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1 | Page Mendelian Genetics Notes Gregor Mendel & His Observations Gregor Mendel was a monk in the 1850s, who also had a strong passion and interest in science. Part of Gregor Mendel’s job for his church, was to tend to the fruit and vegetable garden. With the approval of the church, he used his time in the garden, to study-observe-and try and figure out where different plants’ traits came from. Gregor Mendel Noticed That: o There were different physical traits between the plants. o Each physical trait, had different variations. o He needed to take the sperm and egg from the parent plants, to make offspring plants. o Whatever physical traits came from, must be in the parents’ sperm and egg. o Some variations of traits seemed to show up more frequently than others. o Some variation of traits seemed to be “hidden” in the parent plants, but would show up all of a sudden in some of the offspring plants.
Transcript of Mendelian Genetics Notes - msrozema.weebly.com · Mendelian Genetics Notes Gregor Mendel & His...
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Mendelian Genetics Notes
Gregor Mendel & His Observations
Gregor Mendel was a monk in the 1850s, who also had a strong passion and interest in
science.
Part of Gregor Mendel’s job for his church, was to tend to the fruit and vegetable
garden.
With the approval of the church, he used his time in the garden, to study-observe-and
try and figure out where different plants’ traits came from.
Gregor Mendel Noticed That:
o There were different physical traits between the plants.
o Each physical trait, had different variations.
o He needed to take the sperm and egg from the parent plants, to make offspring
plants.
o Whatever physical traits came from, must be in the parents’ sperm and egg.
o Some variations of traits seemed to show up more frequently than others.
o Some variation of traits seemed to be “hidden” in the parent plants, but would
show up all of a sudden in some of the offspring plants.
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Law of Segregation & Independent Assortment
Law of Segregation:
o Comes from Meiosis (Specifically Anaphase II of Meiosis).
o Only 1 variation of each trait goes into each different sperm / egg cell.
o This guarantees that each sperm / egg cell only have 1n or ½ the genetic
information.
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Law of Independent Assortment:
o Comes from Meiosis (also specifically Anaphase II of Meiosis).
o When each variation, of each trait, separates into the different sperm / egg cell,
they separate randomly.
o Meaning, all of the dominant and recessive variations get all jumbled up.
o Each time sperm / egg cells are made, the way the variations are separated is
completely different.
o This guarantees that each sperm / egg cell is genetically unique, and will make a
genetically unique offspring.
Why Are We Talking About These Now?!
o We now that because each baby comes from a combination of sperm + egg, that
each baby/person ends up inheriting 2 Variations for Each Trait.
o When we are trying to make predictions of future offspring, we need to know
what variations the parents have.
o We also know that we can only predict probability, never know for sure, because
of independent assortment.
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Mendelian Genetics Terms
Allele = Variation of a Trait
o Example: Trait could be hair color. The alleles, or variations, could be blonde-
brown-black.
Dominant Trait/Allele:
o A Trait that always is expressed physically, if inherited.
o These traits appeared to show up more often in his plants.
o CAPITAL LETTER used to represent these traits!
Recessive Trait/Allele:
o A trait that only would be expressed physically, if only recessive variations were
inherited.
o These traits appeared to show up less often in his plants, and often appeared to
be hidden in the parents.
o LOWERCASE LETTER used to represent these traits!
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Homozygous:
o Homo = Same ; Zygous = Genetic Information/Allele
o All of the SAME alleles (or variations)
o Examples:
Homozygous Dominant Dominant Dominant = AA
Homozygous Recessive Recessive Recessive = aa
Heterozygous / Hybrid:
o Hetero = Different ; Zygous = Genetic Information / Allele
o Different alleles (or variations)
o Example: Dominant Recessive = Aa
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Genotype:
o The actual GENetics inherited by the organism.
o Example:
Homozygous Dominant (AA)
Heterozygous (Aa)
Homozygous Recessive (aa)
Phenotype:
o The Physical Appearance an organism has for a trait.
o Example:
Purple
White
Punnett Square:
o Tool used to PREDICT the CHANCE of offspring inheriting different variations of
traits, and the CHANCE of having certain physical traits.
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YouTube Video from Mendelian Genetics & Punnett Squares
Punnett Squares
There are 2 Types of Punnett Squares:
o Monohybrid Crosses When you make predictions on 1 Trait’s Variations
between parents.
o Dyhybrid Crosses When you make predictions on 2 Trait’s Variations between
parents.
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What do I have to do BEFORE setting it up?
o Figure out the trait/s being predicted.
o Figure out the parents’ phenotypes and genotypes.
o Decide what letters you want to use for the parents’ genotypes.
(I recommend only letters where the Upper Case looks totally different from the
Lower Case. For Example: S is a bad choice…. S vs s….. too similar!)
How do I set up my MONOhybrid Punnett Square?
o Draw a square.
o Divide into 4 quadrants.
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o Place 1 Parent at the Top… Write 1 of that parent’s alleles PER column.
Example: If the parent is Heterozygous (Aa), I would write an Uppercase A in one
column, and the Lowercase a in the other column.
o Copy the Top Parent’s alleles into the boxes below.
o Place the 2nd Parent on the Left Hand Side… Write 1 of that parent’s alleles PER
row.
Example: If the parent is also Heterozygous (Aa), I would write an Uppercase A in
one row, and a Lowercase a in the other row.
o Copy the Side Parent’s alleles to the boxes to the right.
A a
A
A
a
a
A
A
A
a
a
a a a a
A A A
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How Do We Interpret This? (Use the Above Punnett Square)
o Each Box is a ¼ or 25% Chance.
o I see ¼ Boxes have AA (Homozygous Dominant)…
This means that if these 2 Heterozygous parents have babies, there is ALWAYS a ¼
or 25% chance that their child could be Homozygous Dominant for that trait.
o I see 2/4 Boxes have Aa (Heterozygous)…
This means that if these 2 Heterozygous parents have babies, there is ALWAYS a
2/4 or 50% chance that their child could be Heterozygous for that trait.
o I see ¼ Boxes have aa (Homozygous Recessive)…
This means that if these 2 Heterozygous parents have babies, there is ALWAYS a ¼
or 25% chance that their child could be Homozygous Recessive for that trait.
“Does it matter if I write the uppercase or lowercase letter first?”
You can still interpret the genotypes and Punnett Square the exact same.
However, scientists always write the uppercase / DOMINANT trait BEFORE the
lowercase / RECESSIVE trait.
Example:
Yes Aa
No aA
YouTube Video on Monohybrid Punnett Squares
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How do I set up my DIhybrid Punnett Square?
o Draw a square.
o Divide into 8 quadrants.
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o Place 1 Parent at the Top…
- Figure out all of the possible combinations.
- Example: The parent is Heterozygous (Aa) for one trait, and Homozygous
Dominant (BB) for the other trait.
- All possible combinations are… AB, aB, AB, and aB
- Write 1 SET per column.
o Copy the Top Parent’s alleles into the boxes below.
AB AB aB aB
A B
A B
A B
A B
A B
A B
A B
A B
a B
a B
a B
a B
a B
a B
a B
a B
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o Place 1 Parent on the Left Side…
- Figure out all of the possible combinations.
- Example: The second parent is Heterozygous (Aa) for one trait, and also
Heterozygous (Bb) for the other trait.
- All possible combinations are… AB, aB, Ab, and ab
- Write 1 SET per column.
o Copy the Top Parent’s alleles into the boxes below.
How Do We Interpret This? (Use the Above Punnett Square)
o Same as a Monohybrid Punnett Square.
o Each Box is a 1/8 or a 12.5% chance.
YouTube Video on Dihybrid Punnett Squares
AB AB aB aB
AABB
AaBB
AABb
AaBb
AaBB
aaBB
AaBb
aaBb
AB
aB
Ab
ab
AABB AaBB
AaBB aaBB
AABb AaBb
AaBb aaBb
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YouTube Video from College Board on Mendelian Genetics
YouTube Video on Mendelian Genetics from Khan Academy
YouTube Video on Punnett Squares from Khan Academy
