Introduction 1289682
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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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Introduction 1289682


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

Introduction 1289682

Mendel’s Breeding Experiment

Monohybrid Inheritance:

The inheritance of just one pair of contrasting characters

Cross 2 pure breeding parents with contrasting characters

Introduction 1289682




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

Introduction 1289682


  • 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

Introduction 1289682