Heredity and Variation
Heredity is the transmission of traits or characteristics from parents to their offspring. Variation refers to the differences in characters among the individuals of a species. Variations are primarily accumulated during sexual reproduction due to the mixing of DNA from two parents and slight inaccuracies in DNA copying, providing an evolutionary advantage by allowing species to adapt to changing environments.
Mendel's Contributions to Genetics
Gregor Johann Mendel, the "Father of Genetics," used pea plants (Pisum sativum) to study the inheritance of traits. He selected pea plants because they have easily observable contrasting characters (like tall/short stems, round/wrinkled seeds, yellow/green seeds) and a short life cycle.
- Monohybrid Cross: Mendel crossed a pure tall plant (TT) with a pure short plant (tt). In the first generation (F1), all plants were tall (Tt), showing that the "tall" trait was dominant and the "short" trait was recessive. When the F1 plants were self-pollinated, the second generation (F2) produced tall and short plants in a 3:1 ratio. This led to the Law of Segregation.
- Dihybrid Cross: Mendel crossed plants with two pairs of contrasting traits (e.g., Round-Yellow seeds and Wrinkled-Green seeds). The F2 generation showed four different combinations of traits in a 9:3:3:1 ratio, demonstrating the Law of Independent Assortment—meaning traits are inherited independently of one another.
Sex Determination in Humans
Human cells contain 23 pairs of chromosomes. 22 pairs are autosomes, while the 23rd pair consists of the sex chromosomes. Females have a perfect pair of sex chromosomes (XX), while males have a mismatched pair (XY). All eggs produced by a female carry an X chromosome. Sperm produced by a male can carry either an X or a Y chromosome. If an X-bearing sperm fertilizes the egg, the child will be female (XX). If a Y-bearing sperm fertilizes the egg, the child will be male (XY). Thus, the sex of the child is determined by the father's genetic contribution.
The Concept of Evolution
Evolution is the gradual process by which the present diversity of plant and animal life arose from the earliest and most primitive organisms. Charles Darwin proposed the theory of evolution by Natural Selection. Nature selects individuals with favorable variations that are best suited to the environment; these individuals survive, reproduce, and pass on those favorable traits.
Speciation and Evidences of Evolution
Speciation is the origin of new species from existing ones, usually caused by geographical isolation (e.g., a mountain or river separating a population), genetic drift, and natural selection. Evidences for evolution come from:
- Homologous Organs: Organs with the same basic structural design and origin but different functions (e.g., the forelimbs of humans, birds, and bats). This indicates common ancestry.
- Analogous Organs: Organs with different structures but similar functions (e.g., wings of birds and insects). This indicates convergent evolution.
- Fossils: The preserved remains or impressions of prehistoric organisms in sedimentary rock. They provide direct evidence of organisms that existed in the past and the missing links between different evolutionary stages (e.g., Archaeopteryx, a link between reptiles and birds).