Principal of Inheritance and Variation Class 12 Notes Biology Chapter 5

Chapter at a Glance

  • Heredity is a process of transmission of traits from parents to their offspring either via asexual reproduction or sexual reproduction. These characteristics or traits are located on the chromosomes in the form of genes.
  • Term genetics was first used by W. Bateson (1905).
  • The two Mendelian factors which occur on the same locus in the two homologous chromosomes of an individual and control the expression of a character are called alleles or allelomorphs.
  • The heterozygous organism produced after crossing to genetically different individuals is called hybrid.
  • Back cross is a cross which is performed between hybrid and one of its parents. Back cross are used by animal and plant breeders to rapidly improve a breed/variety by making a useful gene (trait) homozygous.
  • Test cross is a cross to know whether an individual is homozygous or heterozygous for dominant character. Test cross is crossing of offspring with unknown dominant phenotype with the individual homozygous recessive for the trait.
    • If unknown offspring is homozygous (TT), then crossing with dwarf recessive (tt) gives all Tall offspring i.e., Tt.
    • If unknown offspring is heterozygous tall (Tt), then crossing with dwarf recessive (tt) results in 50% tall (Tt) and 50% dwarf(tt) progeny.
  • True breeding line is the one that has undergone continuous self pollination, shows stable trait inheritance and expression for several generations.
  • Gregor Johann Mendel is called the Father of genetics. His genetic experiments were conducted on garden pea, Pisum sativum in 1856 in the garden at the monastery.
  • Mendel used the pea plant for his experiments because of the following reasons:
    • Life span of pea plant is short.
    • Flowers are self-pollinated.
    • Flowers were hermaphrodite as well as bisexual.
  • Mendel selected 14 ture-breeding pea-plant varieties, as pairs which were similar except for one character with contrasting traits. Some of the contrasting traits selected were smooth or wrinkled seeds, yellow or green seeds, smooth or inflated pods, green or yellow pods and tall or dwarf plants.
  • A cross where transmission of one character is studied is called monohybrid cross.
  • Law of dominance states that only one factor expresses itself in FI generation and expression of both factors expresses in F2 generation in the proportion of 3: 1 ratio.
  • Law of segregation established the fact that alleles do not blend and both the traits are recovered as such in F2 generation.
  • Law of independent assortment states that genes of different characters located in different pairs of chromosomes are independent of one another in their segregation during gamete formation.
  • This law was proposed by Mendel based on the results of dihybrid crosses, where inheritance of two traits were considered simultaneously.
  • Incomplete dominance is a phenomenon where two contrasting alleles are present together but neither of the alleles is dominant over other and the phenotype formed is intermediate of the two alleles.
  • Incomplete dominance was discovered by Correns in inheritance of flower colour in dog-flower (Snapdragon or Antirrhinum majus).
  • In Mirabili s jalapa (four O’clock), there are two types of flower colour in
  • pure state-red & white. When the two types of plants are crossed the hybrid
  • or plants ofF I generation have pink flowers. If the latter are selfed, the plants ofF2 generation are of three types-red, pink & white flowered in the ratio of 1: 2: 1.
  • In codominance, both the alleles of a gene are equally dominant i.e. the dominant character is not able to suppress the recessive character & thus both the characters appear side by side in FI hybrids. FI generation resembles both the parents. E.g., ABO blood group, sickle cell anaemia.
  • Multiple alleles can be defined as a set of three, four, or more allelomorphic genes or alleles, which have arisen as a result of mutation of the normal gene & which occupy the same locus in the homologous chromosomes.
  • The well known example of multiple allele in human is blood group, which also shows codominance.
  • Chromosomal theory of inheritance theory states that Mendelian genes (factors) are located on chromosome and it is the chromosome that segregate and independently assort.
  • Walter Sutton and Theodore Boveri (1902) noted that the behaviour of chromosomes was parallel to the behaviour of genes and used chromosome movement to explain Mendel’s Laws.
  • T. H. Morgan is popularly called as father of experimental genetics. Linkage may be defined as the relationship between genes on the same chromosome that causes them to be inherited together, therefore it will not result in variations among siblings. In linkage there is a tendency to maintain the parental gene recombination except for occasional crossovers.
  • The rearrangements of linked genes due to crossing over is known as recombination.
  • Polygenic inheritance is the inheritance of a characteristic that is controlled by more than one gene. It can produce unexpected phenotypes.
  • In pleiotropy, single gene controls multiple phenotypic expression whereas one gene controls or affects the phenotypic expression in case of Mendelism.
  • Grasshopper has XO type of sex determination in which males have only one chromosome besides autosomes whereas female has a pair of X chromosomes.

MUTATION AND GENETIC DISORDERS

  • Phenomenon that results in alteration of DNA sequence and consequently results in change in genotype and phenotype of an organism is called mutation.
  • Pedigree analysis is the study of an inherited trait in a group of related individuals. It helps to determine the pattern and characteristics of the trait which includes its mode of inheritance, age of onset, and phenotypic variability.
  • A genetic disorder is a disease that is caused by an abnormality in an individual’s DNA.
  • Genetic disorder may be grouped into two categories – Mendelian disorders & chromosomal disorders.
  • Examples of Mendelian disorders are :
  • Haemophilia : It is an X linked recessive disease. It occurs due to deficiency of plasma thromboplastin or antihaemophilia globulin during which the exposed blood does not clot.
  • Sickle cell anaemia : It is an autosomal recessive trait. It is a blood disease where the RBCs becomes sickle shaped as compared to normal one. It is due to inheritance of defective allele coding for -globin. It results in the transformation of HbA into Hb-s in which glutamic acid is replaced by valine at 6th position in each of two -chains of haemoglobin.
  • Chromosomal disorders are of two types – autosomal chromosomal changes & sex chromosomal changes.
  • Autosomal disorders are of two types – euploidy and aneuploidy.
  • Euploidy is the numerical increase of chromosome from normal 2n to multiples of complete haploid set. It is more common in plants, rare in animals & absent in humans.
  • Aneuploidy relates to variation in chromosomes number with respect to one or only a few chromosomes.
  • Examples of autosomal disorders are – Down’s syndrome, Klinefelter’s syndrome, Turner’s syndrome, Edward syndrome, Patau’s syndrome.
  • Down syndrome (trisomy of 21) syndrom is characterised by rounded face, low IQ, mongloid features, folded eyelids, short & broad neck, etc. Klinefelter’s Syndrome (trisomy of ‘X’ chromosome) is caused due to the presence of an additional copy of X-chromosome resulting into a karyotype of 47, XXY. Such an individual has overall masculine development, however, the feminine development (development of breast, i.e., Gynaecomastia) is also expressed. Such individuals are sterile.
  • Turner’s Syndrome (monosomy of ‘X’ chromosome) is caused due to the absence of one of the X chromosomes, i.e., 45 with XO, such females are sterile as ovaries are rudimentary besides other features including lack of other secondary sexual characters.
  • Edwards’s syndrome is caused by having an extra chromosome 18 in some or all of the body’s cells. It is characterised by a low birth weight and certain abnormal features. These include a small, abnormally shaped head; a small jaw and mouth; clenched fists with overlapping fingers; and heart, lung, kidney, intestine, and stomach defects.
  • Patau’s syndrome is a congenital disorder in which there are three copies of chromosome 13, 14, or 15 instead of the usual two. This results in brain, heart, and kidney defects which are usually fatal soon after birth.

Related Articles: