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Inheritance and Mendelian Genetics

Factors That Increase Genetic Diversity

Independent Assortment and Crossing Over

Genetic diversity is increased by independent assortment (genes are inherited independently of each other) and crossing over during meiosis.

A chromosome is a structure that contains DNA, the genetic material that is passed from one generation to the next. Meiosis is the process of cell division during which the number of chromosomes decreases to half the original number by two divisions of the nucleus, resulting in the production of gametes. During meiosis, chromosomes (which are found in pairs) swap large portions of their molecules, causing genetic material to be mixed between them. Crossing over increases genetic diversity in offspring, which in turn may increase the offspring's chances for survival in a changing environment because it is more likely that a unique trait may be necessary for an organism to thrive in an environment different from the one its parents lived in.

Gregor Mendel, the Austrian monk who discovered the underlying principles of genes and inheritance, did not study only one trait; in total, he examined seven different traits in pea plants. He observed that inheritance of alleles for one trait was independent of inheritance of alleles for a different trait. That is, the genes for different traits did not affect each other. For example, one plant might have round green peas, while another had wrinkled yellow peas. Crossing these plants may yield plants with wrinkled green peas or round yellow ones. The genes for pea shape and pea color are independent of each other and are passed to offspring independently of each other. This is known as the law of independent assortment, which states that genes are sorted independently during gamete (sperm and egg) formation and thus have equal chances of occurring together. Today, scientists know that chromosomal crossing over, a process in which chromosomes swap portions of their arms, resulting in the redistribution of genetic material, also occurs during meiosis and increases genetic diversity.

Chromosomal Crossing Over

Chromosomal crossing over increases genetic diversity in offspring. Here, the chromosome coding for dominant alleles A, B, and C crosses over with the chromosome coding for recessive alleles a, b, and c. The new chromosomes have a mixture of dominant and recessive alleles.

Linked Genes

Genes that lie close to each other on chromosomes tend to be inherited together.
There are exceptions to the law of independent assortment. Genes that are frequently inherited together (more than 50 percent of the time) are called linked genes. This means each gene is closely associated with other genes. This happens when genes are close to each other on a chromosome. Although chromosomal crossing over (a process in which chromosomes swap portions of their arms, resulting in the redistribution of genetic material) between these genes can happen, it is infrequent. Genes are said to be linked if the crossover between them occurs less than 50 percent of the time.

Linked Genes

Linked genes are located close to each other on the chromosome, making crossover between them unlikely. Here, genes B and C are linked because they are quite close to each other. Gene A is not linked to B or C; thus, because it is far away, crossover is more likely.
Other genes have a sex-linked trait, a trait that is inherited on either the X or the Y chromosome. These are the chromosomes that code for primary and secondary sex characteristics. Females typically have two of the same kind of sex chromosome (XX), while males typically have two different kinds of sex chromosomes (XY). Examples of sex-linked genes in humans include red-green colorblindness (X-linked) and hemophilia (X-linked). Few Y-linked genes exist because the Y chromosome is much smaller than the X chromosome and thus carries fewer genes on it. Because males inherit only a single X chromosome rather than the two that females inherit, rates of X-linked disorders (such as red-green colorblindness and hemophilia in humans) are higher in males than in females.