Anatomy and Physiology
Genetic Unit
MENDEL'S GENETIC LAWS
 Once upon a time (1860's), in an Austrian monastery, there lived
a monk named Mendel, Gregor Mendel. Monks had a lot of time
on their hands and Mendel spent his time crossing pea plants.
As he did this over & over & over & over & over again, he noticed
some patterns to the inheritance of traits from one set of pea
plants to the next. By carefully analyzing his pea plant numbers
(he was really good at mathematics), he discovered three laws of
inheritance.
 Mendel's Laws are as follows:
 1. the Law of Dominance
2. the Law of Segregation
3. the Law of Independent Assortment
 Now, notice in that very brief description of his work that the
words "chromosomes" or "genes" are nowhere to be
found. That is because the role of these things in relation to
inheritance & heredity had not been discovered yet. What makes
Mendel's contributions so impressive is that he described the
basic patterns of inheritance before the mechanism for
inheritance (namely genes) was even discovered.
Section #1
 GENOTYPE = the genes present in the DNA of an organism. Use a
pair of letters (ex: Tt or YY or ss, etc.) to represent genotypes for one
particular trait. There are always two letters in the genotype
because (as a result of sexual reproduction) one code for the trait
from mom & the other comes from dad, so every offspring gets two
codes (two letters).
 Now, turns out there are three possible GENOTYPES - two big letters
(like "TT"), one of each ("Tt"), or two lowercase letters ("tt").
 When we have two capital or two lowercase letters in the
GENOTYPE (ex: TT or tt) it's called HOMOZYGOUS ("homo" means
"the same"). Sometimes the term "PURE" is used.
 When the GENOTYPE is made up of one capital letter & one
lowercase letter (ex: Tt) it's called HETEROZYGOUS ("hetero" means
"other"). A heterozygous genotype can also be referred to as
HYBRID.
 PHENOTYPE = how the trait physically shows-up in
the organism. What they look like!
 ALLELES = (WARNING - THIS WORD CONFUSES
PEOPLE; READ SLOW) alternative forms of the same
gene. Alleles for a trait are located at corresponding
positions on homologous chromosomes.
Remember genotypes I said that "one code (letter)
comes from ma & one code (letter) comes from pa"?
Well "allele" is a fancy word for what I called codes.
 For example, there is a gene for hair texture (whether
hair is curly or straight). One form of the hair texture
gene codes for curly hair. A different code for of the
same gene makes hair straight. So the gene for hair
texture exists as two alleles --- one curly code, and
one straight code.
The Law of Dominance
In a cross of parents that are pure for contrasting
traits, only one form of the trait will appear in
the next generation. Offspring that are hybrid
for a trait will have only the dominant trait in the
phenotype.
Cross pure yellow and pure green, yellow is dominate
to green?
Cross 2 heterozygous yellow plants?
Let's revisit the three possible
genotypes for pea plant height
Genotype
Symbol
TT
Tt
tt
Genotype Vocab
Phenotype
homozygous
DOMINANT
or pure tall
heterozygous
or hybrid
Tall
homozygous
RECESSIVE
or pure short
Short
Tall
 Note: the only way the recessive trait shows-up in the
phenotype is if the geneotype has 2 lowercase letters
(i.e. is homozygous recessive).
Also note: hybrids always show the dominant trait in
their phenotype (that, by the way, is Mendel's Law of
Dominance in a nutshell).
 ANY TIME TWO PARENT ORGANISMS LOOK
DIFFERENT FOR A TRAIT,
AND ALL THEIR OFFSPRING RESEMBLE ONLY ONE OF
THE PARENTS,
YOU ARE DEALING WITH MEDEL'S LAW OF
DOMINANCE.
Here are the basic steps to using a Punnett
Square when solving a genetics question
 BABY STEPS:
1. determine the genotypes of the parent
organisms
2. write down your "cross" (mating)
3. draw a p-square
4. "split" the letters of the genotype for each
parent & put them "outside" the p-square
5. determine the possible genotypes of the
offspring by filling in the p-square
6. summarize results (genotypes &
phenotypes of offspring)
Step #1: Determine the genotypes of the parent
organisms.
 "Cross a short pea plant with one that is
heterozygous tall. Tall is dominant to
short".
 T= tall
 t= short
 Parent 1 tt
 Parent 2 Tt
Step 2 , 3 and 4
 Step #2: Write down your "cross"
(mating). Write the genotypes of the parents in
the form of letters (ex: Tt x tt).
 Step #3: Draw a p-square.
 Step #4:"Split" the letters of the genotype for
each parent & put them "outside" the p-square.
t
T= Tall
t= short
T
t
t
Step #5: Determine the possible genotypes
of the offspring by filling in the p-square.
T= Tall
t= short
Step #6: Summarize the results (genotypes &
phenotypes of offspring).
 Simply report what you came up with. You
should always have two letters in each of
the four boxes.
 Genotype (what the genes look like) 2=Tt and
2=tt
 Phenotype (what the offspring look like) 2 tall
and 2 short
 You know how, in Step #4, when we "split" the letters of
the genotype & put them outside the p-square? What
that step illustrates is the process of gametogenesis
(the production of sex cells, egg & sperm).
 Gametogenesis is a cell division thing (also called
meiosis) that divides an organism's chromosome
number in half.
 For example, in humans, body cells have 46
chromosomes a piece. However, when sperm or eggs
are produced (by gametogenesis/meiosis) they get only
23 chromosomes each. When the sperm & egg fuse at
fertilization, the new cell formed (called a zygote) will
have 23 + 23 = 46 chromosomes.
Section #2
The Law of Segregation
 During the formation of gametes (eggs or
sperm), the two alleles responsible for a trait
separate from each other. Alleles for a trait
are then "recombined" at fertilization,
producing the genotype for the traits of the
offspring.
 Now, when completing a Punnet Square, we
model this "Law of Segregation" every
time. When you "split" the genotype letters
& put one above each column & one in front
of each row, you have SEGREGATED the
alleles for a specific trait. In real life this
happens during a process of cell division
called "MEIOSIS".
 You can see from the p-square that any time
you cross two hybrids, 3 of the 4 boxes will
produce an organism with the dominant trait
(in this example "TT", "Tt", & "Tt"), and 1 of
the 4 boxes ends up homozygous recessive,
producing an organism with the recessive
phenotype ("tt" in this example).
 Any time two parents have the same
phenotype for a trait
but some of their offspring look
different with respect to that trait,
the parents must be hybrid for that
trait.
The Law of Independent
Assortment
 Alleles for different traits are distributed to sex cells (& offspring)
independently of one another.
 OK. So far we've been dealing with one trait at a time. For
example, height (tall or short), seed shape (round or wrinkled), pod
color (green or yellow), etc. Mendel noticed during all his work that
the height of the plant and the shape of the seeds and the color of
the pods had no impact on one another. In other words, being tall
didn't automatically mean the plants had to have green pods, nor
did green pods have to be filled only with wrinkled seeds, the
different traits seem to be inherited INDEPENDENTLY.
 Please note my emphasis on the word "different". Nine times out
of ten, in a question involving two different traits, your answer will
be "independent assortment". There is a punnet square that
illustrates this law. It involves what's known as a "dihybrid cross",
meaning that the parents are hybrid for two different traits.
 The genotypes of our parent pea plants will be:
RrGg x RrGg
"R" = dominant allele for round seeds
"r" = recessive allele for wrinkled seeds
"G" = dominant allele for green pods
"g" = recessive allele for yellow pods
 Notice that we are dealing with two different traits: (1) seed
texture (round or wrinkled) & (2) pod color (green or
yellow). Notice also that each parent is hybrid for each
trait (one dominant & one recessive allele for each trait).
 We need to "split" the genotype letters & come up with the
possible gametes for each parent. Keep in mind that a
gamete (sex cell) should get half as many total letters
(alleles) as the parent and only one of each letter. So each
gamete should have one "are" and one "gee" for a total of
two letters. There are four possible letter combinations:
RG, Rg, rG, and rg. These gametes are going "outside" the
p-square, above 4 columns & in front of 4 rows. We fill
things in just like before --- "letters from the left, letters
from the top". When we finish each box gets four letters
total (two "are's" & two "gees").
 The results from a dihybrid cross are always the same for 2
heterozygous parents:
9/16 boxes (offspring) show dominant phenotype for both traits
(round & green),
3/16 show dominant phenotype for first trait & recessive for second
(round & yellow),
3/16 show recessive phenotype for first trait & dominant form for
second (wrinkled & green), &
1/16 show recessive form of both traits (wrinled & yellow).
 So, as you can see from the results, a green pod can have round or
wrinkled seeds, and the same is true of a yellow pod. The different
traits do not influence the inheritance of each other. They are
inherited INDEPENDENTLY.
 Interesting to note is that if you consider one trait at a time, we get
"the usual" 3:1 ratio of a single hybrid cross (like we did for the LAw
of Segregation). For example, just compare the color trait in the
offspring; 12 green & 4 yellow (3:1 dominant:recessive). Same deal
with the seed texture; 12 round & 4 wrinkled (3:1 ratio). The traits are
inherited INDEPENDENTLY of eachother --- Mendel's 3rd Law.
In a dihybrid cross…If two heterozygous
round, yellow plants are crossed…yellow is
dominate to green and round is dominate to
wrinkled what are the genotype & phenotype?
1st write your letters…
Y= yellow
y= green
S= spherical/round
s= wrinkled
2nd write your parents…
YySs
YySs
Pair up the letters
1st pairs with 3rd ,
1st with 4th, then
2nd letter with 3rd,
2nd with 4th
Next drop in
your letters
bring both
letters
down
separate
them by
letter S or
Y and
always put
the capitol
letter first…
Now count up the
genotype (letter
combination
square by
square)
Genotypes:
SSYY :1
SSYy :2
SsYY :2
SsYy :4
SSyy :1
Ssyy :2
ssYY:1
ssYy :2
ssyy :1
Then do the
Phenotype what
they look like
Phenotype:
 9 yellow, round
 3 yellow, wrinkled
 3 green, round
 1 green, wrinkled
Summarize Mendel's Laws by listing the cross
that illustrates each.
LAW
PARENT CROSS
OFFSPRING
DOMINANCE
TT x tt
tall x short
100% Tt
tall
SEGREGATION
Tt x Tt
tall x tall
75% tall
25% short
INDEPENDENT
ASSORTMENT
RrGg x RrGg
round & green x
round & green
9/16 round seeds & green pods
3/16 round seeds & yellow pods
3/16 wrinkled seeds & green
pods
1/16 wrinkled seeds & yellow
pods
 Mendel’s work has stood the test of
time, even as the discovery &
understanding of chromosomes &
genes has developed in the 140 years
after he published his findings. New
discoveries have found "exceptions"
to Mendel's basic laws, but none of
Mendel's things have been proven to
be flat-out wrong.
Section #3
IncOMpleTe & COdominANce
 In many ways Gregor Mendel was quite lucky in discovering his genetic
laws. He happened to use pea plants, which happened to have a number of
easily observable traits that were determined by just two alleles. And for the
traits he studied in his peas, one allele happened to be dominant for the trait
& the other was a recessive form. Things aren't always so clear-cut &
"simple" in the world of genetics, but luckily for Mendel (& the science world)
he happened to work with an organism whose genetic make-up was fairly
clear-cut & simple.
 INCOMPLETE DOMINANCE
 If Mendel were given a mommy black mouse & a daddy white mouse &
asked what their offspring would look like, he would've said that a certain
percent would be black & the others would be white. He would never have
even considered that a white mouse & a black mouse could produce a
GREY mouse! For Mendel, the phenotype of the offspring from parents with
different phenotypes always resembled the phenotype of at least one of the
parents. In other words, Mendel was unaware of the phenomenon of
INCOMPLETE DOMINANCE.
 I remember Incomplete Dominance in the form of an example like so:
RED Flower x WHITE Flower ---> PINK Flower
 With incomplete dominance, a cross between organisms with two
different phenotypes produces offspring with a third phenotype that is a
blending of the parental traits.
It's like mixing paints, red + white will make pink. Red doesn't totally
block (dominate) the white, instead there is incomplete dominance, and
we end up with something in-between.
 We can still use the Punnett Square to solve problems involving
incomplete dominance. The only difference is that instead of using a
capital letter for the dominant trait & a lowercase letter for the recessive
trait, the letters we use are both going to be capital (because neither trait
dominates the other). So the cross I used up above would look like this:
INCOMPLETE DOMINANCE
 R = allele for red flowers
W = allele for white flowers
 red x white ---> pink
RR x WW ---> 100% RW
 The trick is to recognize when you are
dealing with a question involving
incomplete dominance. There are two
steps to this:
1) Notice that the offspring is showing a
3rd phenotype. The parents each have
one, and the offspring are different from
the parents.
2) Notice that the trait in the offspring is
a blend (mixing) of the parental traits.
CODOMINANCE
 First let me point out that the meaning of the prefix "co-"
is "together".
Cooperate = work together. Coexist = exist
together. Cohabitat = habitat together.
 The genetic gist to codominance is similar to incomplete
dominance. A hybrid organism shows a third phenotype -- not the usual "dominant" one & not the "recessive" one
... but a third, different phenotype. With incomplete
dominance we get a blending of the dominant &
recessive traits so that the third phenotype is something
in the middle (red x white = pink).
 In COdominance, the "recessive" & "dominant" traits
appear together in the phenotype of hybrid organisms.
 I remember codominance in the form of an
example like so:
 red x white ---> red & white spotted
 With codominance, a cross between
organisms with two different phenotypes
produces offspring with a third phenotype in
which both of the parental traits appear
together.
 When it comes to punnett squares &
symbols, it's the same as incomplete
dominance. Use capital letters for the allele
symbols. My example cross from above
would look like so:
CODOMINANCE
 R = allele for red flowers
W = allele for white flowers
 red x white ---> red & white
spotted
RR x WW ---> 100% RW
 The symbols you choose to use don't matter, in
the end you end up with hybrid organisms, and
rather than one trait (allele) dominating the
other, both traits appear together in the
phenotype. codominance.
 A very very very common phenotype used in
questions about codominance is roan fur in
cattle. Cattle can be red (RR = all red hairs),
white (WW = all white hairs), or roan (RW = red
& white hairs together). A good example of
codominance.
 Another example of codominance is human
blood type AB, in which two types of protein
("A" & "B") appear together on the surface of
blood cells.
MULTIPLE ALLELES
 It makes absolutely no sense whatsoever to continue
if we don't know what the word "allele" means.

allele = (n) a form of a gene which codes for one
possible outcome of a phenotype
 For example, in Mendel's pea investigations, he
found that there was a gene that determined the color
of the pea pod. One form of it (one allele) creates
yellow pods, & the other form (allele) creates green
pods.
 Get it? Two possible phenotypes of one trait (pod
color) are determined by two alleles (forms) of the
one "color" gene.
Y = the
dominant
allele for
yellow
y = the
recessive
allele for
green
Genotypes
RESULTING
PHENOTYPE
Homozygous
Dominant (YY)
Heterozygous (Yy)
Homozygous
Recessive (yy)
Yellow
Yellow
Green
 If there are only two alleles involved
in determining the phenotype of a
certain trait, but there are three
possible phenotypes, then the
inheritance of the trait illustrates
either incomplete dominance or
codominance.
 In these situations a heterozygous
(hybrid) genotype produces a 3rd
phenotype that is either a blend of the
other two phenotypes (incomplete
dominance) or a mixing of the other
phenotypes with both appearing at
the same time (codominance).
THE DEALS ON MULTIPLE
ALLELES
 Now, if there are 4 or more possible phenotypes for a particular trait,





then more than 2 alleles for that trait must exist in the population. We
call this "MULTIPLE ALLELES".
Let me stress something. There may be multiple alleles within the
population, but individuals have only two of those alleles.
Why?
Because individuals have only two biological parents. We inherit half
of our genes (alleles) from ma, & the other half from pa, so we end up
with two alleles for every trait in our phenotype.
An excellent example of multiple allele inheritance is human blood
type. Blood type exists as four possible phenotypes: A, B, AB, & O.
There are 3 alleles for the gene that determines blood type.
(Remember: You have just 2 of the 3 in your genotype --- 1 from mom &
1 from dad).
ALLELE
IA
IB
i
CODES FOR
Type "A" Blood
Type "B" Blood
Type "O" Blood
GENOTYPES
IAIA
IAi
IBIB
IBi
IAIB
ii
RESULTING PHENOTYPES
Type A
Type A
Type B
Type B
Type AB
Type O
 As you can count, there are 6 different
genotypes & 4 different phenotypes for
blood type.
 Note that there are two genotypes for
both "A" & "B" blood --- either
homozygous (IAIA or IBIB) or
heterozygous with one recessive allele
for "O" (IAi or IBi).
 Note too that the only genotype for "O"
blood is homozygous recessive (ii).
 And lastly, what's the deal with "AB"
blood? What is this an example of? The
"A" trait & the "B" trait appear together
in the phenotype. Think think think ....
 Lubey, Steve. Lubey's Biohelp! - Mendel's Genetic
Laws. Aug. 26, 2005
<http://www.borg.com/~lubehawk/mendel.htm>
 Lubey, Steve. Lubey's Biohelp! – Incomplete &
Codominance. Aug. 26, 2005
<http://www.borg.com/~lubehawk/inccodom.htm>
 Lubey, Steve. Lubey's Biohelp! – Multiple Alleles.
Aug. 26, 2005
<http://www.borg.com/~lubehawk/multalle.htm>
 Mendel Image.
http://www.micro.utexas.edu/courses/levin/bio304/genetic
s/genetics.html