Tamil Nadu Board of Secondary EducationHSC Science Class 12

Terminology Related to Mendelism

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Notes

Terminology related to Mendelism:

  1. Factor: Particle present in the organism which is responsible for the inheritance and expression of the character. It is a unit of heredity.

  2. Gene: Specific segment of DNA, which determines a particular character of an organism and is responsible for the inheritance and expression of that character.
    or
    A specific segment of DNA that codes for a polypeptide or protein is called a gene.

  3. Alleles or Allelomorphs: Two or more alternative forms of a gene (factor) present at the same loci of homologous chromosomes and controlling the same character are called as alleles or allelomorphs.

  4. Homozygous: An individual having identical alleles for a particular character is homozygous for that character. It is pure or true breeding. e.g. TT, tt. Homozygous produces only one type of gamete.

  5. Heterozygous: An individual having dissimilar alleles for a particular character is heterozygous for that character. It is a hybrid. e.g. Tt. Heterozygous do not breed true for that trait and produce two types of gametes.

  6. Genotype: It is the genetic constitution of an organism with respect to a single character or a set of characters. e.g. TT, Tt, tt.

  7. Phenotype: The external appearance of an organism. e.g. tallness, dwarfness.

  8. Monohybrid cross: A cross between two pure parents differing in single pair of contrasting character is called a monohybrid cross. The phenotypic ratio for this cross is 3: 1.

  9. Dihybrid cross: A cross between two pure parents differing in two pairs of contrasting characters is called a dihybrid cross. The phenotypic ratio for such a cross is 9: 3 : 3: 1.

  10. F1 generation: The hybrid individuals obtained by a cross between two pure parents with contrasting characters are called F1 generation or first filial generation.

  11. F2 generation: The generation of offsprings obtained by selfing of F1 individuals is called F2 generation or second filial generation.

  12. Dominant: The character expressed in F1 generation is called as the dominant character.
    or
    It is an allele that expresses its trait even in the presence of an alternative allele.

  13. Recessive: The character which is not expressed in F1 generation is called as the recessive character.
    or
    It is an allele that is not expressed in the presence of an alternative allele.

  14. Progeny: All offsprings produced by the parents are called progeny.

  15. Trait: An inherited character and its detectable variant is called a trait. e.g. tallness or dwarfness, yellow or green.

  16. Homologous chromosomes or Homologues: Morphologically and genetically similar chromosomes present in a diploid cell are called homologues or homologous chromosomes. In each pair of homologous chromosome, one chromosome is maternal and the other is paternal.

  17. Pure line: An individual or a group of individuals that is homozygous or true breeding for one or more traits.

  18. Genome: Genome is the total number of genes present on the haploid set of chromosomes.

  19. Hybrid: Heterozygous individual produced by parents having contrasting characters. e.g. Tt.
  • When the gene has two alleles the dominant allele is symbolized with a capital letter and the recessive with a small letter.
  1. When both alleles are recessive the individual is called homozygous recessive (tt) dwarf pea plants.
  2. An individual with two dominant alleles is called homozygous dominant (TT) tall pea plants.
  3. One dominant allele and one recessive allele (Tt) denotes non- true breeding tall pea plants heterozygous tall.
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