Transcript having two different alleles of a gene

Mendel and Genetics, Continued…
Bell Question:
Explain the
difference between
a phenotype and a
genotype.
Answer:
The phenotype is what a trait looks like. The genotype is the
specific alleles that code for the phenotype.
Mendel and Genetics, Continued…
Last time we learned …
… that Gregor Mendel is the Father of Genetics.
… he looked at pea plants and studied various, discrete traits.
… that there were ratios that seemed to consistently describe what
happened in the plants.
… how different alleles affect the phenotype.
… the difference between genotypes and phenotypes.
… how to use Punnett Squares to predict the
ratios of phenotypes.
We’re not done! Onward and upward!
Mendel and Genetics, Continued…
• We can continue to further describe genotypes.
Dominant and recessive are helpful, but they help us
more with phenotypes.
• GG and gg are homozygous. The
alleles are the SAME. GG is for green,
while gg is for yellow.
• For this, we say that GG is homozygous dominant.
• How could we describe gg? homozygous recessive
Mendel and Genetics, Continued…
Word
Definition
having two of
the same
Homozygous
alleles for a
gene
Picture
GG
gg
Mendel and Genetics, Continued…
• What about Gg? Its phenotype is for green peas.
• It has one allele for green peas and one allele for
yellow peas.
• We call this genotype heterozygous.
• Heterozygous is one of each allele. We KNOW
that one of the alleles is dominant, so the plant
would grow green peas.
Mendel and Genetics, Continued…
Word
Definition
having two
different
Heterozygous
alleles of a
gene
Picture
Gg
Mendel and Genetics, Continued…
• This is a German
family tree from the
1500s.
• We can study the
tree and learn about
the relationships
between members.
• We can also create a
genetic tree to learn
about the
phenotypes and
genotypes of families.
Mendel and Genetics, Continued…
Looks a little different, doesn’t it?
• This special tree is
called a pedigree.
• In a pedigree, we
look at specific
phenotypes which
can lead to figuring
out an individual’s
genotype.
Mendel and Genetics, Continued…
• The squares are
males.
• The circles are
females.
• The red
individuals are
affected (have a
specific
phenotype). The
blue individuals are
not.
• Let’s take a closer
look.
Mendel and Genetics, Continued…
Each row is a
new generation
first
generation
second
generation
third
generation
horizontal lines mean
marriage
vertical lines mean
offspring
Mendel and Genetics, Continued…
your
grandfather
If someone has the Ooga Booga disease, then he or she is considered
affected (red). If you are in the third generation, who has Ooga Booga your grandfather or your grandmother?
Mendel and Genetics, Continued…
Do you remember that when
Mendel started his experiments,
he began with mice but soon
switched to pea plants?
It was a great decision since pea
plants reproduce quickly, have
distinct traits, and the phenotypes
are easily recognized.
Mendel and His Pea Plants
Not all phenotypes are expressed so easily or clearly.
• Sometimes, the phenotypes are a mixture.
• Sometimes, more than one trait is expressed.
• Sometimes, it takes more than 2 alleles to express a trait.
With Mendel, if the dominant allele was in
the genotype, the dominant trait was the
phenotype. Not so with these…
Mendel and Genetics, Continued…
Some phenotypes are a blend of alleles. Camellia flowers are like this:
White flower
r
Red flower
r
R
• If you cross a red flower
camellia with a white flower
camellia, you get…
PINK flowers!
R
• This is incomplete
dominance.
Mendel and Genetics, Continued…
Word
Definition
offspring’s
phenotype is
Incomplete
a BLEND of
dominance
the parents’
Picture
+
Mendel and Genetics, Continued…
Codominance can also be expressed in offspring. Codominance is when
the phenotypes of BOTH parents are seen in the offspring.
When the red bull mates
with the white cow, the
offspring have BOTH red
and white hair.
Mendel and Genetics, Continued…
Word
Definition
the
phenotypes
of BOTH
Codominance
parents are
seen in the
offspring
Picture
Mendel and Genetics, Continued…
Sometimes there are more than two alleles. Blood types are like this.
• The four blood types are A, B, O,
and AB.
• The three alleles to code for blood
types are IA, IB, and i.
• The i allele is actually missing the
markers that IA and IB have.
• IA and IB are both dominant over i. The i allele is recessive.
• IA and IB are codominant. What does that mean if a person is IAIB?
The person will show the phenotypes of BOTH alleles.
Mendel and Genetics, Continued…
Can you figure out the phenotypes for these
genotypes? Remember, IA and IB are codominant, and
both these alleles are dominant to i.
Genotype
IAIA or IAi
IBIB or IBi
ii
IAIB
Phenotype
Type A blood
Type B blood
Type O blood
Type AB blood
Mendel and Genetics, Continued…
• Mendel thought that only one gene determines each trait. We now
know that isn’t the case.
• Certain characteristics like height, eye color, and skin color are
determined by more than one gene.
• Polygenic
inheritance
occurs when
more than one
gene determines
the phenotype of
a trait.
Mendel and Genetics, Continued…
Word
Definition
Polygenic
inheritance
when
multiple
genes
determine a
trait
Picture
example:
height
Mendel and Genetics, Continued…
Genes are not the only thing that
impact you. Your environment
also affects your phenotype.
Just because you may come from a long
line of healthy hearts, doesn’t mean that
you shouldn’t eat healthy foods and
exercise.
Your environment is an influence, too!
Mendel and Genetics, Continued…
Exit question:
When an offspring has a BLEND of its
parents’ phenotypes, that is
_______________ dominance.