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Introduction. Heredity: tendency of an individual to resemble his parents. Variation: the differences between individuals of the same species. Genetics: the study of how characters are transferred or inherited from one generation to the next. Mendel’s Breeding Experiment.

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Heredity: tendency of an individual to resemble his parents

Variation: the differences between individuals of the same species

Genetics: the study of how characters are transferred or inherited from one generation to the next


Mendel’s Breeding Experiment

Monohybrid Inheritance:

The inheritance of just one pair of contrasting characters

Cross 2 pure breeding parents with contrasting characters





pure-breeding parents

first filial (F1) generation

all purple

second filial (F2) generation

705 purple

224 white

monohybrid ratio 3:1

  • Genotype - the genetic composition
  • Phenotype - the external appearance
  • Homozygous (pure-breeding)
    • with the same alleles in the same character
  • Heterozygous (hybrid)
    • with different alleles
  • Dominant gene (or character)
    • which expresses its effect even in heterozygous condition
  • Recessive gene (or character)
    • which expresses its effect only in homozygous condition
  • Pedigree
    • it shows the inheritance of one or more characters in different generations
  • Test cross
    • cross the organism with dominant character to an organism with homozygous recessive character
    • to test whether the organism with the dominant character is homozygous or heterozygous
  • Back cross
    • cross an organism with one of its parent
sex determination
Sex Determination
  • in human : by sex chromosomes
  • male : XY
  • female : XX



  • chromosomes other than sex chromosomes  somatic chromosomes which control normal body characters
22 pairs are identical in both sexes (human)
    • autosomes
  • The 23rd pair is different in male and female
    • sex chromosomes
  • identical twins
    • one embryo separates into two and each develops into a new individual
    • they should have same characters and same sex
  • non-identical twins
    • formed from two different zygotes from two separate fertilizations
    • they usually have different characters
    • they may have different sexes
types of variations
Types of Variations
  • continuous variation
    • with a continuous range of intermediate values
    • eg. height, weight, intelligence quotient, heart beat

No. of individuals



human height

types of variations1
Types of Variations
  • discontinuous variation
    • the character is clear-cut & not showing intermediates
    • eg. tongue rolling, ear lobes and colour of corn


causes of variations
Causes of Variations
  • Meiosis
    • homologous chromosomes separate independently from each other and pass into a different gamete ( INDEPENDENT ASSORTMENT). As a result, a great variety of gametes are produced
  • Random Fertilization
    • since fertilization is a random process, there are many possible different combinations of genes in a zygote
causes of variations1
Causes of Variations
  • Mutation
    • genetic make-up may suddenly change
    • sometimes occur naturally
    • rate may greatly increased if the organism is exposed to radiation, certain chemicals or neutron bombardment
    • most mutation are harmful
  • Environmental Factors
    • cause variation in characters with continuous variation