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SS3: BIOLOGY - 2ND TERM

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  1. Reproductive Behaviours | Week 1
    1 Topic
    |
    1 Quiz
  2. Life Cycles of Insects, Toads | Week 2
    6 Topics
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    1 Quiz
  3. Social Insects | Week 3
    2 Topics
    |
    1 Quiz
  4. Biology of Heredity (Genetics) I | Week 4
    4 Topics
  5. Biology of Heredity (Genetics) II | Week 5
    6 Topics
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    1 Quiz
  6. Biology of Heredity (Genetics) III | Week 6
    7 Topics
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    1 Quiz
  7. Variation | Week 7
    1 Topic
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    1 Quiz
  8. Evolution | Week 8
    3 Topics
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Topic Content:

  • Mendel’s First Law (Law of Segregation of Genes)
  • Describe a monohybrid cross as explained by Mendel’s Law of segregation

What is Mendel’s First Law?

Mendel’s first law (Law of Segregation of Genes) states that when two organisms, homozygous (identical alleles) or heterozygous (two different alleles of a given gene), are crossed, the offspring will be hybrid (carry two different alleles) but will exhibit only the dominant trait. The trait that remains hidden is the recessive trait.

  • The law of segregation can be demonstrated in a monohybrid cross, where a cross is made between purebred homozygous plants, e.g. one tall (TT) and one short (tt).
  • Mendel grew tall plants for several generations and observed that the plants produced only tall plants, which means they bred true.
  • Mendel then crossed tall plants with short plants. All the offspring were tall. These he called the first filial generation of F1.
  • He then crossed the F1 plants, collected their seeds, and sowed them. He referred to the offspring as the second filial generation of F2.

First Generation (F1):

law of segregation 1
Monohybrid cross – First generation.
  • The genotype of all the F1 generation plants is Tt.
  • From the figure, all the offspring in the F1 generation are all tall (the dominant trait is T – tall, Hence, the phenotype is all tall plants but they are heterozygous, i.e. not pure breeds.)

The F1 generation plants were then self-pollinated to obtain the second filial generation (F2) as shown below.

Second Generation (F2):

law of segregation 2
Monohybrid cross – Second generation.
  • The genotypes of the F2 plants are:
    • 1 TT
    • 2 Tt
    • 1 tt
  • The genotypic ratio is 1:2:1 (i.e. 1 homozygous dominant tall, TT, 2 heterozygous tall Tt and 1 homozygous recessive tt.)
  • The phenotypes are:
    • 3 Tall plants
    • 1 Short plant
  • The phenotypic ratio is 3:1

These ratios are always true for a monohybrid cross

F1TtTt 
F2TTTttt
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