Mendelian+Laws

sheila.kaliher


 * 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. **

leah.nicholas

http://media-2.web.britannica.com/eb-media/96/118096-004-547374A3.jpg
 * __Gregor Mendel__**

Gregor Mendel is known as the father of genetics. His early work done with pea plants produced a set of laws that helps explain the inheritance of genes.

First Law:Segregation-"During the formation of gametes, the paired unit factors separate or segregate randomly so that each gamete receives one or the other with equal likelihood" (Klug, Cummings, Spencer, pg 41).

Second Law:Independent Assortment-"During the gamete formation, segregating pairs of unit factors assort independently of each other" (Klug, Cummings, Spencer, pg 45).

marqelle.albrecht

Mendelian Laws Postulate 1 is the law of genes. This law states that genetic characteristics are controlled by genes that exist in pairs in individual organisms. Example: P1: round x wrinkled - 3/4 of the F2's were round, 1/4 wrinkle P1: yellow x green - 3/4 of the F2's were yellow, 1/4 were green This shows a ratio of 3:1 inthe F2 generation and therefore, three possible pairwise combinations of the two Factors (Eastern Michigan University)



Postulate 2 is the law of dominance. This law states that when two unlike factors for a single trait are present, one factor is considered to be dominant and the other factor is recessive. The dominant one is expressed while the recessive trait is "hidden." Example: Tall X Dwarf F1 generation: all plants were tall F1 self-fertilization: 3/4 of F2's were tall, 1/4 were dwarf The results show that the factor of tall is dominant and the factor of dwarf is recessive. (Eastern Michigan University)

Example of Postulate 2

Janell.Gietzen

Postulate 3 is the law of segregation. This law states that when gametes are forming, the two alleles that are responsible for a certain trait separate from each other. This way each gamete receives one allele or the other. These traits then come back together at fertilization. This is when the genotype of the offspring is produced.

On the left, in the first generation, a homozygous dominant (red) and a homozygous recessive (white) flower are crossed to produce only hybrid red (Rr) progeny. In the second generation, on the right, the offspring from the first generation are selfed to produce offspring with a genotypic ratio of 1 RR : 2 Rr : 1 rr. This is an example of the law of segregation.

Postulate 4 is the law of independent assortment. This law states that when gametes are forming, the separating pairs of alleles segregate without respect to the other allelic pair. This means the traits are distribted independently of each other.

For example the genotype AaBb could make the following gametes; AB, ab, Ab, and aB.

INDEPENDENT ASSORTMENT

Mendel worked with garden peas to create the following postulates. 1) Unit factors (genes) are in an organism in pairs. 2) If a plant had different alleles, one may be expressed (dominant) and the other masked (recessive). 3) Segregation is defined as random seperation of alleles into gametes. 4) Independent assortment is the segregating of pairs of unit factors (genes) assort independently of each other.

The first postulate says that genes exist as pairs. An example is plant height. The phenotypic or physical appearance possibilities for these would be: Tall/Tall Dwarf/Dwarf Tall/Dwarf Dwarf/Tall These phenotypes would be expressed because of their genotypic make up. The tall gene is dominant because it masks the dwarf trait. Therefore the dominant allele is "D" and the recessive allele is "d." This theory uses the first letter of the recessive trait. The previous plants would have the following genotypes: DD Dd dD dd Postulate 3 says that there's random seperation of alleles into gametes. A way of combing these alleles is to construct a punnet square.This monohybrid cross results with 3 tall plants to 1 dwarf plant. Postulate 4 says that genes assort independently of each other. An example of this is that plant height and seed shape.

Mendel created these postulates to try to explain the results of his pea plant experiments. When he cross-bred two plants with contrasting traits, such as a plant with round seeds and a plant with wrinkled seeds, he noticed that each subsequent generation expressed a specific ratio of the traits being tested. For example, if Mendel cross-pollinated the round seed plant with a wrinkled seed plant, the next generation would produce plants which all had round seeds. Even more interesting, when he cross-pollinated plants from that group together, the next generation produced a 3:1 ratio of round:wrinkled seeded plants, every time.

This type of cross between two individuals with contrasting forms of the same trait (e.g. round vs. wrinkled seed texture) is called a **monohybrid cross**. The very first generation of plants, or the parent plants are called the **parental (P1)** **generation**, their offspring are called the **first filial (F1)** **generation**, and the offspring of the F1 generation are called the **second filial (F2) generation** (Klug, Cummings, Spencer, pg 40).

Here is a link to a video that talks a little bit about Mendel and also discusses his four postulates. It is a podcast for a class. I think it helps to hear someone discuss them. http://www.youtube.com/watch?v=dOs6tLAYcUQ ---Jessica Dewald

I think everyone has done a good job discussing Mendel and his postulates. I found that although Mendel lived and worked 1822-1884, he was not recognized for his work until the 1900's. Interesting to know since he made such an impact on the study of genetics and science in general! ---Jessica Dewald

Under Postulate 3 of Mendelian Genetics, one type of cross established is a Reciprocal Cross. A Reciprocal Cross is used to reverse the phenotypes or genotypes in the male and female parents. This way we can determine if a certain trait is sex dependent or not.

DD x dd and dd x DD are reciprocal crosses

In cases where the alleles are heterozygous, such as Dd x Dd, there is no reciprocal cross.

Another type of cross used for alleles is a Testcross. A Testcross is used to tell if a Dominant trait is homozygous or heterozygous. To do this test, we cross the unknown genotype (D_) with a homozygous recessive (dd)

__If unknown DD__- all tall __If unknown Dd__- 1 tall:1 dwarf
 * || d || d ||
 * D || Dd || Dd ||
 * D || Dd || Dd ||
 * || d || d ||
 * D || Dd || Dd ||
 * d || dd || dd ||

-Derek Martinson

A monohybrid cross was discussed in a previous paragraph. Under Postulate 3, Mendel also used a Dihybrid cross. A dihybrid cross (also called a two factor cross) involves two pairs of contrasting traits, rather than just one pair.

GG yellow WW round Gg yellow Ww round gg green ww wrinkled

P1 GGWW x ggww ↓ F1 GgWw (all yellow, round) ↓ Selfing Yellow, Round = 9 Yellow, Wrinkled = 3 Green, Round = 3 Green, Wrinkled = 1
 * || gw || gw ||
 * GW || GgWw || GgWw ||
 * GW || GgWw || GgWw ||
 * || GW || Gw || gW || gw ||
 * GW || GGWW || GGWw || GgWW || GgWw ||
 * Gw || GGWw || GGww || GgWw || Ggww ||
 * gW || GgWW || GgWw || ggWW || ggWw ||
 * gw || GgWw || Ggww || ggWw || ggww ||

-Derek Martinson

This is just another picture to go along with the independent assortment section above Mendels second law states that for two different characteristics the genes are independently inherited. This particular picture is showing the round and wrinkled, yellow and green characteristics

-Matthew Tarvestad

Mendel's first main conclusion concerned dominance. He observed that crossing parent plants with pure lines—one with green peas, the other with yellow—produced offspring that were always either green or yellow, and not a blend. He also found that yellow peas appeared more often than green peas in a predictable 3-to-1 frequency in the subsequent generation. Mendel called the more common traits "dominant" and the less common ones "recessive."

According to Mendel's second principle, called the principle of segregation, each parent contributes one allele, or part of a gene pair, to an egg or sperm. When fertilization occurs, the offspring's gene pair is determined by which allele each sex cell carried. The allele that gets segregated, or separated, from each parent's pair is a matter of chance. So, for a given trait such as pea color, a heterozygous parent that carries two different alleles would contribute either the dominant allele or the recessive one. A homozygous parent that carries two identical alleles could only contribute the allele for that trait.

Mendel's third principle, the principle of independent assortment, states that the pairs of alleles needed for each trait are passed on to offspring independently of one another. This means that offspring can possess combinations of genes that neither parent possesses. For example, because the inheritance of flower color has no effect on the inheritance of seed color, a white-flowered plant that produces green peas can descend from white-flowered parents that produced yellow peas.

These three principles account for the many combinations of traits seen over several generations of offspring. The fact that some human traits are controlled by more than one gene pair adds even more complexity. Siblings may share a few traits in common, such as hair color and handedness, but they are not likely to share all of their traits.

-Janell Gietzen

Here is a short clip on the basis of Mendel's principles.http://www.youtube.com/watch?v=y9bHYn-1hbU