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Chromosomes and Genes: An Introduction

Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope.  Chromosome is composed of DNA coiled and condensed around nuclear proteins called histones. Humans inherit one set of chromosomes from their mother and a second set from their father. In total, most human cells contain 46 chromosomes with 22 pairs of autosomes, or non-sex chromosomes, and two sex-determining chromosomes. The sex chromosomes in humans are called X and Y. Females carry two X chromosomes, while males carry one X and one Y chromosome. Cells of the body that contain two sets of chromosomes are called diploid.   Gene: It is a linear sequence /segment of  DNA  that provides the coded instructions for synthesis of RNA, which, when translated into protein, leads to the expression of hereditary character. ...
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Natural Selection

Natural selection is one of the basic mechanisms of evolution, along with mutation, migration, and genetic drift  ( Berkeley )  It is a  process  in  nature  in which  organisms  possessing certain genotypic characteristics that make them better adjusted to an  environment  tend to survive, reproduce, increase in number or  frequency , and therefore, are able to  transmit  and perpetuate their essential genotypic qualities to succeeding  generations . This theory was put forward by Charles Darwin, trying to explain how evolution of species occurs . Look at an example to help make the concept natural selection clearer:  Industrial melanism is a phenomenon that affected over 70 species of moths in England. It has been best studied in the peppered moth,  Biston betularia . Prior to 1800, the typical moth of the species had a light pattern. Dark colored or melanic moths were rare and were therefore c...
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Sex Chromosomes

It's not quite accurate to say that everyone inherits two copies of every gene. In most mammals, the X and Y sex chromosomes determine whether an individual is male or female. Females have two X chromosomes, and therefore two copies of every gene. Males, however, have one X and one Y chromosome. For genes that appear only on the X chromosome or only on the Y (some do appear on both), males inherit just one copy. Taken from http://www.eschooltoday.com/science/genetics/images/sex-chromosomes.jpg Other living things have sex chromosomes too. Birds and reptiles have Z and W sex chromosomes. Unlike with X and Y, males have two Z chromosomes, and females have one Z and one W. Some insects and a few mammals have only an X chromosome. Females have two copies and males have one. However, sex is not always determined by chromosomes. With alligators, crocodiles, and most turtles, it's egg incubation temperature. And some fish can change gender in response to cues from the en...
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