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Mendelian Genetics
Gregor Mendel
• Austrian Monk
• Cross breed different varieties
of pea plants in order to create
new characteristics
• Kept records and ratios of
thousands of offspring
• Found specific ratios which led
him to believe that each
fertilization had a chance
outcome with a certain
probability
Gene:
• Hereditary units for specific
traits that are found on a
specific location (locus) on a
chromosome
• Diploid cells have a pair of
genes for each trait, on a pair
of homologous chromosomes
• NON-DUPLICATED
HOMOLOGOUS
CHROMOSOMES 
Allele:
• Different molecular forms for the same trait
Dominant Allele
• A trait that is always expressed
• Masks recessive traits
• Expressed as capital letters, ex: A
Recessive Allele
• A trait that is only expressed when both alleles
are recessive
• Expressed as lower case letters, ex: a
Homozygous:
• When both alleles are the same
• Ex: AA (homozygous dominant) or aa
(homozygous recessive)
Heterozygous
• Having two alleles for different traits
• Aa
Genotype
• The genetic make up or code of an organism
(can not see)
Phenotype
• The physical expression of ones genotype
• (can see)
Mendel’s Pea Traits:
Mendel’s Pea Traits
Trait:
Dominant: Recessive:
Seed Shape Round – R
Wrinkled – r
Seed Color
Yellow – Y
Green – y
Pod Color
Green – G
Yellow – g
Stem Length
Tall - T
Short - t
Mendel’s Pea Traits
Genotype
Phenotype
Homozygous or
Heterozygous
TT
Tall Stem
Homozygous Dominant
Yy
Yellow Seeds
Heterozygous
Rr
Round Seeds
Heterozygous
gg
Yellow Pod
Homozygous Recessive
YY
Yellow Seeds
Homozygous Dominant
rr
Wrinkled Seeds
Homozygous Recessive
Tt
Tall Stem
Heterozygous
Write in the possible genotypes:
Punnett Square
• Method of predicting the probable outcome of a
genetic cross
Monohybrid Cross: Rr & Rr
R
r
R
RR
Rr
r
Rr
rr
• Genotype Ratio: 1:2:1,
Alleles from each
parent are
brought down,
remember alleles
from each parent
are split into
gametes during
meiosis.
Each allele is
brought down
into the boxes to
form the
possible
genotypes of the
offspring
Homozygous Dominant:Heterozygous: Homozygous Recessive
• Phenotype Ratio: 3:1, Round:Wrinkled
• Therefore there is a 75% chance of having offspring with
round seeds and 25% chance of wrinkled seeds
Dihybrid Cross: RrGg vs. RRgg
RrGg vs. RRgg
RG
Rg
rG
rg
Rg
RRGg
RRgg
RrGg
Rrgg
Rg
RRGg
RRgg
RrGg
Rrgg
Rg
RRGg
RRgg
RrGg
Rrgg
Rg
RRGg
RRgg
RrGg
Rrgg
Practice:
1.
2.
3.
4.
5.
Heterozygous round versus
wrinkled
Homozygous yellow seed
versus green
Homozygous tall plant
versus a homozygous short
plant
Wrinkled seed plant versus a
heterozygous round seed
plant
A homozygous tall with
homozygous yellow seeds
versus a homozygous short
with homozygous green
seeds
1.
R
r
2.
Y
Y
r
Rr
rr
y
Yy
Yy
r
Rr
rr
y
Yy
Yy
3.
T
T
4.
R
r
t
Tt
Tt
r
Rr
rr
t
Tt
Tt
r
Rr
rr
5.
TY
TY
TY
TY
ty
TtYy
TtYy
TtYy
TtYy
ty
TtYy
TtYy
TtYy
TtYy
ty
TtYy
TtYy
TtYy
TtYy
ty
TtYy
TtYy
TtYy
TtYy
Practice:
• TtYy vs. TtYy
• State the genotype and
phenotype ratios in addition to a
punnett square
• Genotype Ratios:
• 1 TTYY, 2 TTYy, 2 TtYY,
4 TtYy, 2 Ttyy, 2 ttYy, 1 TTyy,
1 ttYY, 1 ttyy
• 1:2:2:4:2:2:1:1:1
• Phenotype Ratios:
• 9 tall with yellow seeds, 3 short
with yellow seeds, 3 tall with
green seeds, 1 short with green
seeds
• 9:3:3:1
TY
Ty
tY
ty
TY
TTYY
TTYy
TtYY
TtYy
Ty
TTYy
TTyy
TtYy
Ttyy
tY
TtYY
TtYy
ttYY
ttYy
ty
TtYy
Ttyy
ttYy
ttyy
Pedigree
• A method of genetic analysis using a family tree like diagram
• Used to trace and predict outcomes of offspring
Recessive
because it
would have
shown
through here
Ll or LL
Ll
ll
ll
Ll
Ll
Ll
Ll
Ll or LL
ll
1.
2.
Ll
ll
Ll
Pedigree Practice Problem:
Ll
ll
Is lung disease dominant or
recessive? How do you know?
Label as many people as possible
with their genotypes
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