Download Mendelian Genetics

Mendelian Genetics
The Pea Plant
• Peas reproduce sexually (they
produce two different gametes
- male and female)
• Pollination= transfer of male
pollen to pistil
• Fertilization= uniting of male
and female gametes
POLLEN + OVULE  SEED
• Self-pollination= flower
pollinates itself; male and
female gametes come from
same plant
• Cross-pollination= male and
female gametes come from
different plants
Gregor Mendel
• 1850’s Austrian monk
and gardner
• “Father of Genetics”
(study of heredity)
• Heredity= passing on
of traits from parent to
offspring
• Traits= characteristics
that are inherited
Mendel’s garden
Mendel’s
Experimental Methods
• Mendel hand-pollinated
flowers using a
paintbrush
– He could snip the
stamens to prevent
self-pollination
– Covered each flower
with a cloth bag
• He traced traits through
the several generations
Mendel’s Studies
• He first studied one trait at a time (the first
trait he studied was the height of pea plants)
• Monohybrid cross= mating between parents
that only differ by ONE trait
(Other traits he studied: flower position,
flower color, seed color, pod color, seed
shape, pod shape)
Mendel’s Studies
Mendel’s Studies
P1 (Parent)
Generation:
X
Cross-pollinate
F1 (filial)
Generation:
Hybrids= offspring
of parents with
different forms of the
same trait
Self-pollinate
F2 Generation:
Mendel tested 6 more
traits. In every cross,
one trait disappeared
in F1, then reappeared
in a ration of ¼ in the
F2 plants.
*Remember- a good
science experiment
must give
reproducible results!
Mendel’s Laws
From his pea plant experiments,
Mendel came up with rules or
“laws” about how traits are
passed down from parent to
offspring:
1. Rule of Heritable Factors:
•
EACH ORGANISM HAS 2 COPIES OF
A GENE THAT CONTROLS EACH
TRAIT; ONE COPY CAME FROM THE
MALE PARENTS AND ONE COPY
CAME FROM THE FEMALE PARENT
Each chromosome has genes (a gene is a
piece of DNA that controls a trait- ex.
height)
Mendel’s Laws
• There can be different forms
of the same gene; these
different forms are called
alleles.
ex. For the height gene, the
alleles are tall and short
• The offspring can inherit any
combination of alleles from
the parents
ex. Tall + tall, short + short,
tall + short, or short + tall
Mendel’s Laws
2. Rule of Dominance
•
•
•
ALLELES CAN BE DOMINANT OR
RECESSIVE; THE DOMINANT ALLELE,
IF PRESENT, COMPLETELY MASKS
THE RECESSIVE ALLELE
Dominant Trait- remained in the F1
generation (ex. tall)
Recessive Trait- disappearned in F1 (ex.
short)
So we say that tall is dominant to short.
• When writing alleles, follow these rules:
– Use the same letter for different
forms of the same gene.
– Use uppercase letter for dominant
allele.
– Use lowercase letter for recessive
allele.
– Always write dominant allele first.
Example: height
T = tall
t = short
A plant can be:
TT tall
Tt
tall
tt
short
Punnett Square
Used to
help solve
genetics
problems
Mendel’s Laws
3. Law of Segregation
THE TWO ALLELES THAT CONTROL EACH TRAIT WILL
SEPARATE DURING GAMETE FORMATION
TT
X
tt
Result: Tt (all tall)
Then
Tt
X
Tt
Result: TT, Tt (tall) and tt (short)
Mendel’s Laws
4. Law of Independent
Assortment
TRAITS ON DIFFERENT
CHROMOSOMES ARE INHERITED
INDEPENDENTLY OF ONE
ANOTHER
• This can be seen in dihybrid crosses.
For example,
– Round Yellow seeded hybrid crossed
with a Round, Yellow seeded hybrid
(RrYy x RrYy)
Other Important Information:
• Phenotype = written description of
appearance/behavior
– Examples: tall and short
BUT, two organisms can look the same but have
different gene (or allele) combinations*:
• Genotype = 2-letter gene combination
– Examples: TT, Tt, tt
*Note: TT and Tt are both genotypes for the same
phenotype, tall
Other Important Information:
There are 3 different types of genotypes:
homozygous dominant
(TT)
homozygous recessive
(tt)
heterozygous
(Tt)
Homo- means same
Hetero- means different
FYI: homozygous is the same as PUREBRED
heterozygous is the same as HYBRID
JUST A LITTLE PRACTICE
Description
Red Hair
GG
mm
White eyes
Short toe
Dd
Genotype or
Phenotype?
Homo- or
Heterozygous?
Steps to Solving Genetics ProblemsWhen solving genetics problems, ask yourself the following:
1. What’s dominant?
2. What letter do I use? What are my
possible combinations?
3. What am I crossing? (What are parents’
genotypes?)
4. How do I set up my punnett square?
5. What is the answer to the problem I’m
trying to solve?
Example
• Tall pea plants are dominant to short plants.
A farmer crossed a homozygous tall plant
with a short plant. How many of the
offspring will be tall plants?
Steps to Solving Genetics Problems
1. Identify the dominant trait. (ex. Tall is dominant
to short.)
2. Use the first letter of the dominant trait to
represent each possible combination of alleles in
the problem. (TT= tall, Tt= tall, tt= short)
3. Identify the parents’ genotypes, and write out
what the cross would be. (Homozygous tall
crossed with short is TT x tt.)
4. Set up your punnett square. Put one parent’s
alleles on one side, and the other parent’s on the
other side.
T T
5. Solve your punnett square.
Answer the question.
100% will be t Tt Tt
tall plants.
t
Tt Tt