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AP Bio- Information 9: Intro to Mendelian Genetics
Transcript of AP Bio- Information 9: Intro to Mendelian Genetics
Make Sure You Can
The Life Of Mendel
Mendel's Experimental Method
Gregor Mendel was a monk
Monks were among the most educated members of society in the mid 1800's
Mendel studied the sciences extensively.
He also tended the gardens of the Abbey, where he grew/bred peas.
1800's conceptions of inheritance
What did people think about inheritance?
Every offspring has a blend of traits from its parents
Makes sense, right?
If blending were actually the way things worked, then all variations produced would be diluted over the generations.
The organisms we use in biology are chosen for particular reasons
Peas are a great model organism for genetics:
Easy to grow and maintain
Easy to control mating
Lots of babies
Quick generation time.
1. Establish "
" lines for particular traits.
2. Start crossing the lines.
3. Count the offspring that show particular traits.
For any trait:
When two different lines are crossed, the first generation offspring (
) only shows one of those traits.
When the F1 generation is crossed, both traits are shown in the
. Always (!) in a
When an organism has two different alleles, one ("
") will be expressed over the other ("
Dominant vs. Recessive
Sexually reproducing organisms have 2 versions ("
") of any gene.
One from mom, one from dad
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 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 of chromosomes during metaphases and anaphases of meiosis.
Independent assortment can lead to combinations of traits in offspring that are different from the traits of their parents.
This is another example of "
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
2 possibilities, only 1 correct
2 alleles stained on homologous chromosomes
1. Punnet Squares Might Be Useful, Sometimes
exists to help you visualize the alleles an organism can put in a gamete and the combinations that can result from a mating.
It's mostly useful for one-trait ("
Any monohybrid cross will have four possible genotype combinations.
Different kinds of monohybrid crosses have typical kinds of ratios for offspring "
Ex. A heterozygote crossed with a homozygous recessive individual produces a 1:1 ratio of heterozygote genotype offspring to homozygous recessive genotype offspring, and a 1:1 ratio of dominant phenotype offspring to recessive phenotype offspring.
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 ("
") 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. 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
This makes our life much, much easier when dealing with genetics problems.
: 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.
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.
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.