Coupon Accepted Successfully!


Mendel's Contributions

Gregor Johann Mendel (1822-1884) is called the "Father of Genetics". His contributions to the study of inheritance paved the way for our basic understanding of how traits are inherited from one generation to the next. Mendel did much of his work with easily obtained local organisms, especially garden peas. He also did genetic work with other plants and honeybees.
Mendel recognized from observations and early experiments that his experimental organisms had two alternate forms of a trait. He called these alternate forms alleles. According to Mendel, an individual possesses two alleles for each trait. The individual can have two alleles that are the same or one of each form. If the individual possessed two alleles that were the same the individual would be homozygous (or pure) for the trait.
Mendel would indicate this by using letters to represent the alleles. If an individual was homozygous then it would be AA or aa. If the offspring contained one of each allele then it was termed heterozygous or Aa. The uppercase form of the allele was used to indicate that the individual was dominant for the trait and lower case indicated that the individual was the contrasting opposite or recessive for the trait. For example, if the trait was color, and the dominant trait was red, the contrasting trait might be white. The terms dominant and recessive are used to signify when the trait appeared in the offspring of a cross between two homozygous individuals that represented alternate forms of the trait. If the trait appeared in the first generation (F1) then it was designated as dominant. If the trait "skipped" the F1 and appeared in the second generation (F2) then the trait was designated as recessive. The F2 generation was obtained by crossing two F1 individuals.
Mendel could not "see" the alleles for the trait but could see the outward appearance of the offspring. The outward appearance is called phenotype (color, size, etc.). The combination of alleles that caused the phenotype was called the genotype (AA, Aa, or aa). A homozygous dominant individual (AA) reflects the dominant trait. A heterozygous dominant individual (Aa) reflects the dominant trait also. A homozygous recessive individual (aa) reflects the recessive trait.
Mendel's results suggested otherwise. For example, when he crossed pure-bred tall pea plants with pure-bred short pea plants, he got all tall pea plants-no blending there. He concluded that every organism possesses two "factors" (we now call them genes) for a given trait, and passes on just one of these factors-at random-to its offspring.

Mendel's Laws of Inheritance

The results of Mendel's experiments and his generalization and explanation of these results are summarized into four principles. These are now known as Mendel's laws of inheritance, which are, in fact, the fundamental principles of heredity. Mendel's laws of inheritance can be summarized as follows:
  1. Law of Unit Characters
    Genes or Mendelian 'factors' occur in pairs and control the inheritance of traits as a unit.
  2. Law of Dominance
    One gene of a pair may mark or inhibit the expression of the opposite member of that pair. In an individual pea plant that has one gene for tallness (T) and one for shortness (t), the tall one will dominate the other and express itself in the F1 generation. The one that expresses itself is called the dominant gene and the other the recessive gene. When two recessive genes (tt) are together, they will express themselves and develop the recessive trait.
  3. Law of Segregation
    The genes that make up the different pairs are segregated (separated) from each other when gametes are formed in animals or spores are formed in plants. Only one of each pair of genes goes into the gametes or into the spores of plants.
  4. Law of Independent Assortment
    The genes representing two or more contrasting pairs of traits are distributed independently of one another at the time of gamete formation in animals and at the time of spore formation in plants. The gametes unite at random. Sometimes these laws are combined into three laws or into one law.

Reasons for Mendel's Success

Several reasons have been ascribed to the success of Mendel in conducting the breeding experiments as well as in formulating the fundamental principles of heredity. Some of these are as follows:
  1. Mendel's predecessors made observations on the investigated plant or animal taking into account all the characters at once. Mendel confined his attention to a single character at a time. When the behaviour of one character was established, he studied two characters together and so on.
  2. Mendel counted the individuals of each type of progeny that resulted from a cross, and thus studied the inheritance on a quantitative basis.
  3. He kept accurate pedigree records of several generations so that he could trace the ancestry of a given plant back to the beginning of his experiments.
  4. Choice of the plant material proved to be another aid for his success. The garden pea, pisum sativum, has flowers so constructed that pollen from a foreign plant cannot fertilise them. Thus self-pollination and fertilisation is the rule in these flowers. In addition, the flowers are well-suited for artificial cross-fertilisation.
  5. An interesting aspect of Mendel's methodology was that he first formulated a working hypothesis based on the theoretical explanation, and then, tested the validity of the hypothesis by performing experiments.
  6. One of the most important reasons for Mendel's success was realised after the discovery of linkage of genes and characters. The term linkages refers to the tendency of two or more genes being inherited together (hence the characters controlled by them). The basis of linked genes is their location on the same chromosome. Thus, independent assortment of genes is possible only under two situations: (i) the genes should be located on different chromosomes, and (ii) if the genes are present on the same chromosome, they should be located far apart so that they fail to be inherited together.
When Mendel carried out his experiments on heredity, the nature of the nucleus, the chromosomes, and meiosis was not understood. For Mendel, genes were hypothetical entities which explained observed patterns of inheritance. However, the rediscovery of Mendel's work led to a number of investigations which ultimately paved the way for the discovery of the genetic material and the mechanism of gene action. Mendel's laws of inheritance form the fundamental basis of modern genetics and Mendel is aptly referred to as the 'Father of Modern Genetics'

Sex Determination

In maize, a plant much studied by geneticists, every diploid adult gives rise to both male and female structures. These two types of tissues are genetically identical. Plants such as maize and animals such as earthworms, which produce both male and female gametes in the same organism, are said to be monoecious (Greek: 'single house'). Some higher plants such as the data palm or oak tree, and most animals are dioecious, that is male and female gametes are produced, by separate organisms. In most dioecious creatures, sex is determined by differences in the chromosomes. The chromosomal mechanism of sex determination, however, shows many variations in different organisms.
Sex Chromosomes
The sex of a honey bee depends on whether it developed from a fertilised or an unfertilised egg. A fertilised egg is diploid and it gives rise to a female bee-either a worker or a queen, depending on its diet during larval life. An unfertilized egg is haploid and it gives rise to a male drone. In many other animals, including human beings, the sex is determined by a single chromosome or a pair of chromosomes called sex chromosomes.
In every body cell of an organism there are two types of chromosomes namely the autosomes or the body chromosomes, which are not concerned with the determination of sex and the sex chromosomes or allosomes which are concerned with the determination of sex. The sex chromosomes which form a pair normally may either be similar or dissimilar. The similar ones are known as the X-chromosomes while those that are dissimilar consist of an X and a Y-chromosome.
It was only during the last part of the 19th century that significant studies relating the chromosomes to sex were carried out. In 1891 H. Henking, the German zoologist, demonstrated the presence of X-chromosomes in the male of a species of bug. In 1901 McClung, an American zoologist, working on the spermatogenesis of a species of grasshopper, suggested that the X-chromosome was concerned with the determination of sex. Guyer was the first to discover the presence of the X-chromosome in vertebrate cells. In 1905 N. M. Stevens who carried out a thorough investigation on the chromosomal mechanism of sex determination in several insects, demonstrated the occurrence of Y-chromosome in male cells.

Chromosomal Mechanism of Sex Determination

The chromosomal mechanism of sex determination, otherwise known as heterogamesis, was first proposed by Correns in 1906. It emphasises the role of sex chromosomes in the determination of sex of organisms. According to the chromosomal theory, only one sex, either the male or female, is heterogametic, producing two genetically different types of gametes. If the male happens to be heterogametic its genetic constitution or genotype will be 2A+ XY, where A stands for autosomes , X and Y the two dissimilar sex chromosomes and the number 2 the diploid number of autosomes in the body cells. When the heterogametic male forms gametes (sperm) they will be of two different kinds, namely, one type carrying A+X and another type A+Y. The females of the same species on the other hand are homogametic with a genetic constitution of 2A+XX where 2A stands for diploid number of autosomes and X and X the pair of similar sex chromosomes. When the female forms gametes they will all have the same genetic constitution namely A+X. In mammals, including human beings, the males are heterogametic whereas the females are homogametic. In some instances such as butterflies, birds and a few fishes the females are heterogametic with the genetic constitution 2A+XY and produce two types of eggs, one type containing A+X and the other A+Y.


  Idiogram: Metaphase plate of a normal human female.

In a third category exemplified by grasshoppers, bugs and certain insects the Y chromosome is completely absent in the male. Therefore, the genetic constitution of the male will be 2A+XO where 'O' indicates the absence of the Y-chromosome and the female will have the constitution 2A+XX. In these instances the males are heterogametic to sex and the females are homogametic.

The chromosomal mechanism of sex determination can, thus, be classified into two major categories, namely XX-XY type and XX-XO type.

Test Your Skills Now!
Take a Quiz now
Reviewer Name