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Mendelian Genetics 1

1 of 3 of my Mendelian Genetics Unit. Image Credits: Biology (Campbell) 9th edition, copyright Pearson 2011, & The Internet Provided under the terms of a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. By David Knuffke.
by

Heather Botelle

on 3 November 2015

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Transcript of Mendelian Genetics 1

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

The Life Of Mendel
Mendel's Experimental Method
Why Peas?
Mendel's Conclusions
Gregor Mendel was a monk
This often meant a means of education in the mid 1800's
Mendel studied the natural sciences
He also tended the garden
1800's conceptions of inheritance
What did people think about inheritance?


The "
Blending
" hypothesis.

Makes sense, right?
WRONG!
If blending were actually the way things worked, then all variations produced would be diluted over the generations.
"
Model Organisms
":

Peas are a great model organism for genetics:
Easy to grow and maintain
Easy to control mating
Many offspring
Conspicuous traits
Quick generation time.
1. Establish "
true breeding
" lines for particular traits.

2. Perform crosses.

3. Count the offspring that show the identified traits.
Mendel's Data:
For any trait:
When two purebred lines are crossed, in the first generation of offspring (
F1
) only the dominant trait is visible. (D/r)
When the F1 generation is crossed, both traits are shown in the
F2
.
3:1
ratio
When an organism has two different alleles, one ("
dominant
") will be expressed over the other ("
recessive
").
Dominant vs. Recessive
2 alleles!
Sexually reproducing organisms have 2 versions ("
alleles
") of any gene.
One maternal, one paternal
Revisiting genotype vs. phenotype
A few notes:
This is a "modern" restatement of Mendel's conclusions.
We are now aware of many "exceptions" to these rules.
None of the exceptions invalidates these conclusions.
Round vs. Wrinkled
Purple vs. White
Why is it never 3:1 exactly?
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 independently of each other*
*-as long as the alleles are "unlinked" (on separate chromosomes)
The law of independent assortment is also explained by the behavior of 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
"
The Death of the Blending Hypothesis
Mendel's work demonstrates that parental traits are not blended in offspring.

It also demonstrates why people were naively confused into thinking that this was how traits were inherited
Note:
Giant Cross!
2 possibilities, only 1 correct
2 alleles stained on homologous chromosomes
1. Punnet Squares Might Be Useful, Sometimes
The
Punnet Square
exists to help you visualize possible outcomes.

It's mostly useful for one-trait ("
monohybrid
") crosses.
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.

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:
Determine 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.

cross with a recessive phenotype.

analyze phenotypes
How are traits inherited?

How are traits expressed?
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.
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