Heredity

Heredity is the transmission of physical qualities and other characteristics of plants and animals from parent to offspring. Chromosomes, which are part of cells, are responsible for passing on traits from one generation to the next. The genes that comprise the chromosomes carry material that controls what type of plant or animal results and every detail of its formation and potential.

Overview

For thousands of years, people have noticed similarities among family members. A child looked just like his father. His sister was born with her grandmother's musical talent. But it is only within the past 150 years that scientists have unraveled the mystery of heredity and begun to understand the human genetic makeup. Before 1866, when Gregor Mendel presented his study of heredity in pea plants, scientists theorized that traits of both parents blended together to produce offspring that were a mixture of both parents. However, blending did not account for many traits, such as eye color and hereditary diseases. Further studies determined that not only were specific traits passed onto offspring, but also that some traits were more likely to show up than others. Researchers continued to solve pieces of the genetic puzzle and learned to predict the occurrence of certain traits, such as hereditary disease.

Early Studies

In the nineteenth century, most scientists believed that the characteristics of offspring were a combination of their parents' characteristics. For example, if a red rose and a white rose were crossbred, it would result in a pink rose. Gregor Mendel (1822-1884), however, believed that offspring could inherit a given attribute from either one parent or the other, or from both. He experimented with pea plants, tracking seven attributes such as flower color, seed shape, and plant height. He saw that some traits always came through to the next generation and others did not. He also saw that different offspring inherited different combinations of traits. By recording the results of crossing thousands of plants, he proved that some traits were dominant, always appearing, and some were recessive, showing up only when both parents passed on the attribute.

Thomas Hunt Morgan (1866-1945) later showed that chromosomes, structures located in the nucleus of cells, were made up of sequences of genes that carried the traits Mendel had studied. Morgan studied mutations, or spontaneous changes in genes, through extensive work with fruit flies. Although most of the fruit flies in his study had red eyes, one developed with white eyes. Morgan observed and recorded when white eyes appeared in some of the later generations. Subsequent studies identified alternative forms of genes, called alleles. When a pair of alleles is identical, the organism is homozygous for that trait. If the alleles are different, the organism is heterozygous. In a heterozygous organism, the traits of dominant alleles override those of recessive alleles.

The Punnett Square

The geneticist Reginald Punnett designed the familiar square scientists and students use to predict patterns of heredity when pairing homozygous and heterozygous parents. Working with one gene or trait, the researcher lists the alleles of one parent at the top of the square and the alleles of the other parent at the side. As he or she computes the possible combinations, they are inserted inside the boxes to predict the outcomes in offspring. For example, if "A" is the code for dominant brown eyes, then "a" is the code for recessive blue eyes. A heterozygous brown-eyed parent (Aa) and a homozygous blue-eyed parent (aa) would show two squares with "Aa" and two with "aa," meaning that of four offspring, two would likely have brown eyes and two would likely have blue eyes. Such information is particularly useful in predicting the likelihood of offspring inheriting a genetic disease.

Using Genetic Information

Doctors studying heredity have found that some diseases or health conditions are related to mutations in one or more genes. When scientists identify genes that cause or predispose a person to a disease, they are able to help people plan for or avoid the disease. Although certain types of cancers tend to run in families, for example, it is not a simple matter of inheriting a single gene and therefore getting cancer. Instead, several genes are involved, and factors such as diet and environment can significantly affect the outcome. Doctors who identify a cancer gene can advise the patient of ways to minimize the risk of developing the disease.

Other diseases may not affect the carrier but can be inherited by offspring. When the disease genes are recessive, as many are, they might not reveal themselves for generations. Only when both parents carry the gene and only when the two recessives are combined does the disease appear. Because some genetic diseases are much more common within certain ethnic groups, affected couples may choose to undergo genetic screening before deciding to have children. For example, cystic fibrosis is found more often among descendants of northern Europeans, while Tay-Sachs disease is more common among eastern European Jews.

Some people pursue genetic testing for other reasons, such as to establish ancestry. Especially in situations in which an individual knows little of his or her heritage, genetic ancestry testing can provide an estimate of a person's ethnic background. For instance, an American in the twenty-first century might be found to have ancestors who are 50 percent Asian, 40 percent European, and 10 percent Native American; another could be 70 percent African, 20 percent European, and 10 percent unknown. People who want to learn more about their ancestors then have a better understanding of where to start on their quest.

Bibliography

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