Transcript Monohybrid Crosses PPT File

BELLRINGER
1. Is the karyotype below of a diploid or haploid cell? Explain.
2. Why do some organisms need haploid cells?
3. What would happen if humans only produced diploid cells?
4. Give three ways you can refer to ‘haploid cells’.
Essential Question
How is the Law of Segregation demonstrated
in Punnett Squares?
Heredity
• Heredity is the passing on of characteristics
from parents to offspring.
These characteristics are called
traits
Gregor Mendel
• It was not until the 1850’s that Gregor Mendel,
an Austrian monk, carried out important
studies of heredity that we started to
understand how traits are passed down.
• Mendel argued that parents pass on to
their offspring factors (now called genes)
that are responsible for inherited traits.
• Mendel was the first person to succeed
in predicting how traits are transferred
from one generation to the next.
Mendel chose his subject carefully
• Mendel chose to use the
garden pea in his
experiments.
• Garden pea plants reproduce sexually, which
means that they produce male and female sex
cells, called gametes.
Mendel wanted to pick his own parent
plants
Remove
male parts
• When Mendel wanted to
breed, or cross, one
specific plant with
another, he opened the
petals of a flower and
removed the male
organs.
• He then dusted the pollen on the female
organ of the plant he wished to cross it with.
Pollen
grains
• He only wanted to study one trait at a time at first.
SHORT
TALL
• So he picked two plants that were the same in every
way, except ONE, height.
• The tall pea plants he worked with were from
populations of plants that had been tall for many
generations and had always produced tall offspring.
• They were said to be pure-bred tall“pure-bred” means that it comes from two parents with
the same form of a trait.
• Likewise, the short plants he worked with were
pure-bred for shortness.
• A hybrid is the offspring of parents that have different
forms of a trait, such as a tall parent and a short
parent.
Parents
Hybrid Offspring
Monohybrid crosses
• Mendel’s first experiments are called mono-hybrid crosses
• Mono means “one”- for ONE TRAIT
• Hybrid means “parents with different forms of that trait”
•
So we are crossing two parent plants that differ from each
other only by one trait—
which in this case is height.
The First Generation
• He cross-pollinated the tall pea plant with pollen from
a short pea plant.
• All of the offspring grew to be as tall as the taller parent.
P1: Parents
F1: First generation
of offspring
The Second Generation
• Mendel allowed the tall plants in this first generation of
offspring to pollinate.
P1- Parents
F1- first generation of
offspring
F2- second generation of
offspring
What happened?
• Mendel concluded that each organism has two
genes that control each of its traits.
Gene
• We now know that these genes are located on chromosomes,
and that you have TWO for each trait, because you inherit one
from mom and one from dad.
• Genes exist in alternative forms.
These different forms of a trait are called
alleles.
Gene for
Hair Texture
Mom’s allele codes for
Straight hair
Gene for
Hair Texture
Dad’s allele codes for
Curly hair
Chromosome # 1
Chromosome # 1
Inherited from mom
Inherited from dad
• An organism’s two alleles are located on their homologous
chromosomes—one inherited from the female parent and one
from the male parent.
b
B
Bb
Rule of Dominance
• Mendel called the over-powering allele dominant
and
• the allele that can be “over-powered”, recessive.
Bb
• Mendel concluded that the
allele for tall plants is dominant
to the allele for short plants.
• It is customary to use the
same letter
when discussing the
same gene.
Even if the alleles are different
Short plant
Tall plant
t
T T
T
t
for Short
for Tall
t
T
F1
The letter “T” equals height.
Big T is Tall
Little T is Short
t
All tall plants
T t
• An uppercase letter is used for
the dominant allele and a
lowercase letter for the
recessive allele.
Short plant
Tall plant
t
T T
t
T
• The dominant allele for a trait
is always written first.
Tt
t
F1
All tall plants
T t
The Law of Segregation
• The law of segregation states:
every individual has two alleles for each trait.
(one inherited from mom, one inherited from dad)
But, when gametes (or sex cells) are produced, your
alleles separate into separate sex cells.
Bb
bb
B
b
b
b
Phenotypes and Genotypes
• The way an organism looks and behaves
is called its phenotype. (widows peak)
• The allele combination an organism contains
is known as its genotype. (Bb)
• An organism’s genotype can’t always be
known by its phenotype.
Bb
BB
• An organism is homozygous for a trait
if its two alleles for the trait are the
same.
TT
• The pure-bred tall plant that had two alleles for
tallness (TT) would be homozygous for the trait
of height.
• An organism is heterozygous for a trait if its two
alleles for the trait differ from each other.
Tt
• Therefore, the tall plant that had one allele for
tallness and one allele for shortness (Tt) is
heterozygous for the trait of height.
What is the phenotype of the pea plant below?
What is the genotype of the pea plant ?
tt
So therefore, what would you call this trait?
Homozygous short
Punnett Squares
• A Punnett square is a quick way to find the possible
allele combinations in offspring.
• Lets say: one parent has a genotype of Tt
the other parent also has Tt
T
t
T
TT
Tt
t
Tt
tt
Remind me of
what the Law of
Segregation
states.
Let’s Practice:
1. Create a Punnett square that crosses a Homozygous recessive male
with a Heterozygous female.
- What is the genotype ratio:
- What is the phenotype ratio:
2. Create a Punnett square in which all of the children are Heterozygous.
What are the parent’s genotypes?