Chromosomes

Chromosomes are large pieces of DNA that contain genes, other sequences of DNA, and associated proteins. Genes code for traits, and many of the additional sequences and associated proteins, are used to organize and to regulate the expression of the genes.

A typical human cell contains 46 chromosomes, which if stretched out and laid end to end, would be about 1 meter long. There are 23 different types of chromosomes, each containing a different set of genes. Each person has two copies (one from one’s mother and the other from one’s father) of each type of chromosome. The two copies contain the same genes in the same order but may carry different forms of those genes (alleles). Thus, any individual may have two identical copies of a gene (homozygous) or may have two different forms of the gene (heterozygous). Since genes are a sequence of DNA, different alleles vary in their DNA sequence. These variations can affect the presence, severity, or intensity of a particular trait in an individual.

The  sex  chromosomes  make  up  one  set  of  the 23 kinds of chromosomes. There are two forms of the sex chromosome, the X and the Y chromosome. If a zygote receives two X chromosomes, the individual will become female; if a zygote receives one X and one Y chromosome, the individual will become male. The X and Y chromosome pair differs from the other pairs in that they do not contain all of the same genes. The Y chromosome is smaller than the X chromosome and contains genes that determine maleness. Since women have two X chromosomes in their cells and only one is needed for proper functioning, one of the X chromosomes is randomly inactivated in each of the woman’s cells.

During reproduction, gametes are formed that contain one copy of each of the chromosome types. This ensures that each new offspring will receive one copy of each chromosome from each parent. Since males contain one X and one Y chromosome, half of their sperm will carry an X and half will carry a Y chromosome. All eggs carry an X chromosome. The sex of the child is then determined by whether the sperm that fertilized the egg carries an X or a Y chromosome.

Because each parent gives one of each of their chromosomes to each child, there is a wide variety of different combination of genes, and this genetic variability results in the large diversity of people, even those in the same family. Since each child randomly receives half of his or her mother’s genes and half of his or her father’s genes, siblings share one fourth of their  genes  in  common  with  each  other. The  only exception to this is in identical twins, who share 100% of their genes because they arise from a single fertilized egg that splits into two embryos.

Genetic diseases may arise from variant or mutant forms of genes. Many genetic diseases are recessive, meaning that both copies of the gene have to be mutant for the disease to occur. People that have one copy of the mutant and one copy of the normal gene are called carriers, and there is a 50% chance that they will pass the mutant allele to each of their offspring. There are also some dominant diseases that result from a mutation in only one copy of the gene involved in the disease. If a parent has a dominant disease, there is a 50% to 100% chance that he or she will pass this allele to his or her offspring.

References:

1. Farlex, Inc. (n.d.). Chromosome. Retrieved from http://encyclothefreedictionary.com/chromosome
2. Pierce,  (2002).  Genetics:  A  conceptual  approach.  San Francisco: W. H. Freeman.