Mr. Kousen is … WaterMnan.
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 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.
Hail to the "Father of Genetics" !
My name is ma-ma-ma-ma-ma-Mendel.
There are a few importantvocabulary 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:
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
Ma-Ma-Ma-Ma-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:
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.
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, isMendel'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:
(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!
Ma-Ma-Ma-Ma-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:
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.
Thepunnet 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.
Ma-Ma-Ma-Ma-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 / 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.
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" !