GENETICS AND HEREDITY

GENETICS

Genetics is the science which deals with the mechanisms responsible for similarities and differences among closely related species. The term ‘genetic’ was coined by W.Batesmanin 1905. It is derived from the Greek word ‘genesis’ meaning grow into or to become. So, genetic is the study of heredity and hereditary variations it is the study of transmission of body features; i.e., similarities and difference, from parents to offspring’s and the laws related to this transmission.

VARIATION

Any difference between individual organisms or groups of organisms of any species, caused either by genetic difference or by the effect of environmental factors, is called variation. Variation can be shown in physical appearance, metabolism, behavior, learning and mental ability, and other obvious characters.

TYPES OF VARIATION

There are two types of variation.

1. Genotypic variations: Genotypic variations are caused by differences in the number or structure of chromosomes or by difference in the genes carried by the chromosome. Height, eye colour, body forms are some of the genotypic variations. A variation cannot be identified as genotypic by simply observing the organism unless breeding experiments are performed under controlled environmental conditions.
2. Somatic variations: Somatic variations may result from several factors, such as climate, food supply, and actions of other organisms. These variations are not due to differences in genes or chromosomes, and in general are not transmitted to future generations. Hence they are not significant in the process of evolution.

Heredity: Is defined as the transmission of characteristics from parents to offspring’s. It can also be defined as resemblances among individuals related to descent. It also means the inheritance of like qualities or characters from one generation to the next and to successive generations.

MENDELS LAWS OF HEREDITY

Gregor Johann Mendel (1822-1844) is known as the father of genetics as he was the first to demonstrate the mechanism of transmission of characters from one generation to the other. He carried out his work on garden pea, pisum sativum. He selected 7 pairs of contrasting traits of garden peas.

Mendel’s gave three laws or principles of inheritance.

1. Law of dominance: States that in heterozygous condition among two alleles of a character the alleles which expresses itself is dominant and the one which can’t express is recessive.
2. Law of segregation: States that although the alleles of a character remain together for a long time but they do not mix with each other and separate at the time of gametogenesis so that each gamete receives only one alleles of a character either dominant or recessive.
3. Law of Independent assortment: States that alleles of a character can undergo any sort of combination to give rise to a phenotype differing from both the parents.

MENDEL’S EXPERIMENT

In 1857 Mendel began a series of experiments on the pea plant (Pisum Sativum) to study the pattern of inheritance of various characters. He chose pea plant for three reasons. First, pea plants are self-pollinating. Second, they are easy to cultivate. Third, they have a sharply defined characters. Mendels chose to study seven different characters in this plant. Each character such as height, seed shape, seed colour, etc., had two sharply defined and contrasting traits (e.g, dwarfstem and tall stem, wrinkled seed and smooth seed, yellow seed and green seed). He crossed a variety of pea plants, carrying a particular trait (e.g. tallness) of a character (such as height) with another variety having a contrasting trait (e.g, dwarfness) of the same character. These two types were called parental types (p or p1). The generation that was produced out of these two was called first filial generation (F1). When these were self –pollinated, the second generation that was produced was called second filial generation (F2).

The result of Mendel’s experiment followed a pattern, which is as follows:

Whenever two varieties of a character were crossed, the F1 plants showed only one of the traits, the other trait never appeared. It did not matter whether the trait came from the pollen or ova.

The traits that did not appear in F1 reappeared in the F2 but in the ratio 1⁄4 of the total number of the plants.

Mendel called the substance or agent responsible for each trait a ‘factor’. According to Mendel, the trait that did not appear in the F1 was not destroyed or altered. He explained the phenomenon by which one trait appeared in the F1 and the other did not. He said that the trait which appeared in the F1 was dominant and the one which did not appear was recessive.

Mendel called that each genetic character was represented or controlled by a pair of unit factors, or elements. (Later on, the unit factors became known as alleles or allelomorphs. When the term ‘gene’ was coined and defined, the allele became synonymous with the gene.)

One of the alleles came from one parent and the other from the other parent. The first generation plants of his experiments were all tall plants. The allele representing dwarfness could not be expressed in the first generation because it was dominated by the allele representing tallness. In other words, the allele for tallness was dominant and the allele for dwarfness was recessive.

NOTATIONS USED IN BREEDING EXPERIMENTS

The dominant trait is written with a capital letter. For example, tallness is represented as T and darkness is represented with the corresponding small letters t. If tallness is due to both the dominant alleles, it is written as TT. If tallness is due to only one dominant trait then it is written as Tt. If both the alleles are recessive, making the organism draft, then it is written as tt. A homozygous condition is one in which both alleles are of the same nature, for example, Tt or tt. Heterozygous condition (where the two alleles are of different nature) is written as Tt.

In a dihybrid cross, two characters are taken into account. Hence the notation for the homozygous dominant would be AABB, and for the homozygous recessive it would be aabb. When the gamete is formed the traits are separated, as the chromosome number during meiosis is halved.

LAW OF SEGREGATION

When the tall plants in F1 were crossed among themselves, the F2 generation had 75% tall plants and 25% dwarf plants (ratio 3:1). This led Mendel to conclude that the alleles representing darkness were intact and were neither lost nor contaminated. Mendel’s study with one character (monohybrid cross) led to the formulation of the law or principle of segregation. This means that although the alleles of a character remain together, they are separated in subsequent generations.

LAW OF INDEPENDENT ASSORTMENT

After studying the inheritance of one pair of contrasting characters, Mendel went on to take two characters (Dihybrid cross) into account. He crossed a plant having smooth and yellow seeds with a plant having wrinkled and green seeds. All F1 plants had smooth and yellow seeds. When a certain number of F1 plants were sibbed, they showed 9⁄16 of the offspring had smooth and yellow seeds, 3⁄16 had smooth and green seeds, 3⁄16 had wrinkled and yellow seeds and 1⁄16 had wrinkled and green seeds. This ratio was 9:3:3:1.

F2: Mendel’s study with two characters led to the formulation of the law of independent assortment. This means that alleles of one character can undergo any sort of combination to give rise to a phenotype differing from both the parents.

EXCEPTIONS TO MENDEL’S LAWS

Mendel’s laws of heredity are not universally applicable to all the organisms.

• Incomplete dominance: In some organisms, such as four o’clock plant and snapdragon, a new phenotype appears in F1 generation which is intermediate between the two parents. In other words, the offspring does not resemble either of the two parents.
• Co-dominance: In this case, two contrasting characters are inherited simultaneously, e.g., blood groups in human beings.
• Epistasis: This is the phenomenon of interaction between the genes of two or more different characters. This means that an allele of a character inhibits the expression of an allele of another character.
• Pleiotropy: When a single gene affects more than one character, it is called pleiotropy. For example, phenylketonuria is caused by mutation in a single gene but causes mental retardation and inability to metabolise phenylalanine.

Back cross and test cross: The cross between the F1 hybrid and either of the parents is called a backcross. This cross helps in stabilizing the purity of a dominant trait. Test cross helps in determining whether the individual is homozygous dominant or heterozygous.

CHROMOSOMES

Chromosomes are long, thread-like structure present in the nucleus, which contains the genetic information of an individual. They are made up of DNA and proteins. Each species has a fixed number of chromosomes. They are present in pairs. The chromosomes can be seen only when the cell is dividing.

FUNCTIONS OF CHROMOSOMES

1. Chromosomes contain the genes, which are responsible for the inheritance of characters.
2. Chromosomes control the synthesis of structural proteins, enzymes and RNAs.
3. Chromosomes take part in cell division and hence they are responsible for growth, repair and reproduction of the organism.
4. They guide the development of an organism from zygote to adult.
5. They control the differentiation of cells.
6. They are responsible for the reproduction of the organism.
7. They are responsible for determining the sex of the individuals. This is done by sex chromosomes.
8. They bring about variations by crossing over.
9. They undergo mutations and hence they are responsible for evolution.

GENES

Genes are the units of inheritance and are located on chromosomes. They are passed on from parents to offspring’s. They determine the characters of the organism. Genes are located at a particular location on the chromosomes. Genes are made up of DNA. The term gene was introduced by Johannson.

FUNCTIONS OF GENES

1. They are the hereditary units.
2. They code for the synthesis of different types of RNAs, which are essential for the synthesis of proteins.
3. They regulate the process of transcription, leading to the synthesis of proteins.
4. They code for the synthesis of both structural and functional proteins.
5. They take part in the reproduction of organisms.
6. They repair themselves if they get damaged.
7. They cause differentiation of cells during the development of an organism.
8. They undergo mutation and recombination, which leads to evolution.

DNA (DEOXYRIBOSE NUCLEIC ACID)

DNA is the hereditary material in all organisms. It is mainly located in the nucleus. It is also the main constituent of chromosomes. DNA controls all the activities of the cells. The amount of DNA remains constant in all cells of an individual.

FUNCTIONS OF DNA

1. DNA is the hereditary material in all organisms. It carries all the hereditary information from parents to offspring’s.
2. DNA undergoes replication during cell division. Hence, it is responsible for transmitting the hereditary information from parents to offspring’s.
3. DNA controls all the activities of the cell. It controls the cell metabolism through RNA and protein synthesis.
4. DNA produces different types of RNAs by the process of transcription.

SEX CHROMOSOMES

Sex chromosomes are those chromosomes which determine the sex of the individuals. They are of two types X and Y.

SEX DETERMINATION IN HUMANS

The males have one X and one Y chromosome while the females have two X chromosomes. A child gets one X chromosome from his mother and one X or one Y chromosome from his father. If a child gets X chromosome from his father, the child will be a female and if he gets Y chromosome from his father, the child will be a male.

CHARACTERISTICS

Characteristics are the specific traits or behavior of an organism. Characteristics can be inherited from parents or can be acquired from the environment. All organisms, despite their vast diversity, show certain characteristics and processes of life which are similar. The more the number of characteristics shared by two species, the more closely they are related to each other. It also means that they have a common ancestor.

---

Credits: Mr. Riyaz Kathjoo (Dean Academics “GVEI”) Editor: Hilala jan