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Heredity and Variation

Heredity and Variation Synopsis

Synopsis

 

Heredity

  • Living organisms have certain recognisable features such as height, complexion, colour of hair and eyes, shape of nose and chin etc. These peculiar heritable features of living organisms are called characters. 
  • Genetics is the study of transmission of characters from parents to offspring and the laws relating to such transmissions.
  • The alternative forms of a character are called traits. The inheritable characteristics or traits may be morphological, anatomical, physiological or reproductive. 
  • The transmission or passing of genetically based characters or traits from the parents to their offspring is called heredity. The rules of heredity determine the process by which traits and characters are reliably inherited.
  • Hereditary information is present in gamete cells. Gamete cells thus become the link between two generations and transmit characters from parents to their offspring.
  • Different reproductive processes give rise to new individuals which are similar but differ from each other with respect to certain features.

 

Variation

  • The occurrence of small differences or changes among the individuals of a species is called variation.
  • Variation is any change in the traits, e.g. colour of skin or eyes, which the offspring exhibits when compared to parents. For example, human height is a trait which shows variation.

 
1. Somatic or Somatogenic Variations
  • They are variations which affect the somatic or body cells of the organisms. They are also called modifications or acquired characters because they are got by an individual during its life time. 
  • They are caused by three factors: 
  • Environment: The environmental factors are medium, light, temperature, nutrition, wind, water supply, etc. The environmental factors bring about changes in phe¬notype of the individual. 
  • Use and disuse of organs: The organ which is put to continuous use develops more while the organ less used develops little.
  • Conscious efforts: Modifications due to conscious efforts are observed only in those animals which have intelligence. Receiving education, training of some pets, slim bodies, boring of pinna, long neck, small feet, mutilations in pets, bonsai, etc. are some of the examples of conscious efforts.
 
2. Germinal or Blastogenic Variations
  • They are produced in the germ cells of an organism and are inheritable. They may be already present in ancestors or may be formed suddenly.
  • Accordingly, the germinal variations are of two types: 
  • Continuous variations: Con¬tinuous variations are typical of quantitative characteristics. They show differences from the average which are connected with it through small intermediate forms. They are produced by chance separation or segregation of chromosomes, crossing over or chance combination of chromo¬somes during fertilization.
  • Discontinuous variations: They are sudden unpredictable inheritable departures from the normal without any interme¬diate stage. Discontinuous variations are caused by chromosomal aberrations like deletion, duplication, inversion and translocation or change in chromosome number through aneuploidy and polyploidy, or change in gene structure and expression due to addition, deletion or change in nucleotides.
 
Significance of Variation
 
 
 
 
Pre-Mendelian Concept of Heredity
  • A number of viewpoints were put forward prior to Mendel to explain the transmission of characters from parents to offspring. 
  • They are often called theories of blending inheritance as they believed that parents blended or got mixed during their transmission to the offspring.
  1. Moist Vapour Theory: This theory was advocated by Pythagoras. 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. 
  2. 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.
  3. 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 fertilisation to speed up its growth.
  4. Theory of Epigenesis: The preformation theory was discarded by Wolff, a German scientist who came up with the theory of epigenesist. He believed that the organism did not develop as a homunculus in sperms or eggs. The formation of the body parts of the embryo took place step by step. It was only after the fertilization that this formation began.
  5. Theory of Acquired Characters: According to 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.
  6. 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.
  7. The Germ Plasm Theory: This theory was propounded by a German biologist 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 were tissues other than the reproductive ones.
 
Mendels’ Experiments on Inheritance
  • Gregor Mendel systematically studied the patterns of inheritance which involved the transfer of characters from one generation to the next. He is known as the Father of Genetics.
  • Although Mendel described his results in 1866, his work was recognized only in 1900, when Mendel’s laws were rediscovered simultaneously by Hugo de varies - a Dutch biologist, Carl Correns - a German botanist and Erich von Tschermak - an Austrian botanist.
  • Mendel conducted experiments on pea plants (Pisum sativum) and studied the inheritance of certain traits.



 

Some General Terms Used by Mendel

 

Monohybrid Inheritance and the Law of Segregation

  • A cross which involves only a single pair of contrasting characters is called a monohybrid cross.
  • Consider pea plants with a pair of contrasting characters—tallness and dwarfness—with respect to the height of the stem. Because pea plants are self-fertilising, Mendel bred them for several generations till he obtained pure varieties. The parental generation P1 has the genotype TT for a pure tall parent plant and tt for a pure dwarf plant. The two parents are homozygous with one type of gamete each, i.e. T from tall parent and t from dwarf parent. These two plants are crossed with one another.
  • Mendel found that all plants were tall. He called them first filial or F1 generation seeds. The F1 generation has genetic constitution Tt. It is genotypically a hybrid and a heterozygous plant with two different alleles. Phenotypically, the plant is tall because the allele or the gene T for tallness masks the effect of its corresponding recessive gene t.

 
  • F1 plants are self-pollinated to obtain the F2 generation, F1 × F1 = F2.. Now, each of these parents is heterozygous tall with genotype Tt. So, we have Tt × Tt. The gametes produced from each of the parents are of two types, T and t. 
  • The second filial generation F2 has a genotypic ratio of 1 TT:2 Tt:1 tt. In this case, because the allele T for tallness is dominant, the pea plants with genotype Tt will be tall. The phenotypic ratio is 3 tall:1 dwarf. Genotypically, it shows 3 types of plants: 1 TT, which is homozygous tall, 2 Tt which are heterozygous tall and 1 tt which is homozygous dwarf. Thus, the genotypic ratio is 1 TT:2 Tt:1 tt.
  • The ratio 3:1 is known as the monohybrid ratio. The results of the monohybrid cross enabled Mendel to formulate his first law of inheritance which is called the law of segregation.
 
Dihybrid Inheritance and the Law of Independent Assortment
  • A cross which involves plants with two pairs of contrasting characters is called a dihybrid cross.
  • Consider a cross of pea plants with round and yellow seeds and plants with wrinkled and green seeds. In this cross, the two pairs of characters involved are seed shape and seed colour. 
  • The capital letter R is used to denote round-shaped seeds and capital Y to denote yellow-coloured seeds. The genotype of the parent plant is RRYY. The other parent has wrinkled seeds which are green in colour. The letter small r denotes wrinkled seeds and small y denotes green-coloured seeds. The genotype of the parent is rryy. The two parent plants are then crossed, RRYY × rryy. 
  • When the gametes of the P1 generation are formed, each pair of alleles segregates independently of the other. Hence, during gamete formation, RRYY plants produce gametes RY. The gametes RR or YY are not produced by them. Similarly, the other parent rryy produces gametes ry. The gametes rr or yy are not produced by them.
  • In each of the gametes, a pair of alleles is represented by only one of its members. In the P1 generation, the parent RRYY produces two gametes, RY and RY, and the parent rryy produces two gametes, ry and ry. 
  • Mendel observed that the F1 plants were with round yellow seeds phenotypically. The genotype of these F1 hybrids was RrYy. The genotype shows R which is dominant over r with respect to seed shape and results in round seeds. The genotype also contains Y which is dominant over y with respect to seed colour and results in yellow-coloured seeds. All F1 plants are called dihybrids as they have two sets of contrasting characters and have the genotype RrYy. 
  • The F1 plants produced four types of gametes—ry, RY, rY and Ry—during gamete formation. On the other hand, the parental plants P1 produced only two types of gametes RY and ry. The gametes produced by F1 plants which are similar to those produced by P1 plants are called parental combinations. The two other types of gametes rY and Ry produced by F1 plants during gamete formation are called recombinations. 
  • When F1 plants are self-pollinated, they produce a new generation of plants known as the F2 or second filial generation. The four types of male gametes and four types of female gametes produce in all 16 different mating combinations. The 16 different types of F2 individuals can be easily represented by a Punnett square.
  • The F2 generation obtained by self-pollination of F1 plants exhibits 16 plants with different genotypes. Of a total of 16 plants, 9 plants are with round and yellow seeds RRYY, 3 are with wrinkled and yellow seeds rrYY, 3 are with round and green seeds RRyy and 1 is with wrinkled and green seed rryy. Thus, the phenotypic ratio of F2 plants is 9:3:3:1. 
  • Genotypically, there are 9 different types of plant combinations—RRYY, RrYy, Rryy, rryy, RRYy, rrYy, Rryy, Rryy and rrYY. The genotypic ratio is 1:4:1:1:1:2:2:2:2.
  • The genotype RRYY is a pure dominant with homozygous round and homozygous yellow seeds. The genotype rryy is a pure recessive with homozygous wrinkled and homozygous green seeds. 
  • The 9:3:3:1 ratio of each phenotype of the seeds in the F2 generation is called the dihybrid ratio. The results of the dihybrid ratio enabled Mendel to formulate his second law of inheritance which is called the law of independent assortment.