Biology 30 Genetics Unit Mendel Notes
A discussion on Genetics must begin with by looking at the work of Gregor Mendel and a discussion of
independent events and probability.
http://www.bozemanscience.com/science-videos/2012/5/6/genetics.html
The term independent events means that events are independent of each other if the probability of one
occurring does not affect the probability of the others. The probability of something is the measure of
how likely an event is. For instance, when my son was born there was a 1 in 2, or 0.50 probability that
Eli would be a boy. There was also a 1 in 2, or 0.50 probability that he would have been a girl and
hopefully would have a different name. So, if we were to have another child, would the probability still
be 1 in 2, or 0.5 for having another boy? Yes it would because the sex of one child has no impact on the
second child. So, these would be independent events.
Gregor Mendel: "father of genetics"
Blending Theory of Inheritance - offspring of two parents "blend" the traits of
both parents
Particulate Theory of Inheritance - traits are inherited as "particles",
offspring receive a "particle" from each parent.
Evidence for Particulate Theory of Inheritance: A plant with purple flowers is
crossed with another plant that has purple flowers. Some of the offspring have
white flowers (wow!). Mendel set out to discover how this could happen.
Some stuff on Mendel
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parents were farmers
he became ordained as a priest
studied science and mathemathics at the University of Vienna
Mendel's Experiments
Mendel chose pea plants as his experimental
subjects, mainly because they were easy to cross
and showed a variety of contrasting traits (purple
vs white flowers, tall vs short stems, round vs
wrinkled seeds)
1. Mendel chose true-breeding lines of each
plant/trait he studied (true breeding lines always
produced offspring of the same type)
2. He crossed a true breeding plant with a plant of
the opposite trait (purple x white). He called this
the Parental (P) generation. (In this case, he cross-pollinated the plants)
3. He recorded data on the offspring of this cross (First Filial, F1)
4. He self pollinated the F1 offspring
5. He recorded data on the offspring of the second generation, calling it
the Second Filial generation (F2)
Analysis:
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The F1 generation always displayed
one trait (he later called this the
dominant trait)
The F1 generation must have within it
the trait from the original parents - the
white trait
The F2 generation displayed the hidden
trait, 1/4 of the F2 generation had it (he
later called this hidden trait the
recessive trait)
Each individual has two "factors" that
determine what external appearance
the offspring will have. (We now call these factors genes or alleles)
Mendel established three principles (or Laws) from his research
1. The Principle of Dominance and Recessiveness - one trait is masked or
covered up by another trait
2. Principle of Segregation - the two factors (alleles) for a trait separate
during gamete formation
3. Principle of Independent Assortment - factors of a trait separate
independently of one another during gamete formation; another way to look at
this is, whether a flower is purple has nothing to do with the length of the
plants stems - each trait is independently inherited
Modern Genetics
Mendel's factors are now called ALLELES. For every trait a person have, two
alleles determine how that trait is expressed.
We use letters to denote alleles, since every gene has two alleles, all genes
can be represented by a pair of letters.
PP = purple, Pp = purple, pp = white
Homozogyous: when the alleles are the same, the individual is said to be
homozygous, or true breeding. Letters designating a homozgyous individual
could be capital or lowercase, as long as they are the same. Ex. AA, bb, EE,
dd
Heterozygous: when the alleles are different, in this case the DOMINANT
allele is expressed. Ex. Pp, Aa
Monohybrid cross = a cross involving one pair of contrasting traits. Ex. Pp x
Pp
Punnet Square: used to determine the PROBABILITY of having a certain
type of offspring given the alleles of the parents
Genotype: letters used to denote alleles (BB, Pp..etc)
Phenotype: what an organism looks like (brown, purple..)
The Punnett Square
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a simple box-like device that helps us to consider all genetic combinations and show the
expected frequencies of genotypes.
The P and p symbols represent the single allele each gamete receives.
Fertilization provides the two alleles for the new individual, one from the male
(sperm) and one from the female (egg).
The Punnett square shows that the genotypes and associated ratios for a
monohybrid cross are 1 PP :2 Pp : 1 pp.
Any progeny with a P would have the dominant (purple) phenotype, so the
phenotypic ratio is 3 purple to 1 white.
Now it is known that a gene is a portion of the chromosomal DNA that resides
at a particular site, called a locus (plural is loci). The gene codes for a particular
function or trait.
Mendel arrived at the law of segregation with no knowledge of meiosis or
chromosomes. The mechanism of chromosome separation in meiosis I today
explains his law of segregation.
How to Solve a Punnet Square
1. Determine the genotypes (letters) of the parents. Bb x Bb
2. Set up the punnet square with one parent on each side.
3. Fill out the Punnet square middle
4. Analyze the number of offspring of each type.
In pea plants, round seeds are dominant to wrinkled. The genotypes and
phenotypes are:
RR = round
Rr = round
rr = wrinkled
If you get stuck make a "key". Sometimes the problems won't give you
obvious information.
Example: In radishes, a bent root is a dominant trait, though some roots
are straight (which is recessive). If a straight rooted plant is crossed with
a heterozyous bent root plant, how many of the offspring will have
straight roots?
Say what? First of all, this problem doesn't make it easy. Start by
assigning genotypes and phenotypes. It doesn't matter what letter you
pick, but it may be easiest to pick a letter that represents the dominant
trait. In this case, use the letter B for bent.
BB = bent
Bb = bent
bb = straight
Now use the key to figure out your parents. In this case you have a
straight root plant (bb) crossed with a heterozyous bent plant (Bb). Once
you've figured that out, the cross is simple!
If a heteroyzous round seed is crossed with itself (Rr x Rr) a punnett
square can help you figure out the ratios of the offspring.
3/4 round, 1/4 wrinkled
One Trait Test Cross
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A test cross can determine the genotype (heterozygous or homozygous) of an individual with a
dominant trait.
It involves crossing the individual to a true-breeding recessive (homozygous recessive).
If the unknown is heterozygous, approximately half the progeny will have the dominant trait and
half the recessive trait.
If the unknown is homozygous dominant, all the progeny will have the dominant trait
WILD TYPE = refers
to the "normal"
genotype
Often designated
with a + symbol
1. Please go to the following link and complete this assignment in your book:
http://www.biologycorner.com/worksheets/penny%20genetics.pdf
Dihybrid Crosses: Crosses that involve 2
traits.
These type of crosses can be challenging to set up, and the square you
create will be 4x4. This simple guide will walk you through the steps of solving
a typical dihybrid cross common in genetics. The method can also work for
any cross that involves two traits.
Consider this cross
A pea plant that is heterozygous for round, yellow seeds is self fertilized, what
are the phenotypic ratios of the resulting offspring?
Step 1: Determine the parental genotypes from the text above, the word
"heteroyzous" is the most important clue, and you would also need to
understand that self fertilized means you just cross it with itself.
R r Y y
x
R r Y y
Step 2: Determine the gametes. This might feel a little like the FOIL method
you learned in math class. Combine the R's and Ys of each parent to
represent sperm and egg. Do this for both parents
Gametes after "FOIL"
RY, Ry, rY, ry (parent 1) and RY, Ry, rY, ry (parent 2)
Step 3: Set up a large 4x4 Punnet square, place one gamete set from the
parent on the top, and the other on the side
Step 4: Write the genotypes of the offspring in each box and determine how
many of each phenotype you have. In this case, you will have 9 round, yellow;
3 round, green; 3 wrinkled, yellow; and 1 wrinkled green
Some Shortcuts
In any case where the parents are heterozygous for both traits (AaBb x AaBb)
you will always get a 9:3:3:1 ratio.
9 is the number for the two dominant traits, 3 is the number for a
dominant/recessive combination, and only 1 individual will display both
recessive traits.
Another way to determine the ratios is to do it mathematically
3/4 of all the offspring will have round seeds
3/4 of all the offspring will have yellow seeds
3/4 x 3/4 = 9/16 will have round, yellow seeds.
Crosses that Involve 2 Traits
Consider:
R r Y y
x
r r y y
The square is set up as shown
You might notice that all four rows have the same genotype. In this case, you
really only need to fill out the top row, because 1/4 is the same thing as 4/16
A Mathematical Alternative (LAWS OF PROBABILITY)
A punnet square is not needed to determine the ratios of genotypes and phenotypes. Simple statistics
and math can save you the trouble of filling out a square.
In a monohybrid cross Pp x Pp, each parent produced
P gametes and
If you wanted to determine how many of the offspring are pp:
x
p gametes
=
Example 2: H is dominate for long hair (h = short) and B is dominate for black eyes (b = red eyes)
If the parents are: HhBb x hhBb
How many off the offspring will be short haired and red eyed?
Task: Use mathematical analysis to determine the number of short haired, black eyed offspring from the
cross above.
TWO-TRAIT TEST CROSS
Used to determine the genotype of an "unknown" by crossing it with an individual that is homozygous
recessive for both traits.
In flies (Long wings is dominant to short wings, Gray body is dominant to black)
A L __ G ___ is test crossed.
The offspring are 1:1:1:1 --> What is the genotype of the unknown parent?
If the offspring are half long winged & gray, and half long winged and black --> What is the genotype of
the unknown parent?
We will know complete some assignments with the use of punnett squares and our knowledge of
genetics.
2. We will try to complete the following assignment as a group:
http://biologycorner.com/worksheets/genetics_2traits_bio2A.html