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Chromosomes & Inheritance
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How are Chromosomes, Genes, and Inheritance Related?
Mendel’s work showed that genes exist. But scientists in the early 20th century did not know what structures in cells contained genes. The search for the answer was something like a mystery story. The story could be called “The Clue in the Grasshopper’s Cells.”
At the start of the 1900s Walter Sutton, an American geneticist, studied the cells of grasshoppers. He wanted to understand how sex cells (sperm and eggs) form. Sutton focused on how the chromosomes moved within the cells during the formation of sperm and eggs. He hypothesized that chromosomes are the key to learning how offspring have traits similar to those of their parents.
Sutton needed evidence to support his hypothesis. To his surprise, he discovered that grasshopper sex cells have exactly half the number of chromosomes found in grasshopper body cells. He observed what happened when a sperm and an egg cell joined. The fertilized egg that formed had 24 chromosomes. It had the same number of chromosomes as each parent. These 24 chromosomes existed as 12 pairs. One chromosome in each pair came from the male parent and the other came from the female parent.
Recall that alleles are different forms of a gene. Because of Mendel’s work, Sutton knew that alleles exist in pairs in an organism. One allele comes from the female parent and the other comes from the male parent. Sutton realized that paired alleles are carried on paired chromosomes. His idea is now known as the chromosome theory of inheritance. According to the chromosome theory of inheritance, genes pass from parents to their offspring on chromosomes.
The body cells of humans contain 46 chromosomes that form 23 pairs. Chromosomes are made up of many genes joined together like beads on a string. Although you have only 23 pairs of chromosomes, your body cells contain between 20,000 and 25,000 genes. Genes control traits.
The figure shown here shows a pair of chromosomes from an organism. One chromosome if from the female parent and the other from the male parent. Notice that each chromosome has the same genes. The genes are lined up in the same order on both chromosomes. However, the alleles for some of the genes are not identical. For example, one chromosome has allele A and the other has allele a. As you can see, this organism is heterozygous for some traits and homozygous for others.
What happens during Meiosis?
How do sex cells end up with half the number of chromosomes as body cells? The answer to this question is a form of cell division called meiosis. Meiosis is the process by which the number of chromosomes is reduced by half as sex cells form. You can trace the events of meiosis in the figure shown below. Here the parent cell has four chromosomes arranged in two pairs. During meiosis, the chromosome pairs separate into two different cells. The sex cells that from later have only half as many chromosomes as the other cells in the organism.
During meiosis, the chromosomes duplicate only before the first cell division and then the cell divides into two cells. Then each of these cells divides again, forming a total of four cells. Each of these four cells shown below receives two chromosomes—one chromosome from each parent in the original cell. When two sex cells join at fertilization, the new cell that forms has the full number of chromosomes. In this case, the number is four. The organism that grows from this cell got two of its chromosomes from one parent and two from the other parent.
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