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Heredity and Evolution / Mapping Our Genes -Summary


  • Passing down of characters / traits from parents to children is called heredity.
  • Children of same parents differ because they possess different combinations of parental genes. These difference are termed variations. We are all human beings but can be distinguished easily from each other due to variation.
  • Heredity and variation are due to genes and their varied combinations.
  • Study of heredity is called Genetics. Mendel was the first to postulate laws of inheritance (heredity) and he said that heredity was due to “factors” (now called genes) and that every feature was controlled by a pair of factors which separate into different gametes during gamete formation.
  • Another Mendelian law of inheritance stated that in a pair of genes one may be dominant and the other recessive. The dominant gene of the pair masks the effect of the recessive member of the pair.
  • Sutton found out that “Mendelian factors” were the genes and that genes are present on chromosomes.
  • Chromosomes are present in pairs in the nucleus and each is made of one molecule of DNA and proteins. The diploid number of chromosomes in humans is 46, of which 22 pairs are autosomes and 2 chromosomes X and Y are sex chromosomes.
  • A DNA molecule is a polynucleotide. Each of its nucleotides is made of a nitrogenous base, a sugar, and a phosphate. A DNA molecule is made of two strands of DNA helically coiled around each other.
  • Genes are made of DNA. They are segments of the DNA molecule of the chromosome.
  • Sex determination in humans is based on combination of sex chromosomes. Females have two X-chromosomes, while males possess one X and one Y chromosome.
  • Defective genes or absence of genes may cause genetic disorders e.g. thallasemia, haemophilia and colour blindness. Thallasemia is an autosomal genetic disorder while, haemophilia and colour blindness are sex chromosomal disorders.
  • The collection of all the genes of a species constitute its genome.  Human genome has been unravelled that is, location of all human genes on the chromosomes is now known.
  • Genetic engineering involves transfer of a gene from one species into member of another species with the help of plasmids.  Organisms carrying foreign genes, that is, genes of another species are called genetically modified organisms or GMO.
  • DNA fingerprinting is a technique with the help of which the identity of a person can be known from the genetic make up.
  • Evolution is a process that results in heritable changes in a population spread over many generations.


Parents with black hair will likely give birth to children with black hair. Similarly, parents with long noses will have kids with long noses. Once in a while, though, this doesn’t work and parents with black hair will give birth to a blond. This discrepancy can be explained by the principle of segregation.
Thus, the similarities between parents and their offsprings are due to ‘heredity’ and the dissimilarities are due to ‘variation’. Variation is a prerequisite for evolution i.e. in order for evolution to occur, there has to be variation.

♦  ‘A Mix-up at the Hospital’ and ‘A Paternity case’ – Both of these cases use DNA typing to match babies with parents, and to determine potential paternity, respectively.
♦  The long consequences of the accumulation of variations are called evolution.

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