Chapter - 9 Heredity and Evolution

• The rules of heredity determine the process by which traits and characteristics are reliably inherited. Variations arising during the process of reproduction can be inherited.
• These variations may lead to increased survival of the individuals.
• Cellular DNA is the information source for making proteins in the cell. A section of DNA that provides information for one protein is called the gene for that protein.
• Each cell will have two copies of each chromosome, one each from the male and female parents. Every germ cell will take one chromosome from each pair and these may be of either maternal or paternal origin. When two germ cells combine, they will restore the normal number of chromosomes in the progeny, ensuring the stability of the DNA of the species.
• Sexually reproducing individuals have two copies of genes for the same trait. If the copies are not identical, the trait that gets expressed is called the dominant trait and the other is called the recessive trait.
• Traits in one individual may be inherited separately, giving rise to new combinations of traits in the offspring of sexual reproduction.
• Sex is determined by different factors in various species. In human beings, the sex of the child depends on whether the paternal chromosome is X (for girls) or Y (for boys).
• Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair, called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes, both called X. But men have a mismatched pair in which one is a normal-sized X while the other is a short one called Y. So women are XX, while men are XY.
• Variations in the species may confer survival advantages or merely contribute to the genetic drift.
• Changes in the non-reproductive tissues caused by environmental factors are not inheritable.
• Speciation may take place when variation is combined with geographical isolation.
• Evolutionary relationships are traced in the classification of organisms. The frequency of an inherited trait changed over generations. Since genes control traits, we can say that the frequency of certain genes in a population changed over generations. This is the essence of the idea of evolution.
• Tracing common ancestors back in time leads us to the idea that at some point of time, non-living material must have given rise to life.
• Evolution can be worked out by the study of not just living species, but also fossils.
• Complex organs may have evolved because of the survival advantage of even the intermediate stages.
• Organs or features may be adapted to new functions during the course of evolution. For example, feathers are thought to have been initially evolved for warmth and later adapted for flight.
• Evolution cannot be said to ‘progress’ from ‘lower’ forms to ‘higher’ forms. Rather, evolution seems to have given rise to more complex body designs even while the simpler body designs continue to flourish.

Study of the evolution of human beings indicates that all of us belong to a single species that evolved in Africa and spread across the world in stages.