Topics in the Chapter 

  • Introduction
  • Types of Variation
  • Accumulation of Variation during Reproduction
  • Mendel and His Work on Inheritance
  • Monohybrid Cross
  • Dihybrid Cross
  • Sex Determination
  • Evolution
  • Acquired and Inherited Traits
  • Ways by which Speciation takes place
  • Evolution and Classification
  • Evolution by Artificial Selection
  • Human Evolution
Genetics: Study of Heredity and Variation
  • Heredity: The transmission of characters/traits from one generation to the next.
  • Variation: Differences in characters/traits between parent and offspring.
Types of Variations

1. Somatic Variation

  • Occurs in: Body cells.
  • Inheritance: Not inherited or transmitted.
  • Also known as: Acquired traits.
  • Examples: Cutting of tails in dogs, boring of pinna.

2. Gametic Variation

  • Occurs in: Gametes/Reproductive cells.
  • Inheritance: Inherited and transmitted.
  • Also known as: Inherited traits.
  • Examples: Human height, skin color.
Accumulation of Variation during Reproduction
  • Variation occurs during both sexual and asexual reproduction.
Variations in Asexual Reproduction
  • Quantity of Variations: Fewer.
  • Cause: Small inaccuracies in DNA copying (Mutation).
Variations in Sexual Reproduction
  • Quantity of Variations: Large.
  • Cause: Crossing over, separation of chromosomes, mutation.
Importance of Variation
  • Survival Advantage: Different variations provide different advantages.
  • Example: Bacteria that can withstand heat will survive better during a heat wave.
  • Main Benefit: Increases the chances of species survival in a changing environment.

Examples of Variation in Humans

  • Free ear lobes and Attached ear lobes are two variants found in human populations.
Mendel and His Work on Inheritance
  • Gregor Johann Mendel (1822-1884) initiated experiments on plant breeding and hybridization.
  • Father of Genetics: Mendel is recognized for proposing the laws of inheritance in living organisms.
  • Plant used in experiments: Pisum sativum (garden pea). Selected for its contrasting characteristics.
Seven Pairs of Contrasting Characters in Garden Pea

Mendel’s Experimental Material

Garden Pea (Pisum sativum) was chosen due to:
  • Availability of detectable contrasting traits.
  • Short life span of the plant.
  • Ability for self-fertilization and cross-fertilization.
  • Production of a large number of seeds.
Mendel’s Experiments
  • Mendel conducted experiments by cross-pollinating plants to study one character at a time.
Monohybrid Cross
  • Definition: A cross between two pea plants with one pair of contrasting characters.
  • Example: Cross between a tall plant and a dwarf (short) plant.
Generations in Monohybrid Cross

1. First-generation (F1):

  • All plants were tall; no medium-height plants.
  • Indicates that only the tallness trait was expressed, even though both traits (tall and short) were inherited.

2. Second-generation (F2):

  • Progeny of the F1 tall plants were not all tall.
  • Both tall and short traits appeared, showing that both traits were present in the F1 generation, but only the dominant trait (tallness) was visible.
Genetic Conditions
  • Pure or Homozygous Condition: TT (both dominant) or tt (both recessive).
  • Heterozygous Condition (Hybrid): Tt (one dominant, one recessive).
Ratios in Monohybrid Cross
  • Phenotypic Ratio: 3:1 (Three tall plants and one short plant).
  • Genotypic Ratio: 1:2:1 [One TT (pure tall), two Tt (hybrid tall), one tt (short)].
Phenotype vs. Genotype
  • Phenotype: Physical appearance (Tall or Short).
  • Genotype: Genetic makeup (TT, Tt, or tt).
Observations of Monohybrid Cross
  1. F1 Progeny: All were tall, with no medium-height plants.
  2. F2 Progeny: ¼ were short, ¾ were tall. Phenotypic ratio: 3:1 (3 tall : 1 short).
Conclusions
  • TT and Tt both result in tall plants, while tt results in a short plant.
  • A single copy of T is enough to make the plant tall, while both copies must be t for the plant to be short.
  • Dominant Trait: T (because it expresses itself).
  • Recessive Trait: t (because it remains suppressed).
Dihybrid Cross
  • Definition: A cross between two plants with two pairs of contrasting characters.
  • Example: Round green seeds × Wrinkled yellow seeds.
Punnett Square for Dihybrid Cross

Phenotypic Ratio
  • Round, Yellow: 9
  • Round, Green: 3
  • Wrinkled, Yellow: 3
  • Wrinkled, Green: 1
Observations of Dihybrid Cross
  1. F1 Generation: When RRyy was crossed with rrYY, all F1 progeny had RrYy genotype, producing round and yellow seeds.
  2. F2 Generation: Self-pollination of F1 plants resulted in a phenotypic ratio of 9:3:3:1 with both parental and new recombinant phenotypes (round yellow and wrinkled green).
Conclusions
  • Dominant Characters: Round and yellow seeds.
  • Independent Inheritance: The occurrence of new phenotype combinations indicates that genes for round and yellow seeds are inherited independently of each other.
Expression of Traits
  • Cellular DNA: The information source for making proteins in the cell.
  • A section of DNA providing information for one protein is called the gene for that protein.
  • Example: Plant height depends on the amount of a particular plant hormone, which is influenced by the efficiency of the protein-making process.

Summary of Process:

  • Cellular DNA (Information Source) → Synthesis of Proteins (Enzyme) → Efficient Functioning → More Hormone Production → Tallness of Plant
Conclusion: Genes control characteristics/traits.
Sex Determination
  • Definition: The process of determining the sex of an offspring.
Factors Responsible for Sex Determination

1. Environmental Factors:

  • In some animals, the temperature at which fertilized eggs are kept determines the gender.
  • Example: Turtles.

2. Genetic Factors:

  • In animals like humans, gender is determined by a pair of chromosomes called sex chromosomes.
  • XX: Female
  • XY: Male
Sex Chromosomes in Humans
  • Humans have 23 pairs of chromosomes.22 pairs are called autosomes.
  • The last pair is the sex chromosomes, which determine the individual's gender.
  • XX: Female, XY: Male

Inheritance of Sex Chromosomes
  • All children inherit an X chromosome from their mother, regardless of their gender.
  • The sex of the children is determined by the chromosome they inherit from their father:
  • X chromosome from father → XX (Female).
  • Y chromosome from father → XY (Male).
  • Conclusion: Half of the children will be boys, and half will be girls, depending on the sex chromosome they inherit from their father.

Evolution

  • Definition: Evolution is the sequence of gradual changes over millions of years that results in the formation of new species from primitive organisms.
Situation I: Group of Red and Green Beetles
  • Variation: During reproduction, a green beetle arises in a population of red beetles.
Natural Selection:
  • Crows feed on the red beetles, reducing their population.
  • The green beetle, camouflaged in green bushes, is not eaten by crows.
  • The number of green beetles increases as they have a survival advantage.
Conclusion:
  • Green beetles were naturally selected due to their camouflage, which allowed them to survive better in their environment.
  • This natural selection was driven by the crows.

Situation II: Group of Red and Blue Beetles

  • Variation: A blue beetle arises in a population of red beetles.

Population Dynamics:

  • Both red and blue beetles reproduce, but their numbers are kept in check by crows, which eat both colors.
  • An elephant suddenly stamps on the bushes, killing most of the beetles. The surviving beetles are mostly blue.

Conclusion:

  • Blue beetles did not have a survival advantage; instead, the elephant's accident caused a random change in the beetle population.
  • This random change in gene frequency, not due to survival advantage, is called genetic drift, which leads to variation.

Situation III: Group of Red Beetles and Bushes

Environmental Impact:

  • The habitat (bushes) of red beetles suffers from a plant disease, leading to poor nourishment.
  • The average weight of beetles decreases, and their population reduces.
  • Once the plant disease is eliminated, the beetles' population and average weight return to normal.

Conclusion:

  • No genetic change occurred in the beetle population.
  • The changes were temporary and due to environmental factors, not genetic variation.
Acquired and Inherited Traits

Acquired Traits

  • Definition: Traits developed in an individual due to special conditions.
  • Characteristics:
  • Not passed to progeny (next generation).
  • Do not influence evolution.
  • Example: Low weight of starving beetles.
Inherited Traits
  • Definition: Traits passed from one generation to the next.
  • Characteristics:
  • Transferred to progeny.
  • Influential in evolution.
  • Example: Color of eyes and hair.
Ways by Which Speciation Takes Place
  • Speciation: The formation of new species when variation is combined with geographical isolation.

1. Gene Flow:

  • Occurs between populations that are partly but not completely separated.
  • Ensures some genetic exchange between populations.

2. Genetic Drift:

  • Definition: A random change in the frequency of alleles (gene pairs) in a population over successive generations.
  • Causes: Severe changes in the DNA. Changes in the number of chromosomes.

3. Natural Selection:

  • Definition: The process by which nature selects and consolidates organisms that are better adapted and possess favorable variations.
  • Leads to the survival and reproduction of the fittest.

4. Geographical Isolation:

  • Definition: Separation caused by physical barriers like mountain ranges or rivers.
  • Leads to reproductive isolation, preventing gene flow between separated groups of the population.
  • Over time, can result in the formation of new species.

Evolution and Classification

  • Interconnection: Evolution and classification are closely linked, as classification reflects the evolutionary relationships between species.
Key Concepts

1. Classification Reflects Evolution:

  • The classification of species is a reflection of their evolutionary relationships.
  • Species with more shared characteristics are more closely related and likely have a more recent common ancestor.

2. Grouping Organisms:

  • Similarities among organisms allow scientists to group them and study their characteristics, revealing their evolutionary paths.
Evidences of Evolution

1. Homologous Organs:

  • Definition: Organs with the same basic structural plan and origin but different functions.
  • Evidence: Indicates that species with homologous organs are derived from a common ancestor.
  • Examples: Forelimb of a horse (Running), Wing of a bat (Flying), Paw of a cat (Walking/Scratching/Attacking)
  • Conclusion: Same basic structure, different functions.

Analogous Organs:

  • Definition: Organs with different origins and structural plans but the same function.
  • Evidence: Provides a mechanism for understanding evolution, showing that different species can develop similar functions independently.
  • Examples: Wings of a bat (Elongated fingers with skin folds), Wings of a bird (Feathery covering along the arm).
  • Conclusion: Different basic structures, similar function (flight).

Fossils (Paleontological Evidence):

  • Definition: Remains or traces of organisms from the past.
  • Importance: Fossils provide direct evidence of past life forms and their evolutionary history.
  • Example: Archaeopteryx: Possesses features of both reptiles and birds, suggesting that birds evolved from reptiles. Other Fossil Examples: Ammonite: Fossil-invertebrate, Trilobite: Fossil-invertebrate, Knightia: Fossil-fish, Rajasaurus: Fossil-dinosaur skull
  • Determining Age: Deeper fossils are older. Radio-carbon dating (C-14 dating): A method used to detect the age of fossils.

Evolution by Stages

  • Gradual Process: Evolution occurs in stages, with small changes accumulating over generations.

1. Fitness Advantage:

  • Evolution of Eyes:
  • Complex organs like eyes did not evolve suddenly but developed through minor DNA changes over generations.
  • Examples of Eye Evolution:
  • Flatworm: Rudimentary eyes (basic visual advantage)
  • Insects: Compound eyes
  • Humans: Binocular eyes

2. Functional Advantage:

  • Evolution of Feathers:
  • Feathers initially provided insulation in cold weather but later became useful for flight.
  • Examples:
  • Dinosaurs: Had feathers but did not use them for flight.
  • Birds: Later adapted feathers for flight.

Evolution by Artificial Selection

  • Artificial Selection: Humans have played a significant role in modifying wild species through artificial selection, tailoring them to meet human needs over time.

Examples of Artificial Selection

  1. Wild Cabbage: By selectively breeding wild cabbage, humans have developed various varieties: Broccoli, Cauliflower, Red Cabbage, Kale, Cabbage, Kohlrabi.

  2. Wheat: Multiple wheat varieties have been developed through artificial selection to improve yield, resistance to pests, and adaptability to different climates.

Molecular Phylogeny
  • Concept: Molecular phylogeny is based on the idea that changes in DNA during reproduction are fundamental events in evolution.
  • Key Idea: Organisms that are more distantly related will have accumulated greater differences in their DNA sequences over time.
Human Evolution

  • Study Tools: Tools like excavating, time dating, fossil analysis, and DNA sequencing help in understanding human evolutionary relationships.

  • Diversity Yet Unity: Despite the great diversity in human forms globally, all humans belong to a single species, Homo sapiens.

  • Origin:
  • Africa: All humans can trace their origins back to Africa, where the earliest Homo sapiens lived.
  • Migration Path:
  • Early humans spread from Africa to different parts of the world:
  • West Asia → Central Asia → Eurasia → South Asia → East Asia.
  • From there, they traveled to Indonesia, the Philippines, Australia, and across the Bering land bridge to the Americas.

  • Migration Patterns: The migration was not linear; groups sometimes returned and mixed with each other, leading to the diverse human populations seen today.