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Genetics
How Traits are Passed from Parents
to Offspring
I. Gregor Mendel and his
pea plants
A. Studied how traits were
passed from parent to
offspring in pea plants
1. Why did he study pea
plants?
*Large number of
offspring
*7 easily seen traits
(flower color, height, seed color, seed texture, pod
color, pod appearance, flower location)
2. Observations from pea plants:
* Created pure bred pea plants for a particular trait
(always produced offspring with that trait)
Example: If two pure bred purple plants were
crossed you would end up with….
Purple X Purple = all purple offspring (100%)
X
=
Pure bred plants are called the P1 generation
* Performed specific test crosses
Example: What would happen if a pure bred white
flower was crossed with a pure bred purple flower?
Purple (P1) X White (P1)= all purple offspring (100%)
X
P1
P1
=
F1
P1
What if we crossed 2 of the offspring from this cross?
Let’s Try It!
F1 Purple X F1 Purple = 75% Purple and 25% White
X
F1
=
F1
F2
B. Mendel’s Conclusions (4)
1. Traits are controlled by 2 factors
(genes)
Gene for eye
color from
mom
Offspring receives
2 genes for each
trait – one from
each parent
Gene for eye
color from
dad
2. There are dominant and recessive genes
Pure Purple Flower
X
Pure White Flower
X
PP
pp
Pp
All offspring are purple because the P is
dominant over the p
3. Law of Segregation: genes separate
during meiosis (gamete production)
*So, you have one copy of each
gene in the sperm and one
copy of each in the egg
4. Law of Independent Assortment: genes
for different traits are distributed into
gametes randomly
C. Vocabulary:
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Genetics: study of heredity
Heredity: the passing on of traits from parents to offspring
Trait: a specific characteristic (eye color)
Gene: factors that are passed on from one generation to the next
Allele: different forms of a gene (blue vs. brown)
Dominant Allele: gene that will mask the recessive allele
Recessive Allele: not seen if dominant allele is present
Genotype: alleles present for a certain gene
* Homozygous Dominant: both alleles are dom. (TT)
* Homozygous Recessive: both alleles are rec. (tt)
* Heterozygous: one allele is dom. and one is rec. (Tt)
Phenotype: the physical characteristic shown from the genotype
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True-Bred or Pure Bred: an organism who will always produce offspring
identical to themselves
P1: parental generation
F1: offspring from P1
F2: offspring from F1
Law of independent assortment: homologous chromosomes arrange
themselves randomly during meiosis 1
Law of segregation: homologous chromosomes separate during meiosis 1
Probability: the likelihood of an event occurring
Monohybrid cross: cross that involves one trait
Dihybrid cross: cross that involves 2 traits
Punnett Square: used to predict offspring
Genotypic Ratio: ratio of homozygous dominant: heterozygous:
homozygous recessive
Phenotypic Ratio: ratio of dominant trait: recessive trait
Complete Dominance: in the heterozygous form, only the dominant trait is
expressed
Incomplete Dominance: in the heterozygous form, there is an in between
trait
Codominance: in the heterozygous form, both traits are expressed
II. Punnett Squares
A. Used to predict all possible offspring
1. Dad’s genes on top, mom’s on the left
2. Fill in squares to determine possible
offspring
*Example: What are the chances of having a boy?
What can dad give?
What can mom give?
50% chance of having a boy
X
Y
X
XX
XY
X
XX
XY
What would you get if you crossed a homozygous purple flower
with a homozygous white flower?
*First you must figure out what the genes for the
“parents” are.
Homozygous purple= PP
Homozygous white= pp
* Second, place the genes on a punnett square and fill
P
p
p
Pp
Pp
P
Pp
Pp
Third: write the ratios (genotypic and phenotypic)
Genotypic Ratio
Homozygous Dominant (PP): Heterozygous (Pp): Homozygous Recessive (pp)
0
:
p
p
4
:
P
P
Pp
Pp
Pp
Pp
0
Phenotypic Ratio
Dominant Trait: Recessive Trait
4
p
p
:
0
P
P
Pp
Pp
Pp
Pp
Now you set up the following test
cross and solve for the ratios
Cross a homozygous
purple flower with a
heterozygous purple
flower.
P
PP
P
PP
P
Geno. Ratio
2:2:0
Pheno. Ratio
4:0
Pp
p
Pp
Now you set up the following test
cross and solve for the ratios
Cross two heterozygous
tall plants
T
TT
t
Tt
T
Geno. Ratio
1:2:1
Pheno. Ratio
3:1
Tt
t
tt
Dihybrid Cross: cross involving 2 traits
Seed color: Y=yellow, y=green
Seed texture: S= smooth, s=wrinkled
Homozygous green wrinkled seed
Genotype: yyss
Heterozygous yellow, Heterozygous smooth seed
Genotype: YySs
*Now we must find all possible
combinations for each parent plant
Parent #1: yyss
ys
ys
ys
ys
ys
ys
ys
ys
Parent #2: YySs
What are all of the possible allelic combinations from this parent?
YS, Ys, yS, ys
ys
YS
Now fill
in the
chart!
Ys
yS
ys
ys
ys
ys
YS
Ys
ys
ys
ys
ys
YySs
YySs
YySs
YySs
Yyss
?
Yyss
Yyss
yySs
yySs
?
yySs
yyss
?
yyss
yyss
yS
ys
III. Types of Dominance
A. Complete dominance: one trait is completely
dominant over another
(Rr= Red)
B. Incomplete Dominance: heterozygous offspring
will have a phenotype in between the
dominant and recessive trait (Rr= pink)
C. Codominance: heterozygous offspring show
both traits (Rr= red and white hair)
Incomplete Dominance
Codominance