Transcript 2_Mendelian Genetics

1. Why do organisms need haploid cells?
2. What would happen if “crossing over” didn’t occur during Meiosis?
3. What are gametes?
Heredity
• Heredity is the passing on of characteristics
from parents to offspring.
These characteristics are called
traits
Gregor Mendel
• It was not until the mid-nineteenth century that
Gregor Mendel, an Austrian monk, carried out
important studies of heredity.
• 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 plant
with another, he opened
the petals of a flower and
removed the male
organs.
• He then dusted the female organ with pollen
from 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 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 tall and short height.
Parents
Hybrid Offspring
Monohybrid crosses
• Mendel’s first experiments are called mono-hybrid crosses
• Mono means “one”
• Hybrid means “parents with different forms of a trait”
•
So we are crossing two parent plants that differ from each
other by a 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.
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.
• Genes exist in alternative forms.
We call these different gene forms 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 trait that sometimes disappeared “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 for different
alleles of the same gene.
T
t
for Tall
for Short
The letter “T” equals height.
Short plant
Tall plant
t
T T
t
T
F1
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 is always
written first for each trait.
Tt
t
F1
All tall plants
T t
The Law of Segregation
• The law of segregation states:
every individual has two alleles of each gene.
(one inherited from mom, one inherited from dad)
But, when gametes (or sex cells) are produced, each
gamete only receives one of these alleles to pass on.
Bb
bb
B
b
b
b
Phenotypes and Genotypes
• The way an organism looks and behaves
is called its phenotype. (brown eyes)
• 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.
DISCUSS WITH THOSE AROUND YOU:
WHAT ARE TRAITS AND ALLELES, AND
HOW THEY ARE PASSED DOWN
Question 1
What is the relationship between TRAITS and
ALLELES?
DISCUSS WITH THOSE AROUND YOU:
WHAT DOES “MONO-HYBRID CROSS” MEAN
Question 2
What is a Mono-hybrid cross?
DISCUSS WITH THOSE AROUND YOU:
MENDEL’S LAW OF SEGREGATION
Question 3
In your own words, explain the law of
segregation?
How are alleles segregated and passed to
offspring?
Bb
B
b
1. Using the picture above, explain Mendel’s Law of Segregation.
T
2. Why is
t
this Punnett
square set up
incorrectly? S
s
TT
3. Why is
this Punnett Tt
square set up
incorrectly? Tt
TT
Di-hybrid crosses
• Remember, Mendel performed experiments on pea
plants that only differed in one trait: HEIGHT
• Mendel then decided to perform another set of
crosses in which he used peas that differed from each
other only in TWO traits rather than one.
Di-hybrid crosses
These experiments were called Di-hybrid crosses:
• Di means “two” - because there were only two differences
• Hybrid because the parents were different from each other
(in two ways)
• Cross because he chose the parents to breed with each other
The pea plants were the same in every way, EXCEPT
PEA SHAPE (round, wrinkled) and PEA COLOR (yellow, green)
THE TWO DIFFERENCES
• In pea plants:
Yellow is Dominant “Y” and Green is Recessive “y”
Round is Dominant “R” and Wrinkled is Recessive “r”
RR YY
rr yy
Mendel took pure-bred plants with round yellow peas
and crossed them with pure-bred plants with
wrinkled green seeds.
• What happened with the first generation of offspring?
P1- Parents
F1- first generation of
offspring
F2- 2nd generation of
offspring
What happened with the second generation of offspring?
The Law of Independent Assortment
• The Law of Independent Assortment states:
genes for different traits—for example, eye
shape and eye color—
are inherited independently of each other.
bb
Bb
rr
RR
Bb
Rr
Punnett Square of Dihybrid
Cross
Gametes from RrYy parent
Gametes from RrYy parent
RY
Ry
rY
ry
RRYY
RRYy
RrYY
RRYy
RRYy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
Dihybrid crosses
RrYy
RY
Ry
rY
ry
• A Punnett square
for a dihybrid
cross will need to
be four boxes on
each side for a
total of 16 boxes.
• Remember with the Law of Segregation in a
mono-hybrid cross, you had to separate the alleles
because you can only pass ONE ALLELE FOR EACH
TRAIT on to your offspring.
T
t
In a di-hybrid, you still only pass one allele PER trait to your
offspring….
But remember:
in a di-hybrid
you have two
RR YY
traits….
RY
T
TT
Tt
t
Tt
tt
RY
RY
RY
Male parent is Heterozygous for Brown eyes and Homozygous recessive for
almond
Let’s practice one together,
theneyes:
you will do one on your own.
Bb rr
Female parent is Homozygous recessive for Blue eyes and Heterozygous for
Round eyes:
bb Rr
Br
Br
br
br
bR
Bb Rr
Bb Rr
bb Rr
bb Rr
br
Bb rr
Bb rr
bb rr
bb rr
bR
Bb Rr
Bb Rr
bb Rr
bb Rr
br
Bb rr
Bb rr
bb rr
bb rr
Father, CcSs (Curly hair, Slanted eyes)
CS
Cs
cS
cs
CS
Mother, CcSs (Curly hair, Slanted eyes)
CCSS
CCSs
CcSS
CcSs
Cs
CCSs
CCss
CcSs
Ccss
cS
CcSS
CcSs
CcSs
Ccss
ccSS
ccSs
cs
ccSs
ccss
Head, C-curly, c-straight
Eyes, S-slanted, s-straight