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The basis units of inheritance are the genes. These are found
in rod-shaped structures called chromosomes in the nuclei of nearly all cells. Humans have
23 pairs of chromosomes for a total of 46. The only human cells that don't have 23 pairs
of chromosomes are the sex cells, the sperm and the egg. They have 23 unpaired
chromosomes. Each parent contributes 23 to its offspring for the total of 46 chromosomes.
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 chromosomes
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Chromosomes are able to make copies of themselves so that when a cell splits as in mitosis, the new cells will have the same chromosomes. The more complex an organisim is, the more chromosomes it will have. For example, humans are complex organisims and have 46 chromosomes where bacteria have only one.
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More complex organisims not only have more chromosomes, but also tend to reproduce in a sexual way. Many plants don't have clear sex chromosome differences. In many animals, special chromosomes determine the sex of an animal. These special chromosomes are called the X and Y. In a man's cells, the twenty- third pair of chromosomes are made up of one that looks like an X and another that looks like a Y. In a woman's cell, the twenty-third pair of chromosomes both look like an X. |
The twenty-third chromosome in a womans' cell will always be an X, but the twenty-third chromosomes in a mans' cell can be an X or a Y. If the father contributes an X chromosome to his offspring, the baby will be female. If the father contributes a y chromosome to his off-spring, the child will be a male. In this way, it is the fathers' genetic contribution that decides the sex of the young.
CELLS | CHROMOSOMES | DNA | QUIZ
male + female chromosomes