Heredity and Variations
The transmission of characters from parents to the offspring is termed as heredity. The gametes act as vehicles of hereditary transmission. However, not all offsprings are similar to their parents. Certain characters appear to belong to neither of the parents. These dissimilarities are termed as “variations”. The study of heredity and variations is termed as “Genetics”.
Gregor Johann Mendel is the “Father of Genetics”. Children often resemble their parents. They acquire some of the characters from mother and some others from their father. This is due to transmission of “factors” from the parents to children. These factors now termed as genes occur on the chromosomes.
Mendel performed experiments on heredity by using garden pea (i.e. Pisum sativum).
Selection of pea plant : Mendel had following advantages in his choice of Pisum sativum.
1.There were several varieties of pea which showed distinct alternative traits.
2.The plants where easy to cultivate. The short life span was also useful in getting quick results.
3.The flowers self pollinate and the petals completely enclose the reproductive structures . This was helpful to maintain varieties with same characters generation after generation.
4.The varieties could be artificially cross pollinated.
Mendel’s Hybridization Technique
Hybridization is the crossing of two different individuals to produce an offspring or hybrid. Mendel cultivated all the varieties of plant until he was sure that each variety was true breeding or homozygous for a character. Homozygosity was tested by growing seeds, for example, plants with white flower.
Homozygosity or true breeding character was confirmed when the seeds produced only plants with white flowers. These true breeding plants were allowed to self pollinate to maintain their homozygosity. Such true breeding plants were used as parents (P).
The next stage was hybridization by cross pollination between two parents with alternative forms of a character. For example, Mendel cross pollinated true breeding red flower variety with true breeding white flower variety.
He removed anthers from many plants before cross pollination. Emasculated varieties of plants were termed as female parents. The stigmas of the female plants were dusted with pollen grains from the anthers of another plant, now termed as male parent.
The artificially cross pollinated flowers of female parent were then covered with small bags (i.e., bagging) to prevent pollens from other plants reaching their stigmas.
Such a cross where two alternative forms of a single character are used was termed as ‘monohybrid cross’. The hybrid or offspring produced as a result of this cross forms first filial or F1 generation.
The last stage of his experiment consisted of producing second filial generation or F2 generation. This was done by enclosing flowers of F1 hybrid in bags so that self pollination occurs naturally. The seeds from such F1 plants were grown into F2 hybrids.
Based on his observation Mendel proposed the following principles or laws of inheritance.
- Law of dominance
- Law of segregation
- The law of independent assortment
Inheritance of one gene
In monohybrid cross, there is inheritance of only a pair of genes .
Pair of unit factors
A pair of unit factors controls each character. This pair is termed as alleles ( it is also termed as allelomorphic pair). It is a pair of contrasting or alternative characters. For example, for a character such as height, there are two contrasting or alternative characters such as tall and dwarf.
Dominant- Recessive Relationship
Crossing between true homozygous or true breeding varieties ( i.e., parents) results in F1 generation. For, example tall (TT) variety and dwarf (tt) variety. The F1 hybrid plants were all tall. This characteristic was termed dominant. According to Mendel the dwarf characteristic though present in F1 hybrid, fails to express itself in the presence of dominant allele. Mendel termed the unexpressive allele as recessive.
An allele which expresses itself externally when present in homozygous or heterozygous conditions.
An allele which expresses externally when present in homozygous condition but remains suppressed in heterozygous condition.
In F1 or heterozygous organism, both unlike alleles, i.e.,tall (T) and dwarf (t) occur together, but only the tall character is expresses itself . Hence, tall is a dominant allele & dwarf is the recessive alleles in the example.
Law of Dominance
- The characters of an organism are under the control of Mendelian factors, now termed as genes.
- These ‘factors’ or genes always occur in the pairs (alleles).
- In a pair containing two dissimilar factors (i.e., heterozygous condition), only one factor dominates and expresses itself while the other factor though present remains unexpressed termed as recessive.
Law of Segregation
Mendel’s law of segregation states that an individual organism has two alleles for every gene, one inherited from its mother and one from its father. So, when this organism forms its own gametes, each gamete will receive only one of these alleles, and this process is random.
The 2 alleles for each character segregate (i.e., separate) during gamete formation.
Inheritance of Two Genes
Mendel, after working with one pair of allele, worked with two different characters using two pairs of alleles (also known as dihybrid cross).
- Cotyledon colour : yellow (dominant) and green (recessive)
- Seed shape : round (dominant) and wrinkled (recessive)
Law Of Independent Assortment
In dihybrid cross (a cross involving two pairs of alleles), Mendel used cotyledon colour and seed shape of pea as two pairs of alleles.
Crossing between homozygous or true breeding dominant (yellow cotyledons and round seeds) and true breeding recessive (green cotyledons and wrinkled seeds) results in F1 generation.
The F1 hybrid shows dominant phenotypes, i.e, yellow cotyledon and round seeds.
The selfing of F1 plants results in F2 generation. F2 seeds shows the following combinations:
Yellow & round : Parental combination : 9
Yellow & wrinkled : Recombination : 3
Green & round : Recombination : 3
Green & wrinkled : Parental combination : 1
Phenotypic Ratio 9:3:3:1 (dihybrid ratio)
The results thus show that individual alleles still segregate in the same 3:1 ratio obtained from monohybrid cross. This proves that though segregation of 2 pairs of alleles is considered together, each pair segregates independent of the other. This results in new combinations.
Methods of analysis
Punnett Square (Reginald C. Punnett 1875-1967)
It is a graphical representation to calculate the probability of all possible phenotypes of offspring in a genetic cross. The top row and left column represent the possible gametes. All the male gametes occur in the vertical squares and all the possible female gametes occur in the horizontal squares.
It involves cross between F1 hybrid with any one of the two parents i.e., homozygous dominant or homozygous recessive. Thus there would be 2 following possibilities:
1. Cross between F1 hybrid (Aa) and homozygous dominant (AA)
2. Cross between F1 hybrid (Aa) and homozygous recessive (aa).
So, both the crosses are termed as back cross.
Out of the two types of back crosses, a cross between F1 hybrid (Aa) and its homozygous recessive (aa) parent is termed as test cross. This cross is termed as test cross because it helps to find out whether the given dominant phenotype is homozygous or heterozygous.
The test cross produces offsprings, of which 50% are heterozygous and the remaining 50% are homozygous recessive. Thus, a monohybrid test cross between F1 tall plant (Tt) and its homozygous recessive parent (tt) will produce 50% heterozygous tall (Tt) and 50% homozygous recessive dwarf (tt).