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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Mendels Law of Inheritance Page 1

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Introduction to Genetics


  1. The branch of biology which deals with the study of heredity and variation is called genetics.
  2. Gregor Johann Mendel, known as the father of genetics, first made a systematic search into the principles of inheritance, making it an exact and experimental branch of biology.




Mendel's Experiments on Sweet Pea


  1. Mendel performed his experiments on the garden pea plant Pisum sativum. The features of this plant that helped him perform the experiments easily and even faster are:


It is an annual plant with a short life cycle and can be easily cultivated.


It is naturally self-pollinated. Thus, pure lines are available.


The flowers of the plant are bisexual and self-pollinating. However, they can be cross-pollinated and hybridised.


It shows seven pairs of contrasting characters that are controlled by seven genes localised on seven different chromosomes.


In the contrasting characters, one character is completely dominant over the other (recessive character).

  2. Mendel was the first to analyse the results of hybridisation experiments mathematically by applying the laws of probability.
  3. Seven observable contrasting characters in pea:
    1. Length of stem:Tall versus Dwarf
    2. Colour of flowers:Red versus White
    3. Colour of pods:Green versus Yellow
    4. Shape of seeds:Round versus Wrinkled
    5. Colour of cotyledons:Green versus Yellow
    6. Shape of pods:Inflated versus Constricted.
    7. Position of flowers:Axillary versus Terminal.




Terms Used in Genetics


  1. Factor: A unit which is responsible for the inheritance and expression of a particular character.
  2. Gene: Gene is the smallest segment of the DNA molecule that determines the inheritance and expression of a character. Mendel's factor is also known as a gene. A gene is also called cistron since it is a functional unit of the DNA.
  3. Alleles or Allelomorphs: A pair of genes occupying the same locus (position) and determining the same character are called alleles or allelomorphs. Alleles are thus the alternative forms of the same gene. For example, in pea, tallness is determined by gene 'T' and dwarfness is determined by gene 't'. Thus, 'T' and 't' are alleles and can be represented as T | | t.
  4. Homologous Chromosomes: A pair of chromosomes having the same sequence of genes is called a homologous chromosomes.
  5. Homozygous Or Pure Line Or Pure Breed: Homozygous or pure line or pure breed individual is the one possessing identical alleles for any trait. Pure breed is obtained from parents with identical set of characters or genes at least for three generations. They breed true. Example: Tall plant TT.
  6. Heterozygous Or Hybrid: An individual possessing dissimilar alleles for a trait is called heterozygous. An individual obtained from crossing two organisms with dissimilar alleles is called a hybrid.
  7. Genotype: Genetic constitution of an organism is called its genotype. For example, in tall pea plants, TT or Tt can be the genotypes.
  8. Phenotype: The external or morphological appearance of an individual for any trait is called the phenotype. For example, tall and dwarf are phenotypes of the pea plant.
  9. Dominant Allele/Gene/Character: Out of the two alternative forms (alleles) of a trait, the one which expresses itself in the F1 hybrid is called the dominant allele or character. For example, in the hybrid tall plant with genotype Tt, T is the dominant allele since it expresses itself.
  10. Recessive Allele/Gene/Character: Out of the two alternative forms (alleles) of a trait, the one which is suppressed in the F1 hybrid is called the recessive allele or character. For example, in the hybrid tall plant with genotype Tt, 't' is the recessive allele and dwarfness is the recessive character.
  11. Parental Generation And Filial Generation: In an experimental cross, the individuals crossed constitute the parental generation. The first generation of offspring is called the first filial generation or F1. The second generation is called the second filial generation and so on.
  12. Selfing: Selfing is a type of inbreeding where both parents possess the same genetic constitution. In bisexual plants like pea, selfing is achieved by self-pollination. self-pollination is the a type of inbreeding which takes place by fertilisation of the ovule by pollen of the same flower.
  13. Hybrid: A hybrid is an offspring of pure line of parents showing contrasting traits.
  14. Offspring:Individuals produced by the parents.
  15. Pure breed:A homozygous individual obtained from parents with identical characters which breeds true for at least three consecutive generations.
  16. Back cross:A cross between an F1 offspring with either of its parent is called a back cross.
  17. Test cross:A cross between the F1 offspring and its homozygous recessive parent is called a test cross.
  18. Monohybrid cross:A cross which involves a single pair of alleles is called a monohy-brid cross.
  19. Monohybrid ratio:A phenotypic ratio of 3:1 obtained in F2 generation of a monohybrid cross is called a monohybrid ratio.
  20. Dihybrid cross:A cross which involves two pairs of alleles is called a dihybrid cross.
  21. Dihybrid ratio:A phenotypic ratio of 9:3:3:1 obtained in the F2 generation of a dihybrid cross is called a dihybrid ratio.




Law of Dominance


  1. A character that is inherited independently of other characters is called a unit character.
  2. Mendel inferred the principle of dominance and recessiveness by performing seven different crosses with sweet pea using different unit characters.
  3. In a heterozygous condition, the phenotypic trait that is expressed is called dominant, while the trait that is suppressed is called recessive.
  4. Mendel's law of dominance or Mendel's first law of dominance states that in a cross between two organisms pure for their contrasting pair of characters, the character (allele) which is expressed in the F1 generation is the dominant character and the gene/allele determining the dominant character is called the dominant gene/allele.




Monohybrid Cross and Monohybrid Ratio


  1. A cross which involves a single pair of alleles is called a monohybrid cross.
  2. Monohybrid cross gives monohybrid ratios.
  3. Mendel, in one of his experiments, chose homozygous parents with contrasting characters for height. Thus, homozygous tall plants and homozygous dwarf plants were chosen as parents, of which one was used as male parent and the other as female parent.
  4. Thereafter, pollen grains from the male parent was transferred to the stigmatic surface of the female parent. This led to pollination and fertilisation and thus, seeds were set in the fruit.
  5. Mendel counted the seeds and sowed them in the soil. On germination, all the seeds grew into tall plants representing the first filial or F1 generation. They were all hybrids.
  6. Mendel then selfed the F1 hybrid plants. The seeds thus produced on germination produced F2 generation with three tall and one dwarf plant. ie, in 3 : 1 ratio. This ratio is called the monohybrid ratio.




Law of Segregation


  1. The law of segregation states that when a pair of contrasting characters occur together in a hybrid, they remain together without mixing with each other and segregate during the formation of gametes.
  2. Also, the gametes are pure in the sense that they carry factors for one trait only and never a mixture of both. Mendel called this principle 'the purity of gametes'.
  3. The phenomenon of reappearance of the recessive trait in F2 and its separation from the dominant trait in a definite ratio was termed by Mendel as the law of segregation.
  4. Thus, the purity of gametes and the law of segregation can be said to be the two sides of the same coin.




Dihybrid Ratio


  1. A dihybrid cross is a cross between individuals that show contrasting traits in two unit characters. Thus, a dihybrid cross involves two pairs of alleles. The phenotypic ratio of 9 : 3 : 3 : 1 obtained in the F2 generation of a dihybrid cross is called the dihybrid ratio.
  2. Mendel studied this ratio in pea. The two characters considered by him were the shape of the seed and its colour. The pea seeds are either yellow (dominant) or green (recessive), while in shape they are either round or smooth (dominant) or wrinkled (recessive).
  3. One parent belonged to a pure line for both yellow and smooth traits of the seeds. The other parent belonged to the pure line for both green and wrinkled traits of the seeds.

    On cross-pollinating these, the resultant F1 hybrid had all yellow and smooth seeds.

    When F1 was selfed, the resulting F2 generation showed the following constitution:
    315 yellow and round seeds,
    108 green and round seeds,
    101 yellow and wrinkled seeds, and
    32 green and wrinkled seeds.
    Hence, the ratio stood at 9 : 3 : 3 : 1




Law of Independent Assortment


  1. Mendel called this law as 'the law of combination of different characters'.
  2. When two unit characters are inherited together, each one will behave independently of the other while segregating in the F2 generation and will combine with the other in a random manner.
  3. For example, in the parents, yellow had coexisted with round and green with wrinkled. In F2, the factor for yellow combines in equal probability with factors for round and wrinkled. Also, the factors for green combines in equal probability with factors for round and wrinkled.




Test Cross and Back Cross


  1. A cross between F1 and its homozygous recessive parent is called a test cross. The test cross is actually a back cross between F1 hybrid and its homozygous recessive parent. The F2 generation of test cross consists of 50% heterozygous tall plants and 50% homozygous dwarf plants.
  2. Significance of test cross:


Helps in determining the genetic constitution of an organism.


Test cross is useful in determining whether the dominant character of F1 offspring is due to homozygous or heterozygous condition.


Test cross helps a breeder in discarding the unwanted progeny from the breeding programme.

  1. A cross between an F1 offspring with either of its parent is called a back cross. When F1 offspring is crossed with its dominant parent, all the F2 offspring show dominant character.
  2. Significance of back cross:


Back cross is of great significance as it is one of the rapid methods of improving the variety of a crop.


With the help of a back cross, we can easily obtain a desirable character in pure homozygous condition.


Plant breeders introduce desirable characters in crop plants with the help of a back cross.


Back cross is frequently used in hybridisation experiments.

  1. Test cross can also be called a back cross. However, a back cross need not always be a test cross.




The Concept of Factor and its Present Position


  1. Mendel proposed that traits are controlled by discrete particles which he called 'factors' or 'elements'.
  2. In modern terminology, the factors are called genes. The term genes was proposed by Johannsen in 1909. Other modern terms used are allelomorphs or alleles.



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