10.5 Heredity & Evolution

10.5 Heredity & Evolution

Why It Matters

Children resemble their parents, but they are not identical to them. The transmission of traits from one generation to the next is called heredity, and the differences between individuals of the same species are called variation. Together, heredity and variation give rise to evolution — the gradual change in the inherited traits of a population over generations.

Mendel and the Pea Plant

Gregor Johann Mendel (1822–1884), an Austrian monk, is called the Father of Genetics. He performed experiments on garden pea plants (Pisum sativum) in his monastery garden and worked out the basic laws of inheritance.

Mendel chose the pea plant because:

• It grows quickly and easily.
• It has several clearly contrasting traits (tall vs dwarf, round vs wrinkled seeds, yellow vs green seeds, purple vs white flowers, inflated vs constricted pods).
• It normally self-pollinates, so Mendel could control crosses by removing the anthers and dusting pollen from another plant by hand.
• It produces many offspring in one generation.

Mendel's monohybrid cross (one trait at a time): He crossed a pure tall plant (TT) with a pure dwarf plant (tt). The F1 generation was all tall. He let the F1 plants self-pollinate; the F2 generation had a ratio of about 3 tall : 1 dwarf. From this he concluded:

• Traits are controlled by factors (we now call them alleles of a gene).
• Each parent contributes one factor for each trait.
• The factor for tallness (T) is dominant and the factor for dwarfness (t) is recessive — so the heterozygous plant (Tt) looks tall.

A Punnett square shows the combinations:

Result: 3 tall (TT, Tt, Tt) : 1 dwarf (tt). Genotype ratio 1 TT : 2 Tt : 1 tt. Phenotype ratio 3 : 1.

Dihybrid cross (two traits together, e.g., round/yellow seeds RR YY × wrinkled/green seeds rr yy) gives a 9:3:3:1 phenotype ratio in F2, showing that the two traits are inherited independently (Mendel's Law of Independent Assortment).

Sex Determination in Humans

Humans have 23 pairs of chromosomes. 22 pairs are called autosomes and are the same in males and females. The 23rd pair are the sex chromosomes:

• Females have two X chromosomes (XX).
• Males have one X and one Y chromosome (XY).

When gametes form, the parents' chromosome pairs separate. A mother always gives an X chromosome to her child (because she has only X). The father gives either an X (producing a girl, XX) or a Y (producing a boy, XY). Therefore the father's gamete determines the sex of the child. This is a frequent RRB question.

Evolution

Evolution is the gradual change in the inherited traits of a population of organisms over successive generations, giving rise to new species from common ancestors.

Charles Darwin proposed the theory of evolution by natural selection (in On the Origin of Species, 1859). The key ideas:

1. Within any population, there is variation (some individuals are faster, taller, better camouflaged, etc.).
2. More offspring are produced than can survive (over-production).
3. Individuals with traits suited to the environment are more likely to survive and reproduce ('survival of the fittest').
4. These useful traits are inherited by their offspring, so the population gradually changes.

Jean-Baptiste Lamarck proposed an earlier theory: the inheritance of acquired characteristics. He suggested, for example, that giraffes stretched their necks to reach high leaves and passed the long neck on to their young. This theory is now discredited because acquired traits (e.g., a bodybuilder's muscles, a lost limb) are not inherited — only changes in the genetic material (DNA) are passed on.

Speciation is the formation of a new species. It happens when populations of the same species become reproductively isolated for a long time (e.g., by a geographic barrier like a river or mountain) and accumulate enough genetic differences that they can no longer interbreed.

Evidence for Evolution

1. Fossils — preserved remains (bones, teeth, footprints) of organisms that lived long ago, formed when organisms get buried in mud/sediment and mineralized over thousands of years. The deeper the rock layer, the older the fossil. Fossils give a time sequence of how organisms changed. A famous example is the fossil Archaeopteryx — a link between reptiles and birds (it had feathers and wings but also teeth and a long bony tail).
2. Homologous organs — organs with the same basic structure but different functions, suggesting common ancestry. E.g., the forelimbs of a human, frog, bird and horse all have the same set of bones (humerus, radius, ulna, carpals) but are used for grasping, hopping, flying and running respectively. This is called divergent evolution.
3. Analogous organs — organs with different structures but similar functions, suggesting the organisms are NOT closely related but adapted similarly. E.g., the wings of a bird and a butterfly are both used for flying but have totally different structures. This is called convergent evolution.
4. Vestigial organs — organs that are reduced and have no clear function in the present organism but were useful in ancestors. E.g., the human appendix and the small hind limb bones of whales and snakes.

Exam Traps

1. Sex determination: it is the father (sperm) who determines the sex of the child, not the mother. The mother always contributes an X.
2. Mendel's ratio — monohybrid F2 = 3:1 (phenotype), dihybrid F2 = 9:3:3:1. Don't mix them up.
3. Homologous vs analogous: same structure = homologous (common ancestor); same function but different structure = analogous (no recent common ancestor).
4. Lamarck is discredited — acquired traits are NOT inherited. Darwin's natural selection is the accepted mechanism.
5. Evolution does NOT produce 'better' organisms — it produces organisms better suited to their CURRENT environment. If the environment changes, today's 'fittest' may become tomorrow's weakest.