Mendels Laws

1. The Law of Dominance

2. The Law of Segregation

3. The Law of Independent Assortment

Confirmation of Mendels First Law

Reasons for Failure of Mendels Predecessors

Summary Table of Mendel Laws

Genetic Terminology

For centuries, people realized that individual characteristics were passed on from parent to offspring.

When a child is born people look for its resemblance with the parents or close blood relatives.

Questions relating to the nature and the basis for this relationship have occupied the thoughts of man for centuries.

However the mechanism of inheritance was not generally understood until early this century.

Serious systematic attempts to seek answers to these questions began only in the 18th century.

A number of scientists had worked on plant hybridization during the 18th and 19th centuries prior to Mendel.

Koelreuter conducted extensive studies on hybridization and proposed the theory of uniformity and heterosis in F₁ and variations in F_2.

The following important conclusions were available to Mendel from the studies of his predecessors.

1. In F₁ hybrids, some characteristics are identical to those of one of the two parents, some are similar to those of the other parent, while some others are intermediate between those of the two parents.

2. Characteristics of F₁ and F₂ progeny produced by reciprocal crosses are identical. This observation clearly demonstrates that the contributions of male and female parents to the characteristics of the progeny are equal.

3. F₁ progeny from a single cross are uniform in their characteristics. That is, all the plants in F₁from a cross are similar to each other. But F₂generation shows a large variation for different characteristics.

4. In F₂ generation, some plants have characteristics similar to one parent, while some others are similar to the other parent in their appearance. The appearance of parental forms in F₂ was called reversion. But a majority of the plants were intermediate in appearance between the two parents.

5. Some plants in F₂ have entirely new characteristic forms.

Mendels Laws - The three important laws of Mendel include;

1. The Law of Dominance

2. The Law of Segregation

3. The Law of Independent Assortment

Mendels Law of Dominance - The first law of Mendel states that 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 and over and 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 F₁generation) were tall.

Similarly, crossing pure yellow seeded pea plants and pure green seeded pea plants produced an F₁generation of all yellow seeded pea plants.

The same was true for other pea traits. 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, Mendel proposed a law in which he proposed that There is a factor that makes pea plants tall, and another factor that makes pea plants short.

Furthermore, when the factors were mixed, the tall factor seemed to dominate over the short factor.

Now, from our modem 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.

Mendels Law of Segregation - The second law states that 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.

So, he took two of the F₁generation (which are tall) and crossed them. His new batch of pea plants (the F₂ generation) is about 3/4 tall and 1/4 short.

The parent plants for this cross each have one tall factor that dominates the short factor and causes them to grow tall.

To get short plants from these parents, the tall and short factors must separate, otherwise a plant with just short factors could not be produced.The factors must segregate themselves somewhere between the production of sex cells and fertilization.

It can be seen from the p-square that any time two hybrids are crossed, 3 of the 4 boxes will produce an organism with the dominant trait (in this example TI, Tt, and Tt), and 1 of the 4 boxes ends up homozygous recessive, producing an organism with the recessive phenotype.

When 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.

Mendels Law of Independent Assortment - The third law states that Alleles for different traits are distributed to sex cells (offspring) independently of one another.

Mendel noticed during all his work that the height (tall or short), of the plant and the shape (round or wrinkled) of the seeds and the colour (green or yellow), of the pods had no impact on one another.

In other words, being tall did not automatically mean that 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.

It involves what is known as a dihybrid cross, meaning that the parents are hybrid for two different traits.

The genotypes of the 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

Thus we are dealing with two different traits: (1) seed texture (round or wrinkled) and (2) pod colour (green or yellow). Also each parent is hybrid, for each trait (one dominant and one recessive allele for each trait).

The results from a dihybrid cross are always the same: 9/16 boxes (offspring) show dominant phenotype for both traits (round and yellow), 3/16 show dominant phenotype for first trait and recessive for second (round and green), 3/16 show recessive phenotype for first trait and dominant form for second (wrinkled and yellow), 1/16 show recessive form of both traits (wrinkled and green).

So, as can be seen 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 we 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, let us compare the colour trait in the offspring; 12 green and 4 yellow (3: 1 dominant recessive) and the same for the seed texture 12 round and 4 wrinkled (3: 1 ratio). The traits are inherited independently of each other.

Confirmation of Mendels First Law -

With these observations, Mendel could form a hypothesis about segregation. To test this hypothesis, Mendel selfed the F₂ plants.

If his law was correct he could predict what the results would be. And indeed, the results occurred as he expected.

From these results we can now confirm the genotype of the F2 individuals. Thus the F₂ is genotypically 4 Dd : 2 Dd : 4 dd.

This data was also available from the Punnett square using the gametes from the F₁ individual.

So although the phenotypic ratio is 3: 1 the genotypic ratio is 1:2:1.

Mendel performed one other cross to confirm the hypothesis of segregation the backcross.

We must remember that the first cross is between two pure line parents to produce an F₁ heterozygote.

At this point instead of selfing the F₁ Mendel crossed it to a pure line, homozygote dwarf plant.

Reasons for Failure of Mendels Predecessors -

In his 1865 paper, Mendel presented a brilliant analysis of the deficiencies in the experimental approaches of his predecessors. These are summarized below.

1. The scientists studied the plant as a whole, Le., its total appearance consisting of a large number of characters.

2. Therefore, the plants could not be classified into few clear cut classes. These workers did not attempt an exhaustive classification of the different forms of the characteristics present in the progeny.

3. The scientists were more concerned with the description of various forms appearing in the progeny. An attempt to determine the frequencies of different characteristic forms in the progeny was not made.

4. In many cases, the data from different generations were not kept accurately and separately.

5. In many cases, a complete control on pollination in the F₁ was lacking.

6. In many studies, the F₁ was an interspecific hybrid exhibiting partial to considerable sterility.

7. The number of plants studied in F₂ was relatively small.

8. In addition, most of the characters studied by the earlier workers were quantitative in nature.

Summary Table of Mendel Laws

Genetic Terminology -

Gregor Johann Mendel