MENDEL'S GENETIC LAWS
(Source: http://www.hobart.k12.in.us/jkousen/Biology/mendel.htm#vocabkey)
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 there
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.
There are a few important vocabulary terms we should ironout before diving into Mendel's Laws.
 GENOTYPE = the genes present in the DNA of an organism. We will 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 comes from mama
organism & the other comes from papa organism, 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"). Since WE LOVE VOCABULARY, each possible combo has
a term for it.
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
instead of homozygous.
When the GENOTYPE is made up of one capital letter & one
lowercase letter (ex: Tt) it's called HETEROZYGOUS ("hetero"
means "other"). Just to confuse you, a heterozygous genotype
can also be referred to as HYBRID. OK?
Let's Summarize:
Genotype = genes present in an organism (usually
abbreviated as two letters)
TT =
homozygous =
pure
Tt =
heterozygous =
hybrid
tt =
homozygous =
pure
 PHENOTYPE = how the trait physically shows-up in the organism. Want
to know the simplest way to determine an organism's phenotype ? Look
at it. Examples of phenotypes: blue eyes, brown fur, striped fruit, yellow
flowers.
 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 just a second ago when explaining 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.
Let's try & illustrate with a diagram.
In this picture the two "hot
dog" shapes represent a pair of
homologous
chromosomes. Homologous
chromosomes are the same size
& have the same genetic info
(genes). Each letter in the diagram stands for an allele (form
of a gene). What's important to notice is that the letters can
be in different forms (capital or lowercase) --- that is what we
mean by allele --- and that the letters are lined-up in the same
order along each hot dog --- I mean homologous chromosome.
The "a-forms" are in corresponding positions, so are the "B-
forms", the "c" alleles, the "d" alleles, etc. etc. OK?
Reread that "allele" definition again & study the picture.
Getting back to our abbreviations, we could use a "C" for the
curly allele, and a "c" for the straight allele. A person's
genotype with respect to hair texture has three possiblilties:
CC, Cc, or cc. So to review some vocab, homozygous means
having two of the same allele in the genotype (2 big or 2 little
letters --- CC or cc). Heterozygous means one of each allele in
the genotype (ex: Cc).
Now I could tell you which genotypes create curls & which do
not, but then I'd be stealing some of Mr. Mendel's
thunder. More on that in a minute ........
Mendel's First Law
The Law of Dominance
Stated "simply" it goes like so:
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.
While Mendel was crossing (reproducing) his pea plants (over & over &
over again), he noticed something interesting. When he crossed pure tall
plants with pure short plants, all the new pea plants (referred to as the F1
generation) were tall. Similarly, crossing pure yellow seeded pea plants
and pure green seeded pea plants produced an F1 generation of all yellow
seeded pea plants. The same was true for other pea traits:
Parent Pea Plants
F1 Pea Plants
tall stem x short stem
all tall stems
yellow seeds x green seeds
all yellow seeds
green pea pods x yellow pea
pods
all green pea
pods
round seeds x wrinkled seeds
all round seeds
axial flowers x terminal flowers all axial flowers
So, what he noticed was that when the parent plants had contrasting
forms of a trait (tall vs short, green vs yellow, etc.) the phenotypes of the
offspring resembled only one of the parent plants with respect to that
trait. So, he said to himself,
"Greg, there is a factor that makes pea plants tall, and another factor that
makes pea plants short. Furthermore Greg ol' boy, when the factors are
mixed, the tall factor seems to DOMINATE the short factor".
Now, in our modern wisdom, we use "allele" or "gene" instead of what
Mendel called "factors". There is a gene in the DNA of pea plants that
controls plant height (makes them either tall or short). One form of the
gene (allele) codes for tall, and the other allele for plant height codes for
short. For abbreviations, we use the capital "T" for the dominant tall
allele, and the lowercase "t" for the recessive short allele.
Let's revisit the three possible genotypes for pea plant height & add some
MORE VOCABULARY.
Genotype Symbol
Genotype Vocab
Phenotype
TT
homozygous DOMINANT
or
pure tall
tall
Tt
heterozygous
or
hybrid
tall
tt
homozygous RECESSIVE
or
pure short
short
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).
The PUNNETT SQUARE (P-Square for short)
OK, now is as good of time as any to introduce you to a new friend, the
Punnett Square. This little thing helps us illustrate the crosses Mendel
did, and will assist you in figuring out a multitude of genetics problems.
We will start by using a P-Square to illustrate Mendels Law of
Dominance. Recall that he "discovered" this law by crossing a pure tall
pea plant & a pure short pea plant. In symbols, that cross looks like this:
Parents (P): TT x tt
where T = the dominant allele for tall stems
& t = recessive allele for short stems
The P-Square
for such a cross
looks like this:
Inside the 4
boxes are the
possible
genotypes (with
respect to plant
height) of the
offspring from
these parent
pea plants. In
this case, the
only possible
genotype is Tt
(heterozygous).
In hybrids, the
dominant trait
(whatever the
capital letter
stands for) is
the one that
appears in the phenotype, so all the offspring from this cross will have tall
stems.
To "fill in the boxes" of the Punnett Square, say to yourself "letter from
the left & letter from the top". The "t" from the left is partnered with the
"T" from the top to complete each of the four squares.
A summary of this cross would be:
Parent Pea Plants
(P Generation)
Offspring
(F1 Generation)
Genotypes: Phenotypes: Genotypes: Phenotypes:
TT x tt
tall x short
100% Tt
100% tall
Now, a helpful thing to recognize is this:
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.
All the offspring are heterozygous for the trait, one
parent is homozygous dominant, and the other is
homozygous recessive.
Mendel's Second Law
The Law of Segregation
Goes like so: 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.
The way I figure it, Mendel probably got really bored crossing pure
dominant trait pea plants with pure recessive trait pea plants (over & over
& over again)& getting nothing but pea plants with the dominant trait as a
result. Except for gaining more & more evidence for his Law of
Dominance, this probably grew tiresome. So, at one point he takes the
offspring of a previous cross & crosses them. Ooooooooh ............
Recall that his original cross for the tall & short pea plants was:
Parents
F1
Offspring
Genotype(s) TT x tt 100% Tt
Phenotype(s)
tall x
short
100%
tall
So, he takes two of the "F1" generation (which are tall) & crosses them. I
would think that he is figuring that he's gonna get all tall again (since tall
is dominant). But no! Low & behold he gets some short plants from this
cross! His new batch of pea plants (the "F2" generation) is about 3/4 tall &
1/4 short. So he says to himself,
"Greg ol' boy, the parent plants for this cross each have one tall factor
that dominates the short factor & causes them to grow tall. To get short
plants from these parents, the tall & short factors must separate,
otherwise a plant with just short factors couldn't be produced. The factors
must SEGREGATE themselves somewhere between the production of sex
cells & fertilization."
I think it's easier to picture this law by using a p-square. Our cross is two
hybrid parents, Tt x Tt.
The punnet square would look like this:
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". Meiosis
leads to the production of gametes
(sex cells), which are either eggs or
sperm. Sometimes the term
"GAMETOGENESIS" is used instead of
meiosis. Scientists love vocabulary
(sorry).
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).
Our summary:
Parent Pea Plants
(Two Members of F1
Generation)
Genotypes: Phenotypes:
Tt x Tt
tall x tall
Offspring
(F2 Generation)
Genotypes: Phenotypes:
25% TT
50% Tt
75% tall
25% tt
25% short
A helpful thing to recognize:
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.
Mendel's Third Law
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 big ugly punnet square that
illustrates this law so I guess we should take a look at it. 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
where
"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").
This is what it looks like:
RG
RG RRGG
Rg
RRGg
rG
RrGG
rg
RrGg
round
Rg
RRGg
round
round
round
RRgg
RrGg
Rrgg
round
rG
RrGG
round
round
round
RrGg
rrGG
round
round
rrGr
wrinkled
wrinkled
rg
RrGg
Rrgg
rrGg
rrgg
round
round
wrinkled
wrinkled
The results from a dihybrid cross are always the same:
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.
Summary:
I would like to summarize Mendel's Laws by listing the cross that
illustrates each.
LAW
DOMINANCE
SEGREGATION
INDEPENDENT
ASSORTMENT
PARENT CROSS
OFFSPRING
TT x tt
100% Tt
tall x short
tall
Tt x Tt
75% tall
25% short
9/16 round seeds &
green pods
3/16 round seeds &
yellow pods
3/16 wrinkled seeds &
green pods
1/16 wrinkled seeds &
yellow pods
tall x tall
RrGg x RrGg
round & green x
round & green
There you have them, Mendel's huge contributions to the world of
science. A very smart cookie. His 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.
Hail to the "Father of Genetics" !