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Intro to Mendelian Genetics /monohybrid cross

Mendelian Genetics Unit. Image Credits: Biology (Campbell) 9th edition, copyright Pearson 2011, & The Internet
by

Angela Burcham

on 4 April 2014

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Transcript of Intro to Mendelian Genetics /monohybrid cross

Law of Independent Assortment
Mendelian Genetics
Make Sure You Can
Gregor Mendel
Mendel's Laws
Solving Genetics
Problems


The Life Of Mendel
Mendel's Experiments
Why Peas?
Gregor Mendel
Early Theories of Inheritance
1. Establish "true breeding" lines for particular traits.

2. Crossed true breeding varieties exhibiting alternate forms of traits.

3. Permitted hybrid offspring of first cross to self-fertilize
What Mendel Found
When Mendel crossed alternate forms of true-breeding plants
(P1 generation)
The first generation of hybrid offspring (F1) only shows
one
of the parental forms.
Dominant vs. Recessive
2 alleles!
Sexually reproducing organisms have 2
alleles

(versions)

of any gene.
One from each parent that account for variations in inherited characters
Revisiting genotype vs. phenotype
Law of Segregation
Only one allele for a trait goes into a gamete
The law of segregation is explained by the behavior or chromosomes during metaphases and anaphases of meiosis.
The segregation of alleles is a random process.
Alignment of chromosomes at the metaphase plate is random.
Segregation occurs during anaphases
Separate alleles for separate traits are passed on independently of eachother*
*-as long as the alleles are "unlinked" (on separate chromosomes)
The law of independent assortment is also explained by the behavior or chromosomes during metaphases and anaphases of meiosis.
Recombinant Offspring!
Independent assortment can lead to combinations of traits in offspring that are different from the traits of their parents.

This is another example of "
recombination
"
2 possibilities, only 1 correct
2 alleles stained on homologous chromosomes
Punnet Squares are Useful in predicting probability of inherited traits
The
Punnet Square
exists to help you visualize the alleles an organism can put in a gamete and the combinations that can result from a mating.

A simple monohybrid crosses involves inheritance of one-trait


Punnet squares stop being useful when you start looking at more than one trait at a time. You can thank independent assortment for this.

A two-trait ("
dihybrid
") cross analysis involved keeping track of four genotype combinations for each parent.

Since these are independent, there are 4 possible combinations.

This requires a Punnet square with 16 boxes.
NOT A GOOD IDEA


A tri-hybrid cross would need 64 boxes, a tetra-hybrid cross would need 256 boxes.
2

2. Make Probability Work For You
Since the chances of any unlinked allele winding up in a gamete is independent of the chances of any other unlinked allele winding up in the same allele, basic probability can be used to calculate the chances of any number of combinations of alleles.

Put another way: The odds of having any number of unlinked events occur simulataneously is equal to the product of the odds of occurence of each event
MULTIPLIED TOGETHER
.

This makes our life much, much easier when dealing with genetics problems.

BEWARE
: It also means that the questions that can be asked can get quite complex.
F1 predictions from P-cross:
Analyze each trait independently. Then combine probabilities.
PpYyIi x ppYyii

The Test Cross
Let's Try A Few:
Deterime the genotypic and phenotypic ratios of the F1 generation that result from each of the following crosses.
Notes:
A- normal pigment, a- albino
T- tall, t- short
Y- Yellow, y- green
R- Round, r- wrinkly

Capital letter- dominant allele
Lowercase letter- recessive allele
A method of determining the genotype of an organism that expresses the dominant phenotype.

The organism is mated with a recessive phenotype.

The phenotypes of the offspring are then analyzed.
Explain the experimental method that Gregor Mendel Used.

Explain why peas are a good model organism for genetic studies.

Describe each of Mendel's laws, relate them to the events of meiosis, and use them to explain/predict data from genetic experiments and practice problems.
Humuculus
Preformation
Theory
Blending Theory
Theory of Pangenesis
Early education in an Austrian monastery

Studied chemistry, botany and physics at the University of Vienna .

Eventually became abbot

Mendel studied inheritance using garden peas
Easy to control mating
Conspicuous traits
Purple vs White
You can eat your experiments
Peas commonly used in hybridization experiments at the time

Large number of pure varieties


CHARLES DARWIN (1868) reiterated the theory of pangenesis.-Every part of the animal body contributes particles that are concentrated in the reproductive organs and passed to it's offspring.



Lots of offspring
Easy to grow and maintain
with quick generation times
4. Most importantly he counted the numbers of offspring exhibiting each trait in each generation
Quantitative vs Qualitative observation
When the F1 generation was crossed, both traits were present in the F2.
This was true for each of the seven traits
Mendel called the form of the trait that was present in the F1 generation
Dominant

and the form not expressed,
Recessive
The quantification of results is what distinguishes Mendel's research from other investigators of the time.
Mendel then allowed the F1 plants to mature and self-fertilize. The next generation was called F2
And
Always in a 3:1 ratio
Are the F2 generation plants true breeding?
By allowing the F2 plant to self fertilize he was able to determine that the recessive individuals were true breeding, but that the dominant individuals were of 2 types.

Some true breeding and some hybrid
hidden ratio 1:2:1
Mendel's Conclusions

in modern terms
If both alleles are the same the organism is
homozygous
.
If the alleles are different the organism is
heterozygous.

In a heterozygous organism one allele will be
dominant
and be expressed in the
phenotype
and the other allele will be
recessive
(hidden) but present in the
genotype
The Law of Segregation
Allele pairs segregate (separate) during the production of gametes. A gamete will only contain one allele for each inherited character
monohybrid crosses
Failed the examination to become a teacher .... twice!
Lets go to the board so we can all participate !
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