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.