Mendels Three Laws of Genetics

MENDEL'S THREE LAWS

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 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

<answers are here>

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:
TT x tt Phenotypes:
tall x short 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.

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?
If you said "yes", then check this out: "The Punnet Square (in baby steps)".
For some practice Punnett Square problems visit my very own: "P-Square Practice Page".
Don't forget to come back & learn more about Mendel!

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) Offspring
(F2 Generation)

Genotypes:
Tt x Tt

Phenotypes:
tall x tall

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	rg
RG	RRGG round	RRGg round	RrGG round	RrGg round
Rg	RRGg round	RRgg round	RrGg round	Rrgg round
rG	RrGG round	RrGg round	rrGG wrinkled	rrGr wrinkled
rg	RrGg round	Rrgg round	rrGg wrinkled	rrgg 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	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

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" !

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?

A. cD
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)

<back>

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%

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?

A. cD
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

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