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## U2 TRUEMAN'S ELEMENTARY BIOLOGY +2 ### Offspring as per Hardy-Weinberg Equation (p+q)² = p² + 2pq + q² - **p²:** frequency of individuals with homozygous dominant alleles (AA) - **2pq:** frequency of heterozygous individuals (Aa) - **q²:** frequency of individuals with homozygous recessive allel...

## U2 TRUEMAN'S ELEMENTARY BIOLOGY +2 ### Offspring as per Hardy-Weinberg Equation (p+q)² = p² + 2pq + q² - **p²:** frequency of individuals with homozygous dominant alleles (AA) - **2pq:** frequency of heterozygous individuals (Aa) - **q²:** frequency of individuals with homozygous recessive alleles (aa) ### According to this principle, if a population is large: - Individuals have random mating. - Gametes combine at random and undergo no evolutionary changes in genotypic frequencies. - Isolation mechanisms (natural selection, mutation, frequencies and genetic drift) are constant. ### Significance: - It's possible to calculate all allele and genotype frequencies using the expressions **allele frequency p + q = 1** and **genotype frequency p² + 2pq + q² = 1**. - Constant gene frequencies over several generations indicate that evolution is not taking place. - Changing gene frequencies would indicate that evolution is in progress. - Evolution occurs when the genetic equilibrium is upset (evolution is a departure from the **Hardy-Weinberg Principle**). ### Example Of the 200 plants of garden pea produced by crossing: - 150 plants have round seeds. - 50 plants have wrinkled seeds. ### In Hardy-Weinberg equation, Mendelian genetic variations are summed up as: - **Total pea plants produced:** 200 - **Number of pea plants with wrinkled (recessive) seeds:** 50 - **Therefore, frequency of pea plants with wrinkled seeds:** q² = 50/200 = 0.25 - **Or q = √0.25 = 0.5** since, p + q = 1 - **… frequency of occurrence of dominant allele p = 1 - q = 1 - 0.5 = 0.5** - **Using Hardy-Weinberg equilibrium: p² + 2pq + q² = 1** - **Frequency of AA genotype = p² = (0.5)²= 0.25** - **Frequency of Aa genotype = 2pq = 2 x 0.5 x 0.5 = 0.5** - **Frequency of aa genotype = q² = (0.5)²= 0.25** - **Total frequency of three genotypes = 0.25 + 0.5 + 0.25 = 1** - Using above frequency, one would expect **50 individuals with AA genes; 100 individuals with Aa genes and 50 individuals with aa genes.** ## 6.8 A BRIEF ACCOUNT OF EVOLUTION - It's thought that about 2000 million years ago (mya) the first cellular forms of life originated on Earth. - Some of these cells had the ability to release O2. - The reaction could have been similar to light reaction in photosynthesis. - Slowly single-celled organisms became multicellular life forms. ### Early fossils: - Were fewer and of simple organisms. - Complexity and increase in number occured with the passage of time. ### To understand the brief account of evolution, we should have seen the Table 6.1. Time scale of Earth. - Mya = Million - Bya = Billion ## 1. Azoic Era (Age in million years) - Primitive Earth was under formation and there is an absence of life. ## 2. Archaeozoic Era (3500 million years ago) - Has scanty fossils of only prokaryotes which were the first cells. - These organisms formed multicellular organisms. - Some employed water as hydrogen source. ## 3. Proterozoic Era (3000-1500 million years ago) - Had fossils of eukaryotes which were advanced cells. - These eukaryotes led to the formation of green algae and early invertebrates. ## 4. Palaeozoic Era (570-250 million years ago) - Fossils were in good number. - **(i) Cambrian (570 million years ago):** All invertebrate plants were abundant. - **(ii) Ordovician (500 million years ago):** This period is also known as "Age of Invertebrates".