The transmission of characters (or traits) from the parents to their offsprings is called heredity.
The differences in the characteristics (or traits) among the individuals of a species is called variation.
For example, human height is a trait which shows variation. Another example of variation in human beings involves ears. The lowest part of the ear is called earlobe. In most of the people, the earlobe is ‘hanging’ and it is called free earlobe. In some people, earlobe is closely attached to the side of the head and it is called attached earlobe.
Accumulation of variations:
- The significance of a variation shows up only if it continues to be inherited by the offspring for several generations.
- The great advantage of variation to a species is that it increases the chance of its survival in a changing environment.
For example, the accumulation of ‘heat resistant’ variation (or trait) in some bacteria will ensure its survival even when the temperature in its environment rises too much due to a heat wave or some other reasons. On the other hand, the bacteria which did not have this variation to withstand heat would not survive under these circumstances and die.
Some important terms:
Chromosome is a thread-like structure in the nucleus of a cell formed of DNA which carries the gene.
- A gene is a unit of DNA on a chromosome which governs the synthesis of one protein that controls a specific characteristic (or trait) of an organism.
- Genes are actually units of heredity which transfer characteristics from parents to their offsprings during reproduction.
- Genotype is the description of genes present in an organism. Genotype is always a pair of letters such as TT, Tt or tt (where T and t are the different forms of the same gene).
- The characteristic (or trait) which is visible in an organism is called its phenotype.
Rules for the inheritance of trait: Mendel’s contribution:
- Gregor Mandel was the first scientist to make a systematic study of patterns of inheritance which involved the transfer of characteristics from parents to progeny.
- He did this by using different varitieties of pea plant (Pisum sativum) which he grew in his garden.
- Mendel used a number of contrasting visible characters of garden peas- shape of seeds ( round and wrinkled), colour of seeds ( yellow and green), height of pea plants ( tall or dwarf), colour of flowers ( white or violet) and so on.
Mendel chose pea plants for studying inheritance because –
Pea plants had a number of clear cut differences which were easy to identify.
Pea plants were self-pollinating which enabled them to produce next generation early.
Many generations of pea plants can be produced in a comparatively short time span and their study is much simpler than that of animals.
A new form of plant resulting from a cross (or breeding) of different varieties of a plant is known as hybrid.
When we breed two pea plants having one contrasting characteristic each (of one trait each) to obtain new plants, then it is called monohybrid cross.
When we breed two pea plants having two contrasting characteristic each (of two traits each) to obtain new plants, then it is called dihybrid cross.
Inheriyance is the transmission of genetically controlled characteristics (or traits) from one generation to the next.
Monohybrid inheritance and the law of segregation:
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- According to Mendel’s first law of inheritance: The characteristics (or traits) of an organism are determined by internal ‘factors’ which occur in pairs. Only one of a pair of such factors can be present in a single gamete.
- Mendel first crossed pure-bred tall pea plants with pure-bred dwarf pea plants and found that only tall pea plants were produced in the first generation or F1 generation.
- Mendel concluded that the F1 generation showed the traits of only one of the parent plants: tallness, which is dominant trait and the other trait which is suppressed by the dominant trait in f1 generation is called recessive trait that is dwarfness.
- Mendel then crossed the tall pea plants of the F1 generation and found that tall plants and dwarf plants were obtained in the second generation (F2 generation) in the ratio of 3:1 (three fourth plants were tall and one-fourth plants were dwarf).
- Mendel concluded that the dwarf trait of parent pea plant which has seemingly disappeared in the first generation progeny, reappeared in the second generation. Mendel also noted that all the pea plants produced from the hybrid, tall plants of F1 generation were either tall or dwarf, there were no plants with intermediate hight ( or medium height) in-betwwen the tall and dwarf plants. so mendel’s experiments showed that the traits are inherited independently.
- The ratio 3:1 is known as the monohybrid ratio.