Chapter 5: Inheritance

  • Genetics is the study of heredity and variation
  • Chromosomes are thread-like or rod-shaped structures made up of DNA molecules.
  • Chromosomes carry hereditary information and are found in the nucleus of a cell.
  • Chromatin refers to the total collection of DNA and proteins.
  • Deoxyribonucleic acid (DNA) is a double helix strand of polynucleotides.
  • Each nucleotide is made up of a deoxyribose sugar, a phosphate group and a nitrogenous base.
  • A gene is the basic unit of inheritance.
  • A gene consists of a short segment of a DNA molecule.
  • It is found on a specific position on a chromosome.
  • Alleles are different forms of a gene for a trait.
  • Each trait is controlled by at least a pair of genes.
  • Alleles occupy the same relative position on a pair of homologous chromosomes.
  • A dominant allele expresses itself both in the homozygous (AA) or a heterozygous (Aa) condition.
  • A recessive allele expresses itself only in the homozygous (aa) condition.
  • Genotype is the genetic composition of an individual. Example: TT, Tt or tt.
  • Phenotype is the physical trait expressed outwardly for an individual. Example: Height, eye colour.
  • Mendel's Law of Segregation states that the members of each pair of alleles separate or segregate during the formation of gametes. Only one allele can be carried in a single gamete.
  • Mendel's Law of Independent Assortment states that two or more pairs of alleles segregate independently of one another during the formation of gametes.
  • The sex of of an individual is determined by sex chromosomes.
  • Sex chromosomes are present as X and Y chromosomes.
  • A male has 44 autosomes and XY chromosomes.
  • A female has 44 autosomes and XX chromosomes.
  • The probability of a couple getting a baby boy or baby girl is 50%.
  • The blood group phenotypes are A, B, AB and O.
  • The ABO blood group in human is an example of inheritance that is controlled by multiple genes.
  • The ABO blood group is controlled by three alleles.
  • The Rhesus factor is another type of blood group.
  • The Rhesus factor is an antigen present on the surface of red blood cells.
  • The Rhesus factor causes agglutination when it reacts with the antibodies from individuals without this antigen.
  • The Rhesus factor is controlled by a pair of alleles, theRh and rh allele.
  • A person with the Rhesus factor is known as Rh-positive and has genotype Rh-Rh or Rh-rh.
  • A person who is Rh-negative has genotype rh-rh.
  • Sex-linked genes are found on sex chromosomes.
  • Example of sex-linked genes are gene for colour blindness and haemophilia.
  • A hereditary disease is a medical condition caused by an allele inherited from the parents.
  • Examples of hereditary diseases are Huntington's disease, sickle-cell-anaemia, cystic fibrosis and thalassaemia.
  • Down's syndrome victim has an extra chromosome belonging to chromosome pair 21.
  • Down's syndrome victim has 47 chromosomes.
  • The complete set of an organism's gene is called genome.
  • The human genome project is a project to generate the detailed maps of the human genome. The genes in the chromosome are sequenced and identified. This will help in diagnosing diseases in human.
  • Applications of the knowledge in genetics include DNA fingerprinting and genetic engineering.
  • DNA fingerprinting is a procedure which analyses a person's DNA segments.
  • Genetic engineering is a technique of genetic manipulation to create new combinations of genes.
  • Genetic engineering is used in gene therapy and to genetically modify organisms.
  • Gene therapy is a treatment for a disease in which the patient is provided with a new gene.
  • A genetically modified organism is an organism that has acquired one or more genes by artificial means.


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