Gregor Mendal and Genetics
Mendal’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 there hands and Mendel spent his
time crossing pea plants. As he did this 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 iron-out
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.
Wanna 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 possibilities: 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).
***Answer Question Block #1***
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:
offspring from this cross will have tall stems.
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
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)
Genotypes:
TT x tt
Phenotypes:
tall x short
Offspring
(F1 Generation)
Genotypes:
100% Tt
Phenotypes:
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.
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".
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:
25% TT
50% Tt
25% tt
Phenotypes:
75% tall
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
RG RRGG RRGg
rG
RrGG
rg
RrGg
round
round
round
round
Rg RRGg
RRgg
RrGg
Rrgg
round
round
round
round
rG RrGG
RrGg
rrGG
rrGr
round
wrinkled
wrinkled
round
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 each other --- Mendel's 3rd Law.
Summary:
I would like to summarize Mendel's Laws by listing the cross that
illustrates each.
LAW
DOMINANCE
PARENT
CROSS
OFFSPRING
TT x tt
100% Tt
tall x short
tall
75% tall
25% short
9/16 round seeds &
green pods
3/16 round seeds &
RrGg x RrGg
INDEPENDENT
yellow pods
round & green x
ASSORTMENT
3/16 wrinkled seeds &
round & green
green pods
1/16 wrinkled seeds &
yellow pods
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.
SEGREGATION
Tt x Tt
tall x tall
*** Answer Question Block #2***