Download Dihybrid Crosses

CHARACTERISTICS OF LIFE
All Living Things reproduce!!!
All Living Things Have DNA!!!
WHY ARE WE ALL DIFFERENT?
We all inherited different genes from our parents
which determines our traits.
Heredity – Passing on of genes which
determine your traits from parents
to offspring.
23 chromosomes from
each parent.
Genes – Pieces of DNA that carry hereditary
instructions and are passed from parents.
Traits – A distinguishing characteristic that is
passed from parents to offspring.
Genetics – Study of heredity (passing
on of genes)
Johann Gregor Mendel was born July 22, 1822.
Mendel became a friar at the Augustinian monastery
in Brno, Czechoslovakia. From 1868 until his death,
Mendel was the abbot of the monastery.
Mendel was experimenting with flowers in the
monastery's gardens. He wondered how traits were
passed from parent to offspring. He studied the
relations between parents and offspring with
mathematical symbols. His favorite plants to
experiment with were peas.
FATHER
Of
genetics
MENDEL’S EXPERIMENT
PART 1
He bred a tall pea plant with a
pure short pea plant.
But ALL the offspring were TALL.
How could that be?
Maybe a mistake was made?
PART 2
He crossed 2 of the
offspring from the above
cross.
Results – 75% Tall
25% Short
Now wait a minute!! He crossed
two tall and go a short? What’s
going on?????
GENETIC RULES
Every organism has TWO forms of a gene for
each trait. One from each parent. Each form is
called an ALLELE.
You can receive a dominant or recessive gene
from your parent.
GREGOR MENDEL
He realized in the first cross, one
trait appeared and the other seemed to be
masked.
* The trait that masks or is stronger than
the other trait is called the dominant trait.
* The trait that is covered up is called the
recessive trait.
For these reasons, Mendel is called the
Father of Genetics.
GENETICS RULES and SYMBOLS
Scientists use symbols to represent different
forms of a gene.
Capital Letters – Represents dominant
trait. Dominant traits are the stronger
traits.
Lower Case Letters – Represents
recessive trait. Recessive traits are the
weaker traits that are often masked by
dominant traits.
Examples-BB – brown eyes
Bb – brown eyes
bb - blue
Example Dominant Traits
Brown Eyes- B
Widow’s Peak- W
Unattached Earlobes- E
Freckles- F
Example Recessive Traits
Blue eyes- b
Straight hairline- w
Attached earlobes- e
No Freckles- f
Homozygous vs. Heterzygous
If the inherited genes (alleles) are the same,
then they are called HOMOZYGOUS or
purebred. Letters are the same size.
Examples – aa , AA , LL , ll , TT , tt
If the inherited genes are different,
then they are called HETEROZYGOUS
or hybrid. Letters are different sizes.
Examples – Aa , Ll , Tt, Bb
Genotype vs. Phenotype
The genotype is the letter combination or
gene combinations
Example – Tt, Aa, bb,or Ll
The phenotype is the actual appearance of
the organism.
Example – tall, brown hair, blue eyes, long
toes
GENETIC PROBABILITY
Mendal crossed yellow and green pea plants
and discovered that 1 out of 4 were green.
He was using probability.
Probability – The possibility or likelihood that
a particular event will occur.
Used to predict the results of genetics
crosses.
PUNNETT SQUARES
A Punnett square is a special chart used to
show the possible gene combinations in a
cross between 2 organisms. (also called
Developed by
monohybrid crosses)
an English
geneticist by
the
name of
Reginald
Punnett.
PARTS OF A PUNNETT SQUARE
Male Genes
Female
Genes
T= tongue roll
t= cannot roll tongue
Offspring
Combinations
How does a Punnett Square Work?
• Draw a square and divide
it into 4 sections.
• Write the gene pairs across
the top of the box, then the
other down the side.
• In each box, place the
correct gene to see the
possible combinations.
Each square
represents a 25%
possibility of getting
that trait.
Tt
Tt
Tt
Tt
Cross between
homozygous
dominant curly
tail (TT) and a
homozygous
recessive
straight tail (tt).
Tt
Tt
Tt
Tt
What are the percentages of the offspring?
What are the genotypes?
What are the phenotypes?
TT
Tt
Tt
tt
Cross between
two
heterozygous
curly tailed
parents (Tt).
What are the percentages of offspring?
What are the genotypes?
What are the phenotypes?
Mathematical Computations
In a Punnett Square where both parents are
hybrids the percents are listed below:
25% purebred (homozygous) curly – TT
50% hybrid (heterozygous) curly - Tt
25% purebred (homozygous) straight - tt
% of same genotype as parents - 50 %
% of same phenotype as parents - 75%
Dihybrid Crosses
A dihybrid cross
is a cross of two
different traits
Mom
T
t
T- height S-shape
T-tall
S-square
Dad
S
S
s
s
List of gamete genotypes:
MomDad-
t-short
s-round
T
t
Dihybrid crosses- formation of zygote
Mom- ST, St, sT, st
Dad- ST, St, sT, st
When parent gametes (egg/sperm) join to form a
zygote, the possible offspring gene combinations are
as follows:
Formation
of zygote
ST
(egg)
St
(egg)
sT
(egg)
st
(egg)
ST
(sperm)
St
(sperm)
sT
(sperm)
st
(sperm)
Dihybrid Crosses
 Parent generation =
gene pairs given by
parents.
 Gametes = half of
parent gene pair
 1st Generation=
combination of
gametes to produce
offspring.
 2nd Generation=
possible combination
of offspring from 1st
generation cross
 The possible gene
combinations from 1st
generation offspring
Practice
G-eye size
G-large
g-small
B- eyebrows
B-separated
b-attached
The squares contain the gene combinations
that could occur in the cross.
The genotype is the letter combination or
gene combinations in the squares.
Example – Tt, Aa, bb,or Ll
The phenotype is the actual appearance of
the organism.
Example – tall, brown hair, blue eyes, long
toes
INCOMPLETE DOMINANCE
Sometimes, you may notice that traits can blend
together. Two capital letters are used.
For example: Red & White are equally dominant
producing Pink flowers.
Examples – roan color in horses and cows, pink color in
flowers are red and white combined.
Codominance
When both alleles contribute to the
phenotype.
For example: Chickens- black & white feathers are
codominant, therefore they can appear speckled.