What is Mendel’s Theory of Genetics?
Biological genetics, in simple words, is the passing of traits from parents to their offspring. This passing can occur through sexual reproduction or asexual reproduction. The traits are passed onto offspring as genetic information. There are different types of biological genetics. One such type is the Mendelian Genetics, which, discovered in 1900, changed the whole domain of genetics and inheritance forever.
Pre-Mendelian Concept of Heredity
A number of standpoints had already emerged before the Mendelian concept of genetics was discovered. In general, it was believed that the “essences” of parents used to blend during coitus, which was the main reason for inheritance. This theory is termed as the “theory of blending inheritance,” and most of the pertinent views in the pre-Mendelian era were based on this theory: -
Moist Vapour Theory: This theory was advocated by Pythagoras in which he believed that the male body produced some sort of a moist vapour during coitus, which helped in the development of the body parts of the embryo.
Reproductive Blood Theory: This theory was propounded by Aristotle. He was of the belief that both the males and the females produced reproductive blood. But the male reproductive blood was purer than the female reproductive blood. When the two reproductive drops of blood coagulated to form the embryo, it was due to the male’s pure blood that the characteristics of the male contributed more than the impure blood of the female.
Preformation Theory: This theory was given by Swammerdam. He believed that the organism already existed or was pre-formed in the eggs or sperm in a very minute form. This miniature was called the homunculus, which required fertilization to speed up its growth.
Theory of Epigenesis: The preformation theory was discarded by Wolff, a German scientist. He came up with another theory-the theory of epigenesis- in which he believed that the organism did not develop as a homunculus in sperms or eggs. But, the formation of the body parts of the embryo took place step by step. It was only after the fertilization that this formation began.
Theory of Acquired Characters: As per a famous French biologist Lamarck, a new character is passed on to the progeny of an individual once it has been acquired by the same individual. This theory was later rejected by a biologist who experimented on at least 20 generations of a rat.
Theory of Pangenesis: Charles Darwin, the father of evolution, theorized that miniature and invisible body parts exist in the blood called gemmules and are transmitted to sex organs and assembled in the gametes. After the fertilization process, these gemmules develop into natural body parts and organs.
The Germ Plasm Theory: This theory is propounded by a German biologist called August Weismann. He theorized that there were generally types of body tissues- germplasm and somatoplasm. Germplasm tissues were the reproductive tissues that helped in the production of gametes. On the other hand, somatoplasm was tissues other than the reproductive ones.
What was Mendel’s Experiment?
Gregor Mendel experimented on crossbred pea plants with single traits over various generations. In this breeding experiment, he crossed a pair of pea plants, with each having a different trait. Example, if one plant was short, the other was tall; if one had the shorter stem, the other pea plant had a longer stem; if one had round peas; the other plant had wrinkled peas, and if one plant bore white flowers the other pea plant bore purple-colored flowers.
On crossing, Mendel found out that the next generation called F1 consisted of whole individuals showing one trait only. In the next stage, the F1 generation was interbred, and Mendel found that the new F2 generation showed a different result. The traits were in the ratio of 3:1, wherein every three individuals showed similar traits of one parent.
This led Mendel to formulate that the genes in the human body could be combined in three possible forms, and these combinations were made up of different genetic factors or hereditary units- AA, aa, and Aa. The plants in the first stage were AA or aa, i.e., homozygous. The F1 generation Aa and the F2 generation was aa, AA, or Aa.
This led to the formulation of Mendel’s Laws of Inheritance which summarized and concluded his study –
Law of Segregation: This law states that for any trait, every pair of genes called alleles from both the parent splits and one gene from each parent transmits to the offspring. The passing of genes of any trait is a matter of chance.
Law of Independent Assortment: This law states that different pairings of genes or alleles of different traits pass on to the offspring without actually depending on each other. Therefore, the inheritance of one region does not affect the inheritance of another region.
Law of Dominance: While mating, each offspring acquires the trait of one parent only. If a dominant trait or factor is present in a parent, the offspring will exhibit the dominant trait. Recessive factors can only be acquired if both the factors in a gene are recessive in nature.
Mendelian inheritance, or Mendelism, is a collection of hereditary notions proposed in 1865 by Gregor Mendel, an Austrian-born botanist, teacher, and Augustinian monk. These ideas make up the system of particulate inheritance by units, or genes. The discovery of chromosomes as bearers of genetic units later proved Mendel's two primary laws, known as the law of Segregation and the law of independent assortment.
The first of Mendel's laws states that genes are passed down as separate and unique units from generation to generation. The two members (alleles) of a gene pair, one on each of paired chromosomes, split during the generation of sex cells by a parent organism. The progeny produced by these sex cells will reflect these proportions, since half of the sex cells will carry one type of gene and the other half will carry the other.
The Laws of Mendel
The following two principles, or laws, encapsulate Mendel's findings and conclusions.
Law of Segregation
According to the Law of Segregation, each parent's gene pairing (alleles) splits for any trait, and one gene goes from each parent to an offspring. It's entirely up to chance which gene in a pair is passed on.
Independent Assortment Law
According to the Law of Independent Assortment, various pairs of alleles are passed on to children independently of one another. As a result, the inheritance of genes in one part of a genome has no bearing on the inheritance of genes in another part of the genome.
Concept of Heredity
The sum of all biological mechanisms by which certain features are passed down from parents to their offspring is known as heredity. Heredity is a concept that encompasses two seemingly opposing aspects of organisms: a species' consistency from generation to generation and individual variation within a species. Consistency and variance are two sides of the same coin, as genetics illustrates.Genes, the functional units of heritable material found in all living cells, can explain both aspects of inheritance. Every individual in a species has a collection of genes that are unique to that species. This group of genes is responsible for the species' longevity. Variations in the form each gene takes can occur among individuals within a species, providing the genetic basis for the fact that no two people (save identical twins) have exactly the same genome.
Heredity's Fundamental Characteristics
For a long time, heredity was one of nature's most perplexing and mysterious phenomena. This was due to the fact that sex cells, which serve as a bridge for heredity to travel between generations, are normally imperceptible to the naked eye. The fundamentals of heredity could only be appreciated following the introduction of the microscope in the early 17th century and the subsequent discovery of sex cells. Prior to then, Aristotle (4th century BC), an ancient Greek philosopher and scientist, hypothesized that the relative contributions of the female and male parents were quite unequal; the female was considered to supply "matter," while the male was thought to supply "motion."
FAQs on Mendelian Genetics
1. What is Mendelian Genetics?
Mendelian Genetics is a branch of biology that studies the principles of heredity and the variation of inherited characteristics. It is based on the groundbreaking work of Gregor Mendel, who used pea plants to discover how traits are passed from parents to offspring. His findings laid the foundation for modern genetics by introducing concepts like dominant and recessive alleles, which explain how genetic information is transmitted across generations.
2. What are the three fundamental laws of Mendelian inheritance?
The three fundamental laws of inheritance proposed by Gregor Mendel are:
- Law of Dominance: This law states that in a hybrid organism, only one allele (the dominant one) will express itself in the phenotype, masking the effect of the other allele (the recessive one).
- Law of Segregation: This law explains that during gamete formation (meiosis), the two alleles for a trait separate from each other, so that each gamete receives only one allele.
- Law of Independent Assortment: This law states that the alleles for different traits are inherited independently of one another. The inheritance of one trait does not influence the inheritance of another.
3. What is the difference between a genotype and a phenotype?
The terms genotype and phenotype describe two different aspects of an organism. A genotype refers to the specific genetic makeup or the set of alleles an organism carries for a particular trait (e.g., TT, Tt, or tt for height). In contrast, a phenotype is the observable physical expression of that trait (e.g., tall or dwarf). The phenotype is determined by the genotype as well as environmental factors.
4. What are some common examples of Mendelian traits in humans?
While many human traits are complex (polygenic), some follow simple Mendelian inheritance patterns. Common examples include:
- Ear Lobe Attachment: Detached earlobes are a dominant trait, while attached earlobes are recessive.
- Widow's Peak: A V-shaped hairline on the forehead is a dominant trait.
- Blood Type: The ABO blood group system is an example of co-dominance and multiple alleles.
- Cystic Fibrosis: This genetic disorder is caused by a recessive allele.
5. Why did Gregor Mendel choose the pea plant (Pisum sativum) for his experiments?
Mendel's choice of the pea plant was crucial for his success. He selected it for several important reasons:
- It has a short life cycle, allowing him to study multiple generations quickly.
- It produces a large number of offspring, providing ample data for statistical analysis.
- It has many distinct, easily observable traits (e.g., seed shape, flower colour, plant height).
- It is naturally self-pollinating but can be easily cross-pollinated, giving Mendel control over mating.
6. How do incomplete dominance and co-dominance differ from Mendel’s concept of dominance?
Incomplete dominance and co-dominance are non-Mendelian patterns where the dominant-recessive relationship doesn't hold true. In Mendel's dominance, the dominant allele completely masks the recessive one. However, in incomplete dominance, the hybrid's phenotype is an intermediate blend of the two parental phenotypes (e.g., red and white flowers producing pink offspring). In co-dominance, both alleles are expressed fully and simultaneously in the phenotype (e.g., the AB blood type in humans, where both A and B antigens are present).
7. What is a test cross and what is its importance in genetics?
A test cross is a genetic cross between an organism showing a dominant phenotype (but with an unknown genotype, e.g., TT or Tt) and an organism that is homozygous recessive for the same trait (e.g., tt). The importance of a test cross is to determine the unknown genotype of the dominant parent. If any offspring show the recessive phenotype, the parent must have been heterozygous (Tt). If all offspring show the dominant phenotype, the parent was homozygous dominant (TT).
8. How does the concept of gene linkage challenge Mendel's Law of Independent Assortment?
Mendel's Law of Independent Assortment assumes that genes for different traits are inherited independently. However, gene linkage challenges this by showing that genes located close together on the same chromosome tend to be inherited together. They do not assort independently because they are physically linked. This discovery, part of post-Mendelian genetics, added a new layer of complexity, explaining why certain combinations of traits appear more frequently in offspring than predicted by Mendel's laws alone.
