SCIENCE STARTER #3 – 01/07/16
Explain the differences between:
Genotype vs phenotype
Homozygous dominant vs
heterozygous vs homozygous
recessive
TAKE OUT YOUR VOCAB CHART
7.2 PUNNETT
SQUARES
Ch. 9
Pg. 180186
PROBABILIT Y PRACTICE
 A series of genetic crosses results in 750 round seed
pea plants and 250 wrinkled seed pea plants. What
is the probability that the next offspring is a wrinkled
seed pea plant?
 Individuals with desired trait:
 Total Pea Plants:
 Probability Fraction:
 Percent Chance :
PUNNETT SQUARES
 UPPERCASE: dominant traits
 Lowercase: recessive traits
Parent I
A
Parent II
 Punnett square: diagram to
predict the probability of
inheritance of certain traits
 Monohybrid cross: cross
between individuals that
looks at one trait
a
A
AA
Aa
a
Aa
aa
HOMOZYGOUS VS HETEROZYGOUS
Homozygous: Both alleles are the same
Both dominant (CAPITAL) or both lower-case
Example
Homozygous dominant: BB
Homozygous recessive: bb
Heterozygous: Both alleles are different
One capital and one lower-case letter
Example
Bb
Mendel’s Ratios from his experiments
For a cross involving
two heterozygous individuals
Genotype ratio 1:2:1
(1 PP: 2Pp:1pp)
Phenotype ratio 3:1
(3 purple: 1 white)
MENDELS’ LAW OF SEGREGATION
Each gene (allele) separates from the other
so that the offspring get only one gene
from each parent for a given trait.
BB x b b
B
B
b
b
Mendel’s Law of Segregation
PRACTICE PROBLEM #1
Let’s cross a homozygous tall plant (TT) with a short
plant (tt).
Each plant will give only one of its’ two genes to the
offspring.
CROSS TT X tt BY USING A PUNNETT
SQUARE
The genes from one parent go here.
The genes from the other parent go
here.
CROSS TT X tt BY USING A PUNNETT
SQUARE
T
t
t
T
CROSS TT X tt BY USING A PUNNETT
SQUARE
If a box receives a capital
letter and a lower-cased
letter we write the capital
letter first
T
T
t
Tt
Tt
t
Tt
Tt
INTERPRETING THE RESULTS
Possible offspring Genotype = Tt.
What percent of offspring will have the
following genotypes:
TT= 0%
Tt= 100%
tt= 0%
Possible offspring phenotype = tall.
What percent of offspring will have the
following phenotypes:
Tall= 100%
Short= 0%
PRACTICE PROBLEM #2
Black rabbits are dominant to brown. Two
heterozygous rabbits have offspring. Predict
what percentage of their offspring will be brown.
PRACTICE PROBLEM
Black rabbits are dominant to brown. Two
heterozygous rabbits have offspring. Predict
what percentage of their offspring will be brown.
Step 1: Determine the genotype of parents.
Always
use the first letter of the dominant
trait.
____________x _____________
Step 2: Set up a Punnett Square
Step 3: Complete Punnett Square
Step 4: Predict offspring phenotype
B
Genotypic ratio
1:2:1
BB: Bb: bb
B
BB
b
Bb
Phenotypic ratio
3:1
Black:brown
b
Bb
bb
PRACTICE PROBLEM
Black rabbits are dominant to brown. Two
heterozygous rabbits have offspring. Predict
what percentage of their offspring will be
brown?
% Black= _____
% Brown= _____
Genotype ratio ?
4:0
Phenotype ratio ?
4:0
PRACTICE PUNNETT SQUARES
 Cross two heterozygous round-seeded pea
plants (Use the letter R)
Phenotypic Ratio
Genotypic Ratio