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Standard 3
Unit 7 Genetics
(Heredity)
Chapters 6.3-7.4 pages:
177-223
The study of how
genes are passed
from Generation to
Generation!
Unit 7 Genetics
Probability
Chapter 6.3 Pg. 177
Heredity Patterns can be calculated
using the rules of probability.
If 2 plants have certain characteristics and they
are bred together, you can calculate the
probability of the offspring having certain
characteristics.
There are 2 rules to help calculate these
probabilities.
Probability
• The likelihood that an event will occur
• Probability = # of one kind of possible outcome
total # of all possible outcomes
ADDITION RULE!
When the word or is used you add the odds.
So, 1/6 + 1/6 = 2/6
MULTIPLICATION RULE!
When the word and appears you multiply the odds.
So, 1/6 x 1/6 = 1/36
Probability
• What is the probability of rolling a 6 on a die?
– 1/6
• What is the probability of rolling a 6 or a 5 on a dice
cube?
– 1/6 + 1/6= 2/6
• What is the probability of rolling a 6 and a 5
– 1/6 x 1/6= 1/36
– Please do the practice problems on the worksheet HW#1
11-2 Probability and Punnett Squares
Probabilities Predict
Averages
Probabilities Predict Averages
Probabilities predict the average outcome of a
large number of events.
Probability cannot predict the precise outcome
of an individual event.
In genetics, the larger the number of offspring, the
closer the resulting numbers will get to expected
values.
Slide
6 of 21
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End Show
Traits, Genes and Alleles
chap 6.4 pgs 180-182
• A trait is a specific characteristic that varies from
one individual to another
• Genes: the factors that determine traits
• Alleles: alternate forms of a gene
• Homozygous: Two of the SAME alleles (AA)
• Heterozygous: Two different alleles (Aa)
• Genome: all of an organism’s genetic material
• Genotype: genetic makeup of a specific set of
genes
• Phenotype: The physical characteristics or traits
Dominant allele: Gene that is expressed
AA BB
Recessive allele: Gene that is not expressed
aa bb
Mendelian Genetics
Gregor Mendel (1822-84)
• Father of Genetics
• Austrian Monk, spent his life working
with traits of Pea Plants
• Traits passed in “factors” GENES! from
Generation to Generation
• Working during the same time period
as Darwin
• He didn’t know what a GENE was !!
Genes and Dominance
Mendel studied seven pea plant traits, each with
two contrasting characters.
Each original pair of plants is the P (parental)
generation.
• The offspring are called the F1, or “first filial,”
generation.
• The offspring of crosses between parents with
different traits are called hybrids
How do we look at gene
combinations?
We use :
Punnett Squares
The gene combinations that might result
from a genetic cross can be determined by
drawing a diagram known as a Punnett
square.
Punnett squares can be used to predict
and compare the genetic variations that
will result from a cross.
6 steps to solving a problem
Genetics problems require the same 6 steps!
1.
2.
3.
4.
5.
6.
Identify Dominant and Recessive Phenotypes
T=Tall t=Short
Parent phenotype x parent phenotype
tall x tall
Parent genotype x parent genotype
Tt x Tt
Punnett Square
Phenotypic ratio
3 tall: 1 short
Genotypic ratio
1:2:1
TT:Tt:tt
Punnett Squares
Lets try this Mono(1) hybrid(mixture)
cross
A capital letter
represents the dominant
allele for tall.
A lowercase letter
represents the recessive
allele for short.
In this example,
T = tall
t = short
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• Are the parent plants heterozygous or
homozygous?
• Homozygous organisms are TRUE-BREEDING
• Heterozygous organisms are HYBRID
• Are the parents True-breeding or Hybrids?
Punnett Squares
All of the tall plants have the same phenotype, or
physical characteristics.
BUT..All of the tall plants do not have the same
genotype, or genetic makeup.
One third of the tall plants are TT, while 2/3 of the
tall plants are Tt.
The offspring are the
F1 generation
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Punnett Squares
These F1 plants have
different genotypes
(TT and Tt), but they
have the same
phenotype (tall).
Lets find out what
kind of plants we will
get by crossing these!
TT
Homozygous
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Tt
Heterozygous
• Look back at the F1 plant that have the TALL
Phenotype TT, Tt
• How can you determine the genotype of
these plants ? How can you tell the
• Homozygous dominant TT
from the
• Heterozygous Tt
What do you do if you don’t know
the genotype of the dominant
parent?
TEST CROSS
A test cross is used to determine the
genotype of a dominant trait.
 (A dominant trait can be either
homozygous dominant or heterozygous.)
 The unknown trait is crossed with the
recessive trait.
If any offspring show the recessive trait,
the dominant parent had to be
heterozygous.
Dihybrid Crosses…they are twice as
chap 6.5 pg 186
much fun!!!
• Two Traits example: seed color and shape!
• Created by Mendel to test Laws of Independent
Assortment and Segregation
Cross 2 plants that are heterozygous
for the 2 traits (seed shape & pod
color)
Sample #1: Pea Plants
R = round seeds r = wrinkled seeds
Y= yellow pods
y = green pods
1) Identity Dominant and Recessive
2) Parent phenotypes
3) Parent genotypes
4) 1,3 1,4 2,3 2,4
5) Punnet square
6) Possible Phenotypes
7) Phenotypic ratio
• If you are looking at two genes at the same
time, how do you figure out the possible
combinations?
• Look at the positions to determine combos
•R r Y y
1 2 3 4
if this is one parent what alleles
can he contribute?
• 1&3 1&4 2&3 2&4
• RG
Rg
rG
rg
Try this!
• You have a round (R) and yellow (Y) pea plant
and cross it with a wrinkled (r) and green (y)
plant. Both are homozygous, one dominant
and the other recessive
• Please label and do this cross on your notes
• What are the phenotypes for the offspring?
• What are the genotypes for the offspring?
Independent Assortmen
The alleles for round (R) and yellow (Y) are
dominant over the alleles for wrinkled (r) and
green (y).
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Now lets take two of these offspring
and cross them.
•
•
•
•
Parent phenotype(F1 generation)
Parent genotype(F1 generation)
RrYy
Determine the alleles that each parent can
give: 1&3 1&4 2&3 2&4
• Please do the cross and generate the F2
generation
Independent Assortmen
The Two-Factor Cross: F2
Mendel crossed the heterozygous F1 plants (RrYy) with
each other to determine if the alleles would segregate
from each other in the F2 generation.
RrYy × RrYy
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The Punnett square
predicts a 9 : 3 : 3 :1 ratio
in the F2 generation.
Independent Assortmen
Represents:
Independent Assortment
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Independent Assortmen
The alleles for seed shape segregated
independently of those for seed color. This
principle is known as independent assortment.
Genes that segregate independently do not
influence each other's inheritance.
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Independent Assortment
The principle of independent assortment states
that genes for different traits can segregate
independently during the formation of gametes.
Independent assortment helps account for the
many genetic variations observed in plants,
animals, and other organisms.
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Mendel’s Data revealed 3 patterns,
LAWS
1. Law of Dominance: traits can have 2 alleles
(one dominant “A” & one recessive “a”)
Dominant allele:- overpowers or covers up the recessive
- Fully expresses the protein
- Represented by Capital letter A, B EX: Brown eyes
Recessive allele:
- is masked by the other allele
- little or no protein is expressed
- is represented by lower case letter a,b EX: Blue eyes
Mendell’s Laws
2. Law of segregation :
1. Organisms inherit 2 copies of each gene, one from
each parent.
2. Organisms donate only one copy of each gene, thus
the 2 copies of each gene segregate or separate
during meiosis
Mendel’s Laws
• 3. Law of Independent Assortment
The presence of one trait does not affect the presence
of another, genes for different traits segregate
independently of each other during gamete
formation.
(this is seen in a two factor cross, dihybrid cross)
This helps account for many genetic variations
observed in plants, animals and other organisms.
Genetic linkage
Chapter 6.6 page 189-191
• Independent assortment creates a lot of
variation.
• Crossing over in Meiosis also creates variation
Genes located close together on the same
chromosome tend to be inherited together.
This is termed linkage