Terminologies in Genetics
Every living organism produced by sexual method of reproduction resembles its parents and at the same time is different from the parents as well. There is a reason for these similarities and differences. Parents transmit their characteristics to their off springs because of which they look similar. Unlike this, during gamete formation in parents, characteristics between the homologous chromosomes are exchanged due to crossing over because of which off springs are not identical to their parents and with each other.
- Heredity: The phenomenon by which living organisms transmit parental characteristics from one generation to the next is called heredity. Heredity is also known as inheritance.
- Hereditary characteristics: The characteristics that are transferred from parents to their off springs are called hereditary characteristics. E.g. Color of the eye, shape of nose, type of hair, color of skin etc.
- Variation: The difference in characteristics between the off springs and their parents and with each other is called variation. There may be morphological, anatomical, physiological and behavioral differences between them. Variation leads to organic evolution in a long run.
- Genetics: The branch of biological science which deals with the study of heredity and variation is called Genetics. In other words, Genetics is the study of inheritance of characteristics or traits from parents to the off springs. It basically studies about the genes.
- Father of Genetics: Gregor Johann Mendel (1822-1884) is regarded as the Father of Genetics. He was a priest from a church but had the knowledge of science and math. He experimented on garden pea plant (Pisum sativum) for 8 continuous years. He was the first person to find and propose the principles involved in the inheritance of traits or characteristics from one generation to the next. Hence, he is regarded as the Father of Genetics.
- Gene: Gene is the fundamental unit of heredity which is located in DNA in the chromosomes. Actually, genes are transmitted from one generation to the next which code for different characteristics in living organisms. Genes can form their exact copies during the replication of DNA.
- Allele (Allelomorph): An allele is one or more alternative forms of a gene. It has a specific position in a particular chromosome and codes for a particular characteristic. E.g. Tt is a pair of alleles of a gene coding for the height of the pea plant, where ‘T’ is the dominant allele and ‘t’ is the recessive allele.
- Homozygous: The organism having two identical alleles ofr a gene coding for a particular characteristic is called homozygous. Such individuals are said to be pure breeding type as they produce the off springs of the same phenotype. E.g. TT, RR, tt, rr are homozygous forms.
- Heterozygous: The organism in which the two alleles of a gene are not identical is called heterozygous. Heterozygous individuals are also called hybrids. They consist of one dominant and other recessive allele. E.g. Tt, Rr etc. Generally, dominant allele is represented by capital letter and the recessive allele is represented by small letter.
- Phenotype: The external appearance of an organism is called its phenotype. It is expressed in words. E.g. tall, dwarf, red, white etc. Parental characteristics of an organism cannot be known or determined by the phenotype of that organism. i.e. if an off spring is tall phenotypically, we cannot say whether its parents are pure tall or hybrid tall.
- Genotype: The genetic makeup or the genetic composition of an organism is called its genotype. It is expressed in letters. E.g. TT (pure tall), Tt (hybrid tall), tt (pure dwarf) etc. Parental characteristics of an organism can be known from the genotype of that organism.
- Dominant characteristics: The characteristics which are prominent and expressed in the successive generations are called dominant characteristics. They are expressed in the heterozygous form as well. E.g. in a cross between a pure tall pea plant and a pure dwarf pea plant (TT x tt), tall is a dominant characteristic as it is always expressed in the first (F1) generation.
- Recessive characteristics: The characteristics which are suppressed and not expressed in the successive generations are called recessive characteristics. They are expressed only in homozygous form. E.g. in a cross between a pure tall pea plant and a pure dwarf pea plant (TT x tt), dwarf is a recessive characteristic as it is hidden in the first (F1) generation.
- Hybrid: The organisms produced by crossing two genetically different parents are called hybrids. E.g. Tt (hybrid tall), Rr (hybrid red). The process of crossing between two contrasting characteristics in parents is called hybridization. Hybrids are of various types; monohybrid, dihybrid, trihybrid, polyhybrid etc.
- Monohybrid cross: The cross involving only one pair of contrasting characteristics in parents refers to a monohybrid cross and the hybrid thus produced is a monohybrid. E.g. TT x tt (tall vs dwarf), RR x rr (red flower vs white flower), YY x yy (yellow seed vs green seed) etc.
- Dihybrid cross: The cross involving two pairs of contrasting characteristics in parents is called a dihybrid cross and the hybrid thus produced is a dihybrid. E.g. TTRR x ttrr (tall plant with red flower vs dwarf plant with white flower), TTYY x ttyy (tall plant with yellow seed vs dwarf plant with green seed) etc.
- First filial (F1) generation: The first generation of off springs obtained from the cross between parents is called F1
- Second filial (F2) generation: The generation of off springs obtained by interbreeding or crossing the organisms from F1 generation is called F2
*filial: of or related to a son or daughter (derived from a Latin word filius meaning son)
- Why Mendel chose garden pea plant for his experiments?
- The garden pea plant contains large number of contrasting characteristics with pairs, which are as follows
- Height of the plant ( tall and dwarf)
- Color of the flower ( red or purple and white)
- Position of the flower ( axillary and terminal)
- Color of seed ( yellow and green)
- Shape of seed ( round and wrinkled)
- Color of pod ( green and yellow)
- Shape of pod ( inflated and constricted)
- They have a very short life cycle because of which many generations can be studied within few years.
- They produce large number of off springs.
- Both self and cross pollination can easily take place in a pea plant. The flower is big enough to manually control the pollination occurring in pea plant.
- Pea plant is small and doesn’t need much space for its cultivation.
