Comprehensive theory, key formulas, diagrams, and memory aids for Heredity and Evolution.
We have seen that reproduction gives rise to new individuals that are similar to the parents, but with some subtle differences. These similarities are passed down through a process called heredity, while the gradual accumulation of differences over millions of years leads to evolution.
Heredity is the transmission of traits (characteristics) from parents to offspring. It is the reason why dogs have puppies that look like dogs, and why you might have your mother's eyes or your father's nose.
The information for these traits is carried in our DNA. A specific section of DNA that provides the information for one protein (which in turn determines a trait) is called a gene.
Gregor Mendel, an Austrian monk, is considered the "Father of Genetics". In the 19th century, he conducted extensive experiments on garden peas (Pisum sativum) to understand how traits are inherited. He chose peas because they have distinct, easily observable traits (like tall/short plants, round/wrinkled seeds) and have a short life cycle.
Mendel's Monohybrid Cross: Mendel crossed a pure tall pea plant (TT) with a pure short pea plant (tt). * F1 Generation: All the plants in the first generation (F1) were tall. The short trait seemed to have disappeared! * F2 Generation: He then allowed the F1 tall plants to self-pollinate. In the second generation (F2), he observed both tall and short plants in a strict ratio of 3:1 (3 tall for every 1 short).
Conclusions from the Monohybrid Cross: 1. Traits are controlled by units called 'factors' (now called genes). 2. These genes occur in pairs (alleles) in an individual. For example, 'T' for tallness and 't' for shortness. 3. Law of Dominance: In a dissimilar pair of genes (Tt), one gene dominates over the other. The expressed trait (Tall, 'T') is dominant, and the suppressed trait (short, 't') is recessive. A recessive trait is only expressed when both genes are recessive (tt).
Cellular DNA is the information source for making proteins in the cell. If the gene for a specific enzyme is working efficiently, plenty of that enzyme is made, and a specific reaction (e.g., producing a hormone that triggers plant growth) happens efficiently, resulting in a tall plant. If the gene has a mutation and the enzyme is less efficient, the plant remains short. Thus, genes control traits by controlling protein synthesis.
In some animals, sex is determined by environmental factors (like the incubation temperature in some turtles). In humans, sex is determined genetically by specific chromosomes called sex chromosomes. * Humans have 23 pairs of chromosomes. 22 pairs are perfectly matched in both males and females (autosomes). * The 23rd pair is the sex chromosomes. * Females have a perfect pair of X chromosomes (XX). * Males have a mismatched pair (XY). * During reproduction, a child receives one sex chromosome from the mother (always an X) and one from the father (either X or Y). * If the sperm carries an X chromosome, the child will be a girl (XX). If the sperm carries a Y chromosome, the child will be a boy (XY). Therefore, it is the father's genetic contribution that determines the sex of the baby.
Evolution is the gradual change in the inherited characteristics of biological populations over successive generations. It is the process by which modern organisms have descended from ancient ancestors.
Evolution only acts on inherited traits.
A species is a group of organisms that can interbreed to produce fertile offspring. Speciation is the evolutionary process by which new biological species arise. It generally occurs through: 1. Geographical Isolation: A population is physically separated by a barrier (river, mountain). 2. Genetic Drift: Random changes in gene frequency in small populations. 3. Natural Selection: Over time, the isolated populations adapt to their distinct environments. Nature "selects" the traits best suited for survival in each environment. Eventually, the two populations become so genetically different that they can no longer interbreed, forming two new species.
How do we know which organisms are closely related? We look for common characteristics. * Homologous Organs: Organs that have the same basic structural design and origin but perform different functions. (E.g., the forelimbs of a frog, a lizard, a bird, and a human). They indicate a common ancestry. * Analogous Organs: Organs that have different basic structures but perform similar functions. (E.g., the wings of a bat and the wings of a bird). They do not indicate a common ancestry but rather adaptation to a similar environment. * Fossils: The preserved remains or traces of dead organisms from the past. Fossils (like Archaeopteryx, which had reptilian teeth but bird-like feathers) provide direct evidence of evolutionary stages and missing links.
By studying DNA sequences and fossils, scientists have traced human evolution. The roots of human evolution lie in Africa. All modern humans (Homo sapiens) belong to a single species, regardless of skin color or ethnicity, and share a recent common ancestor in Africa. From there, early humans migrated across the globe, adapting to various environments, but remaining genetically one species.
Heredity explains how we are linked to our immediate parents through the precise copying and passing of DNA (genes). Mendel's laws showed us the mathematical predictability of this inheritance. However, DNA copying is not always flawless. These small errors (variations), when subjected to the harsh realities of environmental survival (Natural Selection), slowly accumulate over geological time scales. This interplay between the rigidity of heredity and the flexibility of variation drives the grand process of Evolution, connecting every living organism on Earth to a common, ancient past.