Download Heredity It is all about Life

What controls traits?
 Genes on a chromosomes
Gene:
 a section of DNA on a chromosome that has
genetic information for one trait.
Alleles:
 the different forms of a gene.
How many alleles controlled flower color in Mendel’s
experiment?
 Two
How are alleles represented?
 By letters
Letters are chosen by
 Determining what the dominant trait is. The
letter chosen is usually the first letter of that trait.
Upper case letters represent dominant alleles.
Lower case letters represent recessive alleles.
The dominant allele of seed shape is round;
therefore the symbol will be
R.
The recessive allele of seed shape is wrinkled;
therefore the symbol will be
r.
According to Mendel, an organisms receives two alleles for
each gene. One allele comes from the egg, and the other
from the sperm. Therefore an organism has a combination
of two alleles.
Genotype:
the two alleles that control the
phenotype of a trait
(letter combination)
RR
Rr
Phenotype:
how a trait appears,
or is expressed
rr
Let’s try another example:
In Mendel’s experiment, purple flowers are
dominant over white flowers.


What are the possible genotypes for purple
flowers?
 PP or Pp
What is the possible genotype for white flowers?
 pp
REMEMBER, each parent contributes one allele, so
there should be two in total!!!
When the 2 alleles of a gene are the same, its genotype
is
 homozygous
If the 2 alleles of a gene are different, its genotype is
 heterozygous
Label the following genotypes as either heterozygous
or homozygous.
RR (round) =
 homozygous
 Rr (round) =
 heterozygous
 rr (wrinkled) =
 homozygous

A Punnett Square is a model used to
predict possible genotypes and phenotypes
of offspring.
Example: Tall is dominant over short.
 What possible genotypes could we have
if the parent were tall?
 TT or Tt
 What possible genotype could we have if
the parent were short?
 tt


Let’s say the mother is homozygous
tall. Her genotype would be
___________
And let’s say the father is
heterozygous tall. His genotype
would be _____________
Write the parent’s genotypes in the correct location in the
Punnett Square below.
T
T
Mom’s alleles
T
t
Dad’s alleles
Offspring = __________________________________
You can use the Punnett square to determine the probability
that offspring with a certain combination of alleles will
result. (Use the Punnett square to help you fill in the charts)
Genotypes and Probability
Genotype
Probability
With this information, you can determine the
probability of appearance in the offspring.
Phenotype
Probability
 Unfortunately, it is not
always so black and
white. There are times
that neither allele is
dominant or recessive.
Codominance is when
both alleles can be
observed in a phenotype.
Punnett Squares


Sometimes there can
be a gray area where
neither allele
dominates the other.
Incomplete
dominance is when
the offspring’s
phenotype is a
combination of the
parents’ phenotypes.
Punnett Squares