Mendelian Genetics Introduction
to Genotype and Phenotype
Genetic Terminology
 Trait - any characteristic that
can be passed from parent to
offspring
 Heredity - passing of traits from
parent to offspring
 Genetics - study of heredity
copyright cmassengale
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Genotype is…
our genetic make-up
we cannot see our genes!
We get one copy of our genes
from our mother and one from
our father. The genes on
these chromosomes might
code for different forms of the
same trait. Different forms of
genes are called alleles.
Image retrieved from http://www.emunix.emich.edu/~rwinning/genetics/inherit2.htm
Genotype is determined…
by how the parent’s alleles combine
during reproduction.
Here, the mother has
only the “a” form of the
gene, while the father
has only the “A” form
of the same gene. In
this scenario, the
offspring can have
only one genotype: Aa
Image retrieved from: http://www1.geneticsolutions.com/PageReq?id=3844:1873
Determining Genotypes
Here, each
parent has
both the “A”
and the “a”
allele for this
gene.
The offspring
can have any
of the following
genotypes:
AA, Aa or aa
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So what?
The genotype of an organism is the code for
the physical features of the organism
(phenotype) Phenotype is the physical
appearance of the organism
For example, the allele, “A” might code for a
normal skin pigment, while the “a” allele might
code for albinism.
What is the relationship between
genotype and phenotype?
Image retrieved from http://www.bbc.co.uk/schools/gcsebitesize/biology/variationandinheritance/0dnaandgenesrev5.shtml
Image retrieved from
http://bio1151.nicerweb.com/doc/class/bio1151/Locked/media/ch14/14_06PhenotypeVsGenotype_L.jpg
More Terminology
 Genotype - gene combination
for a trait (e.g. RR, Rr, rr)
 Phenotype - the physical
feature resulting from a
genotype (e.g. red, white)
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How is phenotype determined?
Generally, certain alleles of a gene completely
override other alleles of the same gene (they are
completely dominant).
The allele for a tall plant
is dominant in this
example. If the tall
parent has only tall
alleles, all of its offspring
will receive a tall allele
from this plant. Since
the tall allele is
dominant to the short
allele, all of the offspring
will be tall.
Image retrieved from http://web.pdx.edu/~cruzan/Kid
Punnett Square
Used to help
solve genetics
problems
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Designer “Genes”
 Alleles - two forms of a gene
(dominant & recessive)
 Dominant - stronger of two genes
expressed in the hybrid; represented
by a capital letter (R)
 Recessive - gene that shows up less
often in a cross; represented by a
lowercase letter (r)
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Genotype & Phenotype in Flowers
Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2
alleles affect a characteristic
Possible combinations are:
Genotypes
RR
Rr
rr
Phenotypes
RED
RED
YELLOW
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Genotypes
 Homozygous genotype - gene
combination involving 2 dominant
or 2 recessive genes (e.g. RR or
rr); also called pure
 Heterozygous genotype - gene
combination of one dominant &
one recessive allele (e.g. Rr);
also called hybrid
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P1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round r – Wrinkled
Cross: Round seeds x Wrinkled seeds
RR
x
rr
r
r
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: All alike
Phenotypic
Ratio: All alike
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P1 Monohybrid Cross Review
 Homozygous dominant x
Homozygous recessive
 Offspring all Heterozygous (hybrids)
 Offspring called F1 generation
 Genotypic & Phenotypic ratio is ALL
ALIKE
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F1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round r – Wrinkled
Cross: Round seeds x Round seeds
Rr
x
Rr
R
r
R
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
G.Ratio: 1:2:1
P.Ratio: 3:1
copyright cmassengale
17
Given alleles T(tall) and
t(short)…
-What are the possible genotypes of a tall
plant?
-What are the possible genotypes of
a short plant?
-What would be the phenotype of TT?
-What would be the phenotype of tt?
-What would be the phenotype of Tt?
In a famous
experiment, a man
named Gregor Mendel
mated pea plants.
When he mated a pure
bred smooth seed with
a pure bred wrinkled
seed, he found that the
wrinkled seed
phenotype only
occurred about 25% of
the time.
This happened
because the allele for
smooth seeds (S) is
dominant to the allele
for wrinkled seeds (s).
Image retrieved from http://www.accessexcellence.org/RC/VL/GG/mendel.html
Mendel and the relationship between
genotype and phenotype
Click on the link that appears below to see an animation about
Gregor Mendel and the relationship between genotype and
phenotype
**Once you get to the sight, click on
animation
at the
bottom of the page to see the animation!!**
http://www.dnaftb.org/dnaftb/2/concept/index.html
Go to the following link and see how
changing the genotype of a fictional
dragon will change its phenotype.
http://biologica.concord.org/webtest1/web_
labs_genophenotype.htm
Then go to the following link and watch
the video. This may help you remember
what you have learned.
http://youtube.com/watch?v=EvR_Sdm1or
U