Mendel's Law

Mendel’s Laws of Inheritance

Gregor Johann Mendel (1822 – 188 hi4) is appropriately called father of genetics. With the help of his experiments on garden pea (Pisum sativum) was able to formulate laws, which explain the manner of inheritance of characters. The results of his work were published in Germany in 1866 paper entitled “Experiments in Plant Hybridization”.

 

i.                        Used mathematical principles of probability for explanations he gave in his results. This was something new and unacceptable to biologists of that time. They believed that biological phenomena were too complex to be reduced to a mathematical treatment.

ii.                        Mendel studies the inheritance of contrasting pairs of characters exhibiting discontinuous variation. Many of his contemporaries i.e., Darwin, Galton etc, were pre occupied with characters exhibiting continuous variation. They regarded discontinuous variation as being unimportant in evolution.

iii.                        Mendel failed to demonstrate the validity of his conclusions in other species. He was unlucky in selecting Hieraceum (hawkweed) which showed apomixes (embryos arised directly from diploid tissue in the ovary without fertilization) and honey bees (having haploid males) as the experimental material.

His work was not appreciated by the rest of the scientific community until 1900, when three botanists, Carl Correns (Germany) working on Oenothera, Hugo de Vries(Netherlands) working on Xenia,  and Erich von Tschermak(Austria) working on various flowering plants, rediscovered his work after each had apparently independently reached similar conclusions. Mendel’s original paper was republished in Flora, 89, 364 (1901).

Mendel chose the garden pea as his experimental organism because

i.                        it is an annual plant with well-defined characteristics, and it can be grown and crossed easily.

ii.                        Its short life cycle makes it possible to study several  generations within a short period.

iii.                        Moreover, garden peas, have perfect or bisexual  flowers, flowers that contain both female and male sex organs, and

iv.                        they are ordinarily self-fertilized, i.e., the ovule (female gamete) is fertilized by pollen (male gamete) from the same plants.

v.                        Because of self-fertilization, plants are homozygous. It is therefore, easy to get pure lines for several generations.

vi.                        Pollen from another plant can be experimentally introduced to the stigma of flower, but cross-pollination is rare without human intervention.

vii.                        Mendel was fortunate in choosing a diploid plant because diploid organisms contain only two sets of chromosomes. If he had chosen a polyploidy organism, an organism with more than two sets of chromosomes, he would not have obtained simple, understandable results.

In seven pairs of contrasting traits or characters Mendel chose to study, two parental forms exhibited well-defined, contrasting alternative or visible morphologies on the plants. The seven characters that Mendel used were:

i.                        Stem tall (6-7 feet) or dwarf (1/4 to 1 1/2 feet)

ii.                         

iii.                        Unripe pods may be green or yellow

iv.                        Ripe pods inflated or constricted

v.                        Seeds round or wrinkled

vi.                        Seed coat white or brown

vii.                        Cotyledons green or yellow.

Much of Mendel’s success in his first experiments may be attributed to his good judgment in making crosses, as far as possible, between parents that differed in only one trait.

For each of the seven pairs of characters, plants with one alternative trait was treated as female, and those with other alternative as male. Reciprocal crosses were also made i.e., each of these crosses was made in two ways.

Example: Tall (♂)X Dwarf (♀)

                       

The parental generation is called P (or P₁ and P₂, when the two parents need to be distinguished). The progeny obtained as a result of crossing parents is called as first filial or F generation. The progeny obtained by self-fertilization of F₁ plants is called as second filial or F_, F₄ etc. generations can also be obtained.

To prevent self-fertilisation, the anthers were removed from the female plant. This is known as emasculation. He collected pollen from male parent and dusted on the feathery stigmas of emasculated female flowers. This is called crossing or hybridization. These cross pollinated flowers were enclosed in separate bags to avoid further deposition of pollen from other sources.

A cross between two parents differing in a single pair of contrasting characters is known as monohybrid cross.

A cross between two parents differing in two pair of contrasting characters is known as dihybrid cross.

Phenotype (Gr: form that is shown) : A class of individuals recognised based on outward appearance of a trait in an individual is the phenotype, e.g. Smooth-seeded shape or wrinkled shape of seeds represent two different phenotypes.

Genotype : A class of individuals recognised based on its genetic constitution and breeding behaviour is called the genotype, e.g., the genotype of pure smooth seeded parent pea plant is SS and it will always breed true for smooth-seeded character, but plants having Ss on selfing would give rise to a population represented by 3 : 1 ratio for smooth seeded plants and wrinkled seeded plants.

Trait : is the morphologically or physiologically visible character, e.g. colour of flower, and shape of seed.

Factor : The unit of inheritance and expression of a particular character is controlled by inheritable units called factor (gene) which are present in pairs in parental cells and singly in the gametes. Z

Gene : A segment of DNA molecule which determines the unit of inheritance and expression of a particular character.

Alleles or Allelomorphs : Two or more alternative forms of a gene are called alleles. For example in pea plant, the gene for producing seed shape may occur in two alternative forms: smooth (S) and wrinkled (s). Genes for smooth wrinkled seeds are alleles of each other, and occupy same locus on homologous chromosomes.

Homozygous : An individual possessing identical alleles for a trait is termed homozygous e.g. SS is homozygous condition for smooth seeded character in garden-pea.

Heterozygous : An individual with dissimilar alleles for a trait is termed heterozygous for e.g. Ss represents the heterozygous condition for smooth seeded character in garden pea.

Dominant trait : Out of the two alleles or allelorrorphs of a trait, the one which expresses itself in a heterogygous organism in the F1 hybrid is called the dominant trait (dominant allele) and the one that remains masked in F1 individual but gets expressed in the next generation (F2), is called recessive. Thus, if the allelic combination in an organism is Tt, and T (tallness) expresses itself but t (dwarfness) cannot, so T is the dominant allele, and tallness is dominant on dwarfness represented by “t’.

  : Out of the two alleles for a trait, the one which is suppressed (does not express) in the F1 hybrid is called the recessive trait (recessive allele). But the Recessive allele does express itself only in the homozygous state (e.g. tt).

Parent generations : The parents used for the first cross represent the parent (or P1) generation.

F1 generation : The progeny produced from a cross between two parents (P1) is called First filial or F1 generation.

F2 generation : The progeny resulting from self pollination or inbreeding of F1 individuals is called Second Filial or F2 generation.

Monohybrid cross : The cross between two parents differing in a single pair of contrasting characters is called monohybrid cross and the F1 offspring is Monohybrid. The phenotypic ratio of 3 dominants : 1 recessive obtained in F2 generation from the monohybrid crosses by Mendel was mentioned as 3:1 monohybrid ratio.

Dihybridcross: The cross in which two parents differing in two pairs of contrasting characters are considered simultaneously for the inheritance pattern is called dihybrid cross. The phenotypic ratio obtained in the F2 generation from a dihybrid cross is called Mendelian dihybrid ratio (9 : 3 : 3 : 1), and the F1-individual is called dihybrid (SsTt).

Hybridisation: Crossing organisms belonging to different species for getting desirable qualities in the offspring. z Test cross : is the Crossing of the F1 progeny with the homozygous recessive parent. If F1 progeny is heterozygous, then test cross always yields the ratio of 1 : 1 between its different genotypes and phenotypes.

Reciprocal cross : Is the cross in which the sex of the parents is reversed. That is if in the first cross father was dwarf and mother tall, then in the reciprocal cross, dwarf parent will be female and tall parent male.

Principle of Segregation:

In one experiment, Mendel crossed

When F

On basis of

Mendel self-fertilised F

It means, F

i.                    Tall homozygous (pure) – 25% (TT)

ii.                  Tall heterozygous (hybrid) – 50% (Tt)

iii.                Dwarf homozygous (pure) – 25% (tt)

From his observation, Mendel concluded that F

An important feature of Mendel’s results was that the F

The other trait that disappeared or was masked in the F

Similarly, the factor (now called a gene) that specifies the trait expressed in the F

Mendel concluded that, particles or factors were transmitted intact through the gametes from parents to progeny. He suggested that these factors existed in several alternative forms (now called alleles), determining the different phenotypes observed.

During sexual reproduction, the members of each pair of alleles separated into different reproductive cells or gametes of the male and female parents, that fuse and give rise to progeny. Mendel called this separating or segregation process as “splitting of hybrids”. Thus, paired factors or genes (allelic pairs) separate from one another and are distributed to different sex cells or gametes. This is called as law of segregation.

According to this law, in a heterozygote a dominant and a recessive allele remain together throughout the life (from zygote to the gametogenesis stage). Without contaminating or mixing with each other they finally separate or segregate from each other during gametogenesis. So that each gamete receives only one allele. As the gametes are pure for a given character, this law is also known as Law of purity of gametes.Principle of Independent Assortment :

Mendel crossed plants that differed in two pairs of alleles. In this cross, designed to clarify the relation of different pairs of alleles.

Mendel crossed a homozygous pea plant having yellow and round seeds (YYRR) with the homozygous pea plant having green and wrinkled seeds (yyrr). The F₁ plants were yellow and round seeds (YyRr) just like the homozygous parent, displaying complete dominance

This type of cross in which the parents have different parental contributions for two traits is called a dihybrid cross. This shows that yellow color and round shape were dominant and green and wrinkled condition recessive.

When these F₁ plants were self fertilized, they produced four types of gametes with two parental and two new combinations i.e., YR, Yr, yR and yr are formed in approximately equal number. Thus, recombination of genes takes place at the time of gamete formation in F₁ plants.  

From a total of 556 seeds, the following distribution was observed: 315 – round and yellow, 108 – round and green, 101 – wrinkled and yellow, and 32 – wrinkled and green. These results closely fit a ration of 9:3:3:1. This ratio is simply a multiple of 3:1 ration i.e., (3:1)

Two of these traits were similar to parental combination (round and yellow seeds, wrinkled and green seeds), while the other two were new combinations (round and green seeds and wrinkle and yellow seeds).

The result showed the assortment of two independent pairs of alleles, each showing dominance of one number. Not only did the members of each pairs of alleles segregate, but the allelic pairs of different genes behaved independently with respect to each other i.e., members of different pairs of alleles assort independently into gametes.

According to law of independent assortment “ the factors or genes for different pairs of contrasting characters present in a parent assort (separate) independently from one another during gamete formation”.

Thus, any allele of one gene is equally likely to combine with any allele of the other gene and pass into the same gamete. A random union among these gametes gives rise to 16 possible zygotes. These zygotes yield a 9:3:3:1 phenotypic ratio, which is known as the typical dihybrid ratio. The nine different types of genotypes expected in F

Back Cross and Test Cross

Mendel used two types of tests to distinguish homozygous one from heterozygous having the same phenotype ( e.g. TT and Tt for tall phenotype). If a homozygous tall plant (TT) is selfed, it will breed true, producing only tall plantsl. But when heterozygous tall plant (Tt) are selfed, tall and dwarf appear in 3:1 ratio.

Back Cross:

When F

When F

                                               

                                   

                                               

                                   

Test Cross:

When F

Monohybrid Test Cross

                                               

                                   

                                               

                                   

Dihybrid Test Cross