Mendel’s Law of Heredity | Genetics

In this article we will discuss about the Mendel’s law of heredity.

The modern science of heredity came into existence with the experiments of Mendel which he made on garden pea plants. His breeding experiments were carried out during summer in a small garden plot.

He choose the pea as his favorite material because of the following reasons:

1. It is an annual plant and takes little space for growth.

2. It can be grown easily.

3. Pea seeds are large and present no problem in germination.

4. It was easily available.

5. It had several well defined characters.

6. The several well defined characters had two contrasting forms which were easily distinguishable from each other. For example, some varieties had red flower colour while others white flower colour. These two flower colour forms were easily distinguishable. This helped in easy classification of F₂ and F₁ progeny.

7. Pea were obtainable in many pure breeding varieties.

8. The flowers were well protected from the influence of foreign pollen due to close encasement of the reproductive organs inside the petals (keel or carina) of the flower. This was experimentally verified by Mendel.

9. The hybrids resulting from crossing two varieties were perfectly fertile.

10. Hybridization is easy.

11. These are normally self-fertilizing or self-pollinated.

Seven Pairs of Contrasting Characters of Pea Studied by Mendel:

Reasons for Mendel’s Success:

(1) He recognised the need of keeping adequate records of his observation from generation to generation.

(2) He realised unlike most of his predecessors that the laws of heredity in a complex organism could only be discovered by concentrating on one character at a time. This permitted him to make classification of F₂ and F₁ progeny in to two distinct groups. Later on, he studied double and triple characters inheritance at a time after he had formulated the law governing the inheritance of single pairs of contrasting characters.

(3) The most significant factor behind his success was his ability to analyse the reasons of un-success of earlier workers. He detected sincerely the drawbacks or weakness of their experimental materials, techniques and he avoided them in his own experiments.

(4) Mendel selected pea varieties that had clearly different forms of one or more characters. The two clear cut forms of a character red and white flower colour, round and wrinkled seeds, are termed as contrasting characters.

(5) Mendel conducted his experiments skillfully. For instance, he grew the pea varieties used as parents for two seasons to verify homozygosity of the varieties, avoid mechanical mixtures and stability of the character differences. His major carefulness or precautions avoided various experimental errors which may come from cross pollination.

(6) His knowledge of mathematics proved as a boon for the interpretation of his findings. As a result he was able to understand that ratios varying from 2.82: 1 to 3.15: 1 were all estimates of 3: 1 and not other ratios.

(7) Mendel was undoubtedly most fortunate in-spite of all brilliancy that:

(i) The seven characters chosen by Mendel were all qualitative, not a single inherited quantitatively

(ii) The contrasting or opposite forms of each of the seven characters were governed by single pair of genes or factors and in each case one form was completely dominant over another form

(iii) All the genes were located on different chromosomes

(iv) He observed that the movement of the sperm to the eggs and their union in fertilization constitute or make a complex process extending over a considerable period of time.

The Reasons for Failure of Mendel’s Predecessors:

Mendel (1865) in his research paper entitled “Experiments in Plant Hybridization” presented the following reasons for their failure; these are as follows:

(1) The progenies were not classified on the basis of contrasting factors where as Mendel classified the progenies on the basis of shape of seed in to two different classes, round and wrinkled seeds, yellow and green cotyledon colour, red and white flower colour etc.

(2) They did not maintain the adequate records of their experimental findings i.e., data received in different generations.

(3) The scientists only described the various forms of a character found in the progeny. No attempt was being made to determine the frequency of different forms of characters of any generation present in the progeny. Truly speaking, no data from different generations were kept accurately and separately.

(4) No complete control on pollination was kept in F₁ generation. Therefore, the result obtained from F₂ generation were not reliable.

(5) F₁ progenies were sterile in considerable amount because these were produced mostly by interspecific crosses. Therefore, it became a matter of doubt not to get expected ratios of various character forms in F₂ generation.

(6) In F₂ the number of plants studied was relatively very small i.e., 100-200. Thus, the result found from such a small number of plants are not likely to be conclusive or decisive.

(7) Unfortunately, most of the characters studied by the earlier scientists were quantitative in nature.

(8) The earlier scientists studied many characters at a time.

No doubt, Mendel presented a brilliant analysis of deficiencies in the experimental approach of his predecessors. The above comments are surprisingly true.

Reasons for the Neglect of Mendel’s Findings:

1. The work of Mendel was really most original. Mendel used mathematics to explain the principles of inheritance. This was some thing new to biologists of those days as they believed that biological phenomena may not be classified by mathematical treatment.

2. At that time, scientists had insufficient knowledge of cell.

3. Mendel worked on discontinuous variation where as Darwin, Dalton and others exhibited continuous variation in their attempts. They regarded discontinuous variation insignificant in evolution.

4. The work of Mendel appeared to establish the constancy of characters of a species from generation after generation. In F₂ generation from a cross, no new forms or variation appeared, only the parental forms came in the light. This seemed to run opposite to the theory of evolution, which needed variation or new forms to be borne or generated for natural selection. Thus, Mendel’s result did not seem to fit the scheme of biological evolution.

5. Mendel corresponded with his contemporary botanist, Nageli but he did not give any importance to his work since he was more influenced by Charles Darwin’s theory of evolution.

6. Mendel did not put his findings through further writings on the subject after his initial paper.

7. The cell division, mitosis and meiosis, fertilization, behaviour of chromosomes were unknown at that time when Mendel presented his findings. No doubt, Mendel was much ahead of his time to be understood and appreciated.

Mendel’s Experiments on Pisum sativum:

Mendel collected 34 distinct varieties of pea (Pisum sativum) for his studies. He grew each of these varieties in different plots for a period of two years to test the stability of their characters and purity. Finally he selected 22 varieties and made numerous crosses during the next six years by means of artificial pollination.

Each cross was carried through three generations. Mendel selected seven pairs of contrasting characters for his study. Each of these characters had two different forms known as alternative or contrasting forms of a character e.g., yellow and green cotyledon colour, length of the stem, tall and dwarf etc.

So he started his experiments by crossing plants for one pair of contrasting character at a time. He selected two plants of peas, one with tall stem (6′-7′ in length) and the other with dwarf or short stem (3/4′-1 1/4′). Each of these plants were chosen out of the stock that had produced for many generations nothing but tall and dwarf plants respectively. These are called pure bred plants.

When tall and dwarf pure bred plants are crossed, the hybrid offspring’s were tall and there were no intermediate forms. Mendel could open a flower bud and remove the stamens before any pollen had been shed, thus preventing self- pollination. This removal of stamen in immature stage before dehiscence of pollens from anther lobe is called as emasculation. The purpose of emasculation is to prepare a female parent.

Mendel could then place on the stigma of this castrated or emasculated flower, pollen from the plant which he wished to use as the other (male) parent in a cross. The artificially fertilized flowers were protected from contamination by foreign pollen of unknown origin.

Parental Generation:

The first hybrid generation was called first filial generation (F₁). The tallness that was seen in this experiment was called the dominant character and the character that was suppressed i.e., dwarfness was said recessive character. Each character in individual is determined by a pair of factors. These are called allelomorphs.

In the hybrid, the two contrasting factors remaining as a pair thus form allelomorphs or alleles. With a view to find out what became of dwarfness in heredity, he carried on his experiments further. He allowed the hybrid tall plants to self-fertilize i.e., he allowed the pollen from a flower to fall on the stigma of the same flower.

F₁ Generation:

It clearly showed that dwarfness had not completely disappeared from the race but only was suppressed in the F₁ generation. When the dwarf varieties of the F₂ generation were inbred, the offspring’s in F₃ were all dwarf and in further generations were also dwarf. So, these were said Extracted Recessives as these were recovered from the hybrid generation.

The tall plants (3/4) of the F₂ generation were selfed.

The offspring that resulted showed two sets:

(i) 1/3 of them produced tall plants which in successive or further generations produced only tall plants i.e., pure or extracted tall

(ii) 2/3 of them behaved like F₁ generation (hybrid tall) producing both tall and dwarf plants in the ratio 3: 1.

Mendel further noted that the results from reciprocal crosses were same. When one parent (Parent A) is used as female in a cross with another parent (Parent B) and then it (Parent A) is used as male in another cross with the same parent (Parent B), such crosses are known as reciprocal crosses.

A cross between two parents differing for one character is termed as monohybrid cross while those differing for two characters are known as di-hybrid crosses and differing for three characters are said as tri-hybrid crosses. The results obtained from monohybrid crosses for the rest six characters were the same with those for length of stem.

Limitations:

Mendel’s principle of independent assortment is not applicable for genes and chromosomes equally because maternal and paternal chromosomes are segregated independently at the time of gamete formation during meiosis but linked genes are not segregated or separated independently found on the same chromosome. Therefore, the law of independent assortment is applicable on chromosomes only, not on genes or factors.

In brief, genes located in the same chromosome pair do not behave independently due to linkage. In other words, Mendel law of independent assortment’s holds good in the case of genes located in separate pairs of chromosomes.