x Hetero
H
H
H
HH
Dominant Phenotype
HH
Dominant Phenotype
h
Hh
Dominant Phenotype
Hh
Dominant Phenotype
Sample cross patterns: Homo. Rec.
hh x Hh
Homo. Rec x Hetero
H
h
h
Hh
Dominant Phenotype
hh
Recessive Phenotype
h
Hh
Dominant Phenotype
hh
Recessive Phenotype
hh x HH
Homo Dom x Hetero
H
H
h
Hh
Dominant Phenotype
Hh
Dominant Phenotype
h
Hh
Dominant Phenotype
Hh
Dominant Phenotype
Clues
• If in a cross:
– A trait disappears in the offspring, that trait is
probably caused by a recessive allele, and one of
the parents was homozygous dominant.
– A trait appears that was not visible in parents
appears, that trait is recessive, and both parents
were heterozygous.
Incomplete Dominance
• When two alleles of a gene both produce
functional proteins, the resulting trait can be a
blend of what each protein does.
• In this case, organisms that are heterozygous
but have two DIFFERENT dominant alleles will
appear to have a blend of both traits.
• This changes the visible phenotypes.
•
•
•
•
Incomplete Dominance
Allele R makes a red pigment.
Allele W makes a white pigment.
Allele Z makes no pigment.
The background color without pigment is green.
RZ x RZ
Hetero Dom x Hetero
Dom.
R
Z
W
WR
Incomplete Dominant
Phenotype:
red+white=pink
ZW
Dominant Phenotype:
white+nothing=white
Z
RZ
Dominant Phenotype:
red+nothing=red
ZZ
Recessive Phenotype:
Nothing+nothing=gre
en background color is
revealed.
1 Pink: 1 Red: 1 White: 1 Green