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.
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.
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 ........
Vocabulary
Review Questions
1. Which of the following is a possible abbreviation for a genotype?
A.
BC
B. Pp
C. Ty
D. fg
2. What is the best way to determine the phenotype of the feathers on a bird?
A.
analyze the bird's DNA (genes)
B. look at the bird's
feathers
C. look at the bird's beak
d. examine the bird's
droppings
3. Which of the following pairs is not correct?
A.
kk = hybrid
B. hybrid = heterozygous
C. heterozygous = Hh
D. homozygous = RR
4. The genes present in an organism represent the organism's __________.
A.
genotype
B. phenotype
C. physical traits
5. Which choice represents a possible pair of alleles?
A.
k & t
B. K & T
C. K & k
D. K & t
6. How many alleles for one trait are normally found in the genotype of an organism?
A.
1
B. 2
C. 3
D. 4
7. Which statement is not true?
A.
genotype determines phenotype
B. phenotype determines
genotype
C. a phenotype is the
physical appearance of a trait in an organism
D. alleles are different
forms of the same gene
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 |
tall |
Tt |
heterozygous |
tall |
tt |
homozygous
RECESSIVE |
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:
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 |
Offspring |
||
Genotypes: |
Phenotypes: |
Genotypes: |
Phenotypes: |
Now, a helpful thing to recognize is this: ANY
TIME TWO PARENT ORGANISMS LOOK DIFFERENT FOR A TRAIT, All the offspring are heterozygous for the trait, one parent is homozygous dominant, and the other is homozygous recessive. |
Does
setting up & using the Punnett Square confuse you? Would you
like to see a step-by-step "how to" about the good ol'
p-square? For some practice Punnett Square problems visit my very own: "P-Square Practice Page". Don't forget to come back & learn more about Mendel! |
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:
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 |
Offspring |
||
Genotypes: Tt x Tt |
Phenotypes: tall x tall |
Genotypes: |
Phenotypes: 75%
tall |
A helpful thing to recognize: Any
time two parents have the same phenotype for a 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 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 |
rg |
RG |
RRGG |
RRGg |
RrGG |
RrGg |
Rg |
RRGg |
RRgg |
RrGg |
Rrgg |
rG |
RrGG |
RrGg |
rrGG |
rrGr |
rg |
RrGg |
Rrgg |
rrGg |
rrgg |
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.
I
would like to summarize Mendel's Laws by listing the cross that
illustrates each.
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.
|
Review
Questions
1. Which cross
would best illustrate Mendel's Law of Segregation?
A.
TT x tt
B. Hh x hh
C. Bb x Bb
D. rr x rr
2. In the cross Yy x Yy, what percent of offspring would have the same phenotype as the parents?
A.
25%
B. 50%
C. 75%
D. 100%
3. In a certain plant, purple flowers are dominant to red flowers. If the cross of two purple-flowered plants produces some some purple-flowered and some red-flowered plants, what is the genotype of the parent plants?
A.
PP x Pp
B. Pp x Pp
C. pp x PP
D. pp x pp
Base questions #4-8 on the following information:
A white-flowered plant is crossed with a pink-flowered plant. All of the F1 offspring from the cross are white.
4.
Which phenotype is dominant?
5. What are
the genotypes of the original parent plants?
6. What is the
genotype of all the F1 offspring?
7. What would
be the percentages of genotypes & phenotypes if one of the white F1 plants
is crossed with a pink-flowered plant?
8. Which of
Mendel's Laws is/are illustrated in this question?
9. Crossing two dihybrid organisms results in which phenotypic ratio?
A.
1:2:1
B. 9:3:3:1
C. 3:1
D. 1:1
10. The outward appearance (gene expression) of a trait in an organism is referred to as:
A.
genotype
B. phenotype
C. an allele
D. independent
assortment
11. In the
homologous chromosomes shown in the diagram, which is a possible alleleic
pair?
B. Ee
C. AB
D. ee
12. The phenotype of a pea
plant can best be determined by:
A.
analyzing its genes
B. looking at it
C. crossing it with a
recessive plant
D. eating it
13. Mendel formulated his Law of Segregation after he had:
A.
studied F1 offspring
B. studied F2 offspring
C. produced mutations
D. produced hybrids
14. Which cross would produce phenotypic ratios that would illustrate the Law of Dominance?
A.
TT x tt
B. TT x Tt
C. Tt x Tt
D. tt x tt
15. The mating of two curly-haired brown guinea pigs results in some offspring with brown curly hair, some with brown straight hair, some with white curly hair, and even some with white straight hair. This mating illustrates which of Mendel's Laws?
A.
Dominance
B. Segregation
C. Independent Assortment
D. Sex-Linkage
<Answers to Review Questions>
Answers Area
Vocabulary Term Review Questions - CORRECT ANSWERS ARE UNDERLINED
1. Which of the following is a possible abbreviation for a genotype?
A.
BC
B.
Pp
- genotypes are made up of 2 of the same letter (either 2 capital, 2
lowercase, or one of each)
C. Ty
D. fg
2. What is the best way to determine the phenotype of the feathers on a bird?
A.
analyze the bird's DNA (genes)
B. look
at the bird's feathers
- "phenotype of the feathers" means what the feathers look like, so
look at 'em
C. look at
the bird's beak
d. examine
the bird's droppings
3. Which of the following pairs is not correct?
A.
kk = hybrid - Kk would be hybrid (one capital, one lowercase of the
same letter)
B. hybrid =
heterozygous
C.
heterozygous = Hh
D.
homozygous = RR
4. The genes present in an organism represent the organism's __________.
A.
genotype
B.
phenotype
C. physical
traits
5. Which choice represents a possible pair of alleles?
A.
k & t
B. K &
T
C. K
& k - allele means 2 forms of the same gene. so this choice shows 2
forms of the same letter K or k
D. K &
t
6. How many alleles for one trait are normally found in the genotype of an organism?
A.
1
B. 2
- one allele is inherited from each parent
C. 3
D. 4
7. Which statement is not true?
A.
genotype determines phenotype - (note that the environment does play a role in
influencing phenotype too)
B.
phenotype determines genotype
C. a
phenotype is the physical appearance of a trait in an organism
D. alleles
are different forms of the same gene - (see question #5)
Review
Questions
- ANSWERED & EXPLAINED
1. Which cross would best illustrate Mendel's Law of Segregation?
A.
TT x tt
B.
Hh x hh
C.
Bb x Bb
- both parent show dominant trait, but some recessive offspring will be
produced (each parent carries a "b")
D.
rr x rr
2. In the cross Yy x Yy, what percent of offspring would have the same phenotype as the parents?
A.
25%
B.
50%
C.
75%
- in the completed p-square, 3 of 4 boxes will have at least 1 "Y",
producing the dominant phenotype (same as parents)
D.
100%
3. In a certain plant, purple flowers are dominant to red flowers. If the cross of two purple-flowered plants produces some some purple-flowered and some red-flowered plants, what is the genotype of the parent plants?
A.
PP x Pp
B.
Pp x Pp
- for any offspring to be recessive, each parent MUST have at leat one
"p"
C.
pp x PP - only
one parent is purple, this CAN'T be an answer
D.
pp x pp - neither
parent is purple, this CAN'T be an answer
Base questions #4-8 on the following information:
A white-flowered plant is crossed with a pink-flowered plant. All of the F1 offspring from the cross are white.
4.
Which phenotype is dominant? white
5.
What are the genotypes of the original parent plants? WW (pure white) x ww
(pink)
6.
What is the genotype of all the F1 offspring? Ww (white)
7.
What would be the percentages of genotypes & phenotypes if one of the
white F1 plants is crossed with a pink-flowered plant?
50%
heterozygous white & 50% homozygous recessive pink.
The
cross for this question would be "Ww (white F1) x ww (pink)".
The
alleles of the white parent are above the columns & those of the
pink parent are in front of the rows. 2 of 4 boxes (50%) are "Ww",
which is heterozygous & would have the dominant trait (white). The
other 2 of 4 boxes (50%) are "ww", which is homozygous recessive
& would have the recessive trait (pink).
8. Which of Mendel's Laws is/are illustrated in this question? Dominance is illustrated by the original cross (WW x ww).
9. Crossing two dihybrid organisms results in which phenotypic ratio?
A.
1:2:1 - genotype
ratio of a hybrid cross, ex: Tt x Tt
B.
9:3:3:1-
dihybrid means hybrid for two different traits. An example could be GgYy x
GgYy.
C.
3:1 - phenotype
ratio of a hybrid cross
D.
1:1
10. The outward appearance (gene expression) of a trait in an organism is referred to as:
A.
genotype
B.
phenotype
C.
an allele
D.
independent assortment
11.
In the homologous chromosomes shown in the diagram, which is a possible
alleleic pair?
B. Ee- a
possible allelic pair but NOT SHOWN IN THE DIAGRAM, so this CAN'T be an answer
C. AB
D. ee
- an "allelic pair" is always two forms of the same letter. In
this example they are two lowercase "e's".
12. The
phenotype of a pea plant can best be determined by:
A.
analyzing its genes
B.
looking at it
C. crossing
it with a recessive plant
D. eating
it
13. Mendel formulated his Law of Segregation after he had:
A.
studied F1 offspring -
B.
studied F2 offspring
- he crossed two hybrids (F1's) and got a second generation --- the F2.
C. produced
mutations - Mendel knew NOTHING about mutations
so this CAN'T be an answer
D. produced
hybrids
14. Which cross would produce phenotypic ratios that would illustrate the Law of Dominance?
A.
TT x tt
- one parent tall, the other short, all offspring would be tall
B. TT x Tt
C. Tt x Tt
- illustrates Segregation
D. tt x tt
15. The mating of two curly-haired brown guinea pigs results in some offspring with brown curly hair, some with brown straight hair, some with white curly hair, and even some with white straight hair. This mating illustrates which of Mendel's Laws?
A.
Dominance
B.
Segregation
C.
Independent Assortment
- the question involves two different traits (hair color &
hair texture), this is the only law that deals with two different traits
D.
Sex-Linkage - Mendel knew NOTHING about
sex-linkage so this CAN'T be an answer