Mendel & Genetics
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)
Parent Generation - the two organisms whose
genes produce offspring.
F1 generation - the offspring from parents.
F2 generation - the offspring produced by
crossing two F1 individuals
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
Gene - a segment of DNA on a chromosome
that controls a particular trait.
Mendel's factors are now called ALLELES. For
every trait a person have, two alleles determine
how that trait is expressed.
Allele - any of the several alternative forms of a
Gene.T and t each are alleles of each other.
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
Homozygous(Pure breed): 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 (Hybrid): 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.
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
(Examples: BB, Pp, CcTt)
Phenotype: what an organism looks like
(brown, purple..)
To understand genetics, one must
understand probabilities
The traits that are passed from parents to
offspring are determined by the combination of
genes that are in the gametes. This is a random
event in nature. The probability of a particular
event occurring is the fraction of outcomes in
which that event occurs.
What is the probability of the number 5 coming
up when a dice is rolled?
 A single roll of a dice has the possible
outcomes 1, 2, 3, 4, 5, 6.
 There are a total of 6 possible outcomes,
one of those being the number 5.
 The probability of the number 5 coming
up is 1/6.
What is the probability of rolling an even
number with a dice?
 The even numbers are 2, 4, 6.
1
 Each of these has a
/6 chance of
occurring.
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This gives us three chances in six of
rolling an even number. 3/6 = ½
How to Use Punnett Squares to Determine
Probability
In pea plants, round seeds are dominant to
wrinkled.
Assign genotypes:
RR = round
R r = round
r r = wrinkled
If two heterozygous plants are crossed (R r x R
r ), set up the square as shown below.
In this case 25% of the offspring will be
wrinkled, and 75 % will be round.
Dihybrid cross - a cross between individuals with
two pairs of contrasting traits. Ex. PpTt x PpTt
Dihybrid Crosses:
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 "heterozygous" 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
xR 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
INCOMPLETE DOMINANCE: ONE TRAIT IS NOT
COMPLETELY DOMINANT OVER THE OTHER, SO
BOTH TRAITS ARE EXPRESSED AT THE SAME TIME
IN THE HETEROZYGOUS CONDITION
CODOMINANCE: NEITHER TRAIT IS DOMINANT
OVER THE OTHER…SO BOTH ARE EXPRESSED AT
THE SAME TIME
INHERITANCE OF TRAITS
Heredity: passing traits from one generation to the next.
Polygenic traits: controlled by two or more genes. Skin
and eye color are both influenced by the additive effects
of from three to six genes.
Multiple-allele traits: controlled by three or more alleles of
the same gene. Human blood type is an example.
There are three forms of the blood type gene, IA, IB, and i
(IO). An individual's blood type is determined by two of the
three genes.
Blood
Type
Rh
Type
O
+
O
-
A
+
A
B
B
AB
AB
+
+
-
How Many Have Percentag
It
e
37.4
1 person in 3
%
44%
1 person in 15
6.6%
35.7
1 person in 3
%
42%
1 person in 16
6.3%
1 person in 12
8.5%
10%
1 person in 67
1.5%
1 person in 29
3.4%
4%
1 person in 167 .6%
Sex-linked traits: genes located on a sex chromosome.
Genes on the X chromosome are X-linked & genes on
the Y chromosome are Y-linked. Genes are X-linked,
males are much more likely to show recessive traits since
there is no complementary gene on the Y chromosome.
(Color blindness & hemophilia)
Sex-influenced traits: located on autosomes, but express
themselves differently in the sexes because of sex
hormones. (Pattern baldness)
Important Terms you should already know:
Law of Dominance: Within any characteristic one allele
appears more often than the other. Gives appearance
that allele is stronger and other weak…but, it has nothing
to do with strength. The dominant allele is naturally
selected to appear more often than the other allele.
Dominant: the gene that is expressed.
Recessive: the gene that is hidden…only seen in
homozygous condition.
New Terms
Pleiotropy: A gene can sometimes affect another
characteristic. This ability of having multiple effects is
called pleiotropy. Genes that control fur pigmentation in
cats may have an influence on the cats eyes and brain.
Epistasis: One gene may interfere with the expression of
another gene that is independently inherited. In flower
color a P is required for it to exhibit purple color. PP and
Pp = purple colored flowers. This can only happen if a
dominant allele is present for another characteristic.
PPCc =purple Ppcc = white. The C characteristic has an
effect on the color of the flower.
Polygenic Traits: Quantitative traits or having 3 sets of
alleles for a characteristic. Skin color is polygenic. There
are 6 genes responsible for this characteristic. BBBBBB=
Very dark pigmentation where as bbbbbb = the opposite
very light pigmentation. All the other genotypes are
intermediates of these combinations.
Epigenetics –
Test cross – is the crossing of an individual of unknown
genotype with a homozygous recessive individual to
determine the unknown genotype.