Herdity and Evolution
Introduction
Heredity: The transfer of traits from parents to offspring.
Genetics: A branch of biology focusing on heredity and variation.
Gene: The unit of heredity responsible for phenotype, through protein synthesis and enzyme control.
Asexual reproduction leads to minimal variation due to minor DNA changes during replication.
Sexual reproduction introduces greater variation due to the contribution of genes from two parents.
Gregor Johann Mendel
Mendel, an Austrian monk, laid the foundation for modern genetics with his heredity experiments in 1866.
His theories were initially rejected but later rediscovered in 1900 by Hugo de Vries, Tschermark, and Correns.
Known as the "Father of Genetics."
Mendel’s Experiments
Conducted (1856–1865) using the pea plant (Pisum sativum).
Key reasons for selecting Pisum sativum:
Short life cycle, enabling quick results.
Suitable for year-round growth.
Options for self or cross-fertilization.
Presence of seven pairs of contrasting traits.
Large offspring numbers allowed statistical analysis.
Characteristics of Pisum sativum
Alleles: Alternative forms of a gene located at the same position (loci) on homologous chromosomes.
Trait | Dominant | Recessive |
Height | Tall (T) | Dwarf (t) |
Seed Color | Yellow | Green |
Pod Color | Green | Yellow |
Flower Color | Purple | White |
Seed Shape | Round | Wrinkled |
Pod Shape | Inflated | Constricted |
Flower Position | Axial | Terminal |
Terminology in Genetic
Factor: Unit responsible for inheritance and expression of traits.
Gene: Modern term for Mendel's factor; a segment of DNA determining a specific trait.
Alleles: Alternative forms of a gene (e.g., R for round seeds and r for wrinkled seeds) that occupy the same loci on homologous chromosomes.
Dominant: Trait that appears in the F₁ generation (e.g., round seeds).
Recessive: Trait suppressed in the F₁ generation but reappears in F₂ (e.g., wrinkled seeds).
Genotype: Genetic constitution inherited from parents (e.g., RR).
Phenotype: Observable characteristics of an individual (e.g., round seeds).
Homozygous: Possessing identical alleles for a trait (e.g., RR).
Heterozygous: Possessing different alleles for a trait (e.g., Rr); also called a hybrid.
Parent Generation (P₁): Organisms used for the initial cross.
F₁ Generation: Offspring from the first cross (P₁).
Inbreeding: Cross between individuals of the same progeny (e.g., F₁ × F₁).
F₂ Generation: Offspring from self-hybridization or inbreeding of F₁ individuals.
Monohybrid Cross: Cross between parents differing in a single pair of traits (e.g., seed shape).
Monohybrid Ratio: 3:1 phenotypic ratio in F₂ generation.
Dihybrid Cross: Cross studying two pairs of contrasting traits simultaneously (e.g., seed shape and color).
Test Cross: Cross between an individual with a dominant phenotype and a homozygous recessive individual to determine the genotype of the dominant.
Back Cross: Cross between a hybrid and one of its parents.
Monohybrid Cross
Definition: A genetic cross involving one trait studied at a time.
Experiment:
Mendel crossed a pure tall plant (TT) with a pure dwarf plant (tt).
In the F₁ generation, all plants were tall (Tt), showing no blending of traits.
In the F₂ generation, offspring appeared in a phenotypic ratio of 3 tall : 1 dwarf.
The recessive trait (dwarfness) reappeared in the F₂ generation.
Mendel’s Postulates:
Each character is controlled by a pair of factors.
During gamete formation, paired factors segregate into different gametes.
Gametes fuse to restore paired state in offspring.
Dominance: In heterozygous conditions (Tt), only one factor (dominant) is expressed, while the other (recessive) is suppressed.
Ratios:
Phenotypic Ratio (F₂): 3 tall : 1 dwarf.
Genotypic Ratio (F₂): 1 pure tall (TT) : 2 hybrid tall (Tt) : 1 pure dwarf (tt).
Dihybrid Cross
Definition: A genetic cross involving two traits studied simultaneously.
Experiment:
Mendel crossed a plant with yellow, round seeds (YYRR) and another with green, wrinkled seeds (yyrr).
In the F₁ generation, all offspring had yellow, round seeds, indicating dominance of these traits.
In the F₂ generation, the phenotypic ratio was 9 : 3 : 3 : 1:
9 yellow, round.
3 yellow, wrinkled.
3 green, round.
1 green, wrinkled.
Explanation:
Besides parental combinations (YR and yr), two new combinations (Yr and yR) were observed, indicating independent assortment of traits.
Genotypic Ratio (F₂): 1: 2: 1: 2: 4: 2: 1: 2: 1.
Example: Cross between round, yellow seeds (RRYY) and wrinkled, green seeds (rryy).
F₁ Generation: All offspring were round and yellow (RrYy).
F₂ Generation: Gametes segregate independently, producing offspring in a 9:3:3:1 phenotypic ratio:
9 round, yellow.
3 round, green.
3 wrinkled, yellow.
1 wrinkled, green.
Laws of Inheritance
Law of Dominance:
In a cross between organisms pure for contrasting traits, only one trait (dominant) appears in the F₁ generation.
The suppressed trait is termed recessive.
Law of Segregation:
Alleles remain distinct and segregate during gamete formation.
This principle is also known as the Law of Purity of Gametes.
Law of Independent Assortment:
Inheritance of one pair of contrasting traits is independent of the inheritance of another pair.
Example: Seed shape inheritance is not influenced by seed color inheritance.
Sex Determination
Mechanism: Determined by sex chromosomes:
Female: XX.
Male: XY.
Outcomes:
Fusion of X (male) with X (female) produces a girl.
Fusion of Y (male) with X (female) produces a boy.
Co-Dominance and Multiple Allelism
ABO Blood Group:
Controlled by more than one allele (multiple allelism).
Example of co-dominance: When both alleles are present (e.g., IAIB), both are expressed.
Blood Group Inheritance
Human blood groups are classified into A, B, AB, and O, determined by antigens (glycoproteins) on the surface of red blood cells (RBCs):
A: Antigen A.
B: Antigen B.
AB: Both antigens A and B.
O: No antigen.
Genetics of Blood Groups
Controlled by three alleles: IA, IB, and IO.
IA and IB: Dominant over IO.
IA and IB: Codominant (both expressed when present together).
IO: Recessive (does not produce antigens).
Genotype and Phenotype Relationship
Genotype | Phenotype (Blood Group) |
IAIA | A |
IAIO | A |
IBIB | B |
IBIO | B |
IAIB | AB |
IOIO | O |
Examples of Inheritance
Blood Group A × Blood Group O:
Parent Genotypes: IAIA × IOIO.
Offspring Genotype: IAIO (heterozygous for blood group A).
Offspring Phenotype: All offspring have blood group A.
Blood Group B × Blood Group O:
Parent Genotypes: IBIB × IOIO.
Offspring Genotype: IBIO (heterozygous for blood group B).
Offspring Phenotype: All offspring have blood group B.
Blood Group Inheritance (Extended Examples)
Homozygous Blood Group A × Homozygous Blood Group B:
Parent Genotypes: IAIA × IBIB.
Offspring Genotype: IAIB.
Result: All offspring have blood group AB.
Homozygous Blood Group A × Heterozygous Blood Group B:
Parent Genotypes: IAIA × IBIO.
Offspring Genotypes: IAIB, IAIO.
Result:
50% blood group AB.
50% blood group A (heterozygous).
Blood Group AB × Blood Group AB:
Parent Genotypes: IAIB × IAIB.
Offspring Genotypes: IAIA, IAIB, IBIB.
Result:
25% blood group A.
50% blood group AB.
25% blood group B.
Evolution
Definition:
Gradual development of complex life forms from simpler ones over geological time.
Described as "Descent with modification."
Darwinism
Charles Robert Darwin:
Visited regions including South America, South Africa, Australia, and the Galápagos Islands.
Influenced by:
"Principle of Population" by Malthus.
"Principle of Geology" by Charles Lyell.
Authored The Origin of Species, introducing the Theory of Natural Selection.
Darwin’s Observations and Conclusions
Facts | Consequences (Conclusions) |
1. High reproduction rate among animals; constant species number | Struggle for existence |
2. Struggle for existence and heritable variations | Survival of the fittest (natural selection) |
3. Survival of the fittest and environmental changes | Continuous natural selection → Evolution of species |
Charles Darwin: Proposed the mechanism of new species origin via natural selection but couldn't explain the source of heritable variations.
Mutation Theory
Hugo de Vries: Dutch botanist who explained that heritable variations arise due to gene mutations in the germplasm.
Lamarckism
Proposed by Jean-Baptiste Lamarck in Philosophie Zoologique:
Theory: Inheritance of acquired characteristics.
Use or disuse of an organ causes changes inherited by offspring.
Discarded by August Weismann, who showed experimentally (cutting tails of mice over generations) that acquired traits aren't inherited.
Haeckel’s Theory of Recapitulation
Proposed Biogenetic Law: "Ontogeny (development of an individual) repeats phylogeny (evolutionary history)."
Chemical Evolution of Life
Theory:
Proposed by Oparin (1924) and J.B.S. Haldane (1929).
Life originated via chemosynthesis:
Lighter elements (H, C, N, O) on early Earth formed compounds like water, methane, ammonia, and CO₂.
Interactions formed organic molecules (e.g., sugars, amino acids, nucleotides).
Aggregation led to coacervates → Protocells (primitive cells).
Experimental Proof:
Miller-Urey Experiment (1953):
Simulated early Earth conditions using ammonia, methane, hydrogen, and water.
Exposed mixture to electric sparks and varying temperatures.
Results: Formation of organic compounds like amino acids, sugars, and purines.
Variations:
Definition: Variations are differences in the structure, function, or behavior within a species or between parents and offspring, arising due to DNA copying errors during reproduction. These can be either inheritable or non-inheritable.
Types:
Somatogenic (Acquired) Variations: Occur in somatic cells during an individual’s lifespan, influenced by environmental factors (e.g., temperature, light). They are non-inheritable.
Germinal (Blastogenic) Variations: Occur in germinal cells and are passed down to offspring. These result from mutations or gene recombination.
Genetic Drift:
A random change in gene frequency in a population over generations. Its effect is larger in small populations and leads to genetic diversity without adaptation.
Illustrations:
Population of Red Beetles: A population of 12 red beetles lives in bushes with green leaves. They reproduce sexually, leading to variations in traits.
First Situation (Green Beetle): A green beetle arises, which is harder for crows to spot and eat on green leaves. This beetle passes the green trait to its offspring, leading to a higher number of green beetles. Over generations, the frequency of green beetles increases due to the survival advantage, illustrating genetic drift and evolution.
Second Situation (Blue Beetle): A blue beetle appears, but both red and blue beetles are equally visible to crows. As crows kill most beetles, the population of blue beetles increases by chance. This situation shows how genetic drift can change gene frequency in small populations, even without survival advantage or adaptation.
Third Situation (Plant Disease): A disease reduces food for beetles, leading to poor nourishment and smaller beetles. When the disease clears, beetles regain normal size. This change in size is somatic (non-heritable) and doesn't affect the gene pool, illustrating how non-inherited changes can occur due to environmental factors.
Conclusion: Genetic drift leads to random changes in gene frequency, contributing to population diversity without adaptation. It has a larger impact on small populations and can result in traits becoming more or less common by chance.
Speciation:
Speciation is the process by which new species form from an existing species.
Biological Species Concept: A species is defined as a group of organisms that can interbreed and produce fertile offspring, but cannot normally breed with individuals of other species. Genetic exchange does not occur between species.
Process of Speciation: Speciation occurs when a population becomes reproductively isolated from other populations of the same species. This isolation prevents gene flow between them, leading to the development of independent species that are unable to interbreed.
Geographical Isolation: In this example, a large beetle population lives on a mountain range, with some beetles moving to a nearby area. Over time, the beetles in these areas become isolated due to a river, causing gene flow to decrease and eventually stop. As the populations are affected by genetic drift and natural selection, they evolve separately and become reproductively isolated, eventually forming two new species. Speciation can also occur due to changes in chromosome number.
Microevolution: Small genetic changes can lead to significant evolutionary effects, influencing speciation.
Factors of Speciation: Inbreeding, genetic drift, and natural selection contribute to speciation in sexually reproducing animals. However, geographical isolation does not play a role in the speciation of asexually reproducing animals and self-pollinating plants.
Conclusion: Speciation is the formation of new species through isolation and genetic divergence, with geographical isolation and reproductive barriers being key factors, especially in sexually reproducing organisms.
Tracing Evolutionary Relationships:
Homologous Organs: These organs share a similar basic structure and common origin but have evolved to perform different functions. Homologous organs indicate divergent evolution, helping identify evolutionary relationships between species.
Example: The forelimbs of reptiles, frogs, lizards, birds, and humans are homologous organs. Despite their differences in appearance and function, they share a common origin.
Forelimb in Vertebrates:
Seal: Flippers (Swimming)
Bird: Wings (Flying)
Bat: Patagia (Support and flying)
Horse: Elongated limbs (Running)
Human: Thumb (Grasping)
Plant Example: Leaves in plants can also be homologous. Cactus spines and pea tendrils are both modified leaves, despite their differences, because they originated from the same organ.
Analogous Organs: These organs have different origins and structural plans but perform similar functions. This relationship is known as convergent evolution.
Example: The wings of insects, birds, and bats are analogous organs. Though they serve the same purpose (flying), they evolved independently.
Other Example: Sweet potato and potato both have tubers for food storage, but the sweet potato’s tuber is a root structure, while the potato’s tuber is a stem structure. This is an example of analogy, where similar functions arise due to different evolutionary paths, not common ancestry.
Conclusion: Homologous organs reveal divergent evolution, showing a shared ancestry, while analogous organs highlight convergent evolution, where different species develop similar functions independently
Fossils:
Fossils: Fossils are the remains or impressions of hard parts of extinct organisms preserved in sedimentary rock or other media. They provide evidence of life from past geological periods.
Formation of Fossils:
Fossils form over millions of years through layers of sediment. For example, invertebrates die and are buried in sand, with soft parts decomposing, leaving only skeletons. Over time, more sand accumulates, forming sandstone under pressure.
Later, dinosaurs and other creatures die and their remains get buried in mud, which also turns into rock. Over time, further layers of rock form above older ones. Eventually, erosion exposes these fossils at the surface, allowing for their discovery.
As deeper layers are dug, older fossils are found.
Living Fossils: Some species, like the Ginkgo plant, Limulus (horseshoe crab), and Peripatus (a velvet worm), have changed very little over long periods, surviving relatively unchanged for millions of years.
Determining the Age of Fossils:
Relative Method: Fossils found in deeper layers of rock are generally older than those found closer to the surface. This allows for the relative dating of fossils based on their position in the earth's layers.
Radioactive Dating: Some rocks contain radioactive elements that decay at a constant rate. By measuring the decay of these elements, the age of fossils can be determined. An example is Carbon Dating.
Carbon Dating:
The air contains radioactive carbon (C14) which is absorbed by organisms during their life. After death, the C14 decays at a constant rate with a half-life of about 5730 years.
By measuring the remaining radioactivity in a fossil, scientists can determine how long it has been since the organism died. For example, if a fossil has one-quarter of the original C14 radioactivity, it is about 11,460 years old (two half-lives).
Conclusion: Fossils provide crucial evidence of past life and are used to understand the history of life on Earth. They can be dated using relative positioning or radioactive decay, particularly through techniques like carbon dating.
Evolution by Stages:
Evolutionary Changes: Evolutionary changes are fundamental to the characteristics of living organisms, shaping their traits over time.
Convergent Evolution: This refers to the emergence of biological structures or species that share similar functions and appearances but evolved through different evolutionary paths.
Example: The eyes of insects, octopuses, and vertebrates may look similar, but they have distinct structures and separate evolutionary origins. This suggests convergent evolution, where similar traits arise independently.
Feathers: Feathers initially evolved for insulation in cold weather, but over time, they became adapted for flight in birds.
Example: Dinosaurs had feathers, but they were not used for flight. Later, birds evolved the ability to fly using feathers, showing a close evolutionary relationship between birds and reptiles.
Artificial Selection: Humans have created different types of vegetables from the wild cabbage by selectively breeding plants. This process, known as artificial selection, alters the genetic makeup of a population through selective breeding.
Conclusion: Evolution by stages shows how traits can evolve for different purposes over time, with convergent evolution highlighting similar traits in different species, and artificial selection showing human influence on evolution.
Human Evolution:
Human Evolution: The evolutionary history of humans has been built through the study of fossils (paleontology) and molecular biology, particularly DNA changes.
Anthropology: This branch of science is dedicated to studying and tracing human evolution.
Origin of Humans: The study of human evolution shows that all humans belong to a single species, Australopithecus africans, which evolved in Africa and spread across the world in stages.
Conclusion: Human evolution is understood through fossil records and genetic studies, with Australopithecus africans being the common ancestor of all modern humans.