Download Mendelian Genetics

Mendelian Genetics
Inheritance
*Gregor Mendel
The Father of Genetics
Born in Austria
Entered a monastery at
21
University of Vienna
– Use mathematics to
explain phenomena
– Variation in plants
1857- started
experimenting with pea
plants
*Gregor Mendel
Peas were a good
choice
– Many varieties of traits
– Ability to control
mating pairs
He began with truebreeding strains
Experimented with
hybridization
*Mendel’s crosses
Mendel cross bred 2 true-breeding
varieties of a trait and examined their
offspring
– Purple flowers x white flowers
P = parental generation
F1 = First filial generation (P X P)
F2 = Second filial (F1 X F1)
*Mendel crossed
the F1 to get the F2
He repeated this
many times
Results: 705:224
Ratio of 3.15: 1
*Why this pattern?
Why was the F2 so significant?
If you were Mendel, what
information could you gather from
this experience?
Mendel’s 4 part hypothesis:
1. Alternative versions of genes account
for variations in inherited chromosomes
– Today we know this as an allele
– Allele: alternative version of a gene that
produces distinguishable traits
– Example: There are more that one color of
eyes. Each color is produced by a different
code so each color is considered its own
allele.
• Blue, Brown, Green etc.
Mendel’s 4 part hypothesis:
2. For each character, an
organism inherits 2
alleles, one from each
parent
– 2 copies of each
chromosome, each with
same gene loci
These alleles can be the same
or different
Mendel’s 4 part hypothesis:
3. If two alleles at a locus differ, then the
dominant allele determines the
organism’s appearance; the other, the
recessive allele, has no noticeable affect
on appearance
Translation: If alleles are different, then one will
be expressed - the dominant one
Dominant trait: trait that will always be
expressed
Recessive trait: trait that can be hidden
Mendel’s 4 part hypothesis:
4. The two alleles for a heritable
character separate (segregate) during
gamete formation and end up in different
gametes
– What known phenomenon has Mendel
discovered?
– The Law of Segregation
Who is Gregor Mendel?
What did he do?
What else did you learn?
4 parts of his research
Alternative versions of genes account for
variations in inherited chromosomes
For each character, an organism inherits 2
alleles, one from each parent
If two alleles at a locus differ, then the
dominant allele determines the organism’s
appearance; the other, the recessive allele,
has no noticeable affect on appearance
The two alleles for a heritable character
separate (segregate) during gamete
formation and end up in different gametes
Really important vocabulary
Allele
Dominant allele
Recessive allele
Law of segregation
Law of Segregation
What is it?
How did Mendel find this out?
He did crosses
Data!
More really important vocabulary!
Homozygous: Organism has identical alleles
Heterozygous: Organism has different alleles
Genotype = genetic makeup of an organism,
represented by letters
Phenotype = Visible or measurable traits
that are based upon genotype
Using our vocab- modern
notation
Homozygous
Heterozygous
Genotype
Phenotype
Ok, now what?
The value of Mendel’s discoveries
– Predictable patterns of inheritance
• Parent to offspring
– Law of segregation seen in single trait
crosses
Punnett Squares!
What are they
good for?
– Easy way to
predict genetic
outcomes!
– Illustrate the Law
of Segregation
– Shows all
possible gametes
Punnett Square Set up
Monohybrid cross crossing one
heterozygote with
another
Notice anything?
Count up genotypes and phenotypes
Ratios can be predicted
– Genotypic ratio = 1:2:1
– Phenotypic ratio = 3:1
Lets do some practice Punnett
squares!
Count up genotype and phenotype
ratios
Can we compute probabilities?
PP x pp
Pp x Pp
Yy x YY
Parents:
Parents:
Test Cross
Can we determine the genotype of a
parent when it is unknown?
P_ X pp
All purple offspring
50% purple, 50% white
Mendel also discovered the Law
of Independent Assortment
How did he discover this law
without modern knowledge of cell
division or chromosomes?
Results can be seen in his
experimental crosses for 2 traits
Dihybrid crosses
Two Factor Crosses- 2 traits
Both parents are heterozygous for both
traits
– Let’s set up the parents!
– Y and R
Parents
Y = yellow seed
y = green seed
R = round
r = wrinkled
What are the parent phenotypes?
Calculation time
Phenotypes:
Phenotype Ratio:
Probability:
Genotype ratio:
Is there anything predictable about
these crosses?
Parents:
What is independent
assortment?
Look it’s Independent
Assortment!
Gamete possibilities
– RY
– Ry
– rY
– Ry
Each parent would have four possible
gametes, creating 16 possible
combinations for fertilization
*An important side note
Mendel’s laws of segregation and
independent assortment only work for
genes located on separate chromosomes
What about genes on the same
chromosome?
– They move by different rules
• More on this later
Partner practice
Practice Problems
Pea pod shape: I = inflated, i = constricted
Parents: Ii x Ii
– Find phenotype ratio
– Find genotype ratio
– What is the probability of getting a
inflated pea pod in the offspring?
Practice Problems
Flower position: A= axial, a = terminal
Stem height: T= tall, t= short
Parents: AaTt x AaTt
– Find phenotype ratio
– What is the probability of getting an
axial flower and a short stem?
– What is the probability of getting a
terminal flower and a tall stem?
Practice Problems
Flower position: A= axial, a = terminal
Stem height: T= tall, t= short
Parents: Aatt x aaTt
– Find phenotype ratio
– What is the probability of getting an
axial flower and a tall stem?
– What is the probability of getting a
terminal flower and a short stem?