How does genes determine the traits of an organism

Go Paperless with Digital. Stuart E. Ravnik, assistant professor of cell biology and biochemistry at the Texas Tech University Health Sciences Center, summarizes the answer to this seemingly simple question: Image: Nikolaj Blom and Michael Lappe. Get smart. Sign Up. Support science journalism. Knowledge awaits. See Subscription Options Already a subscriber?

Create Account See Subscription Options. Continue reading with a Scientific American subscription. Subscribe Now You may cancel at any time. Human DNA comes in 23 pairs of packages called chromosomes. These chromosomes are large bundles of tightly packed DNA.

Your mother and father each donate 23 chromosomes, which pair up to give you your full set of 23 chromosomes.

Within these 23 pairs of chromosomes, there ar e certain sections that determine different physical features. These sections of DNA th at contain information that determine your physical features are called genes.

Since you have two pairs of chromosomes, you also have two pairs of genes, one from your fath er and one from your mother. These pairs of genes then determine certain physical features or traits.

The genes that you have in your body right now make up your genotype. This genotype then determines your physical appearance, which is called your phenotype. In this activity, you will be given two sets of chromosomes.

One set is labeled male chromosomes while one is labeled fema le chromosomes. You will drop these chromosomes from above your head and they will randomly mix in different ways giving you a genotype. From this genotype, you will then have the detailed instructions to make a sketch of a human face. Before you begin, you should know a few more things about how genes determine your appearance. Genes can come in two different forms or alleles. A gene can be either dominant or recessive.

In this activity, dominant forms of a ge ne appear in capital letters while recessive forms of a gene a ppear in lower case letters. Since you get one gene from your mother and one from your father for each trait, you may have a combination of dominant and r ecessive genes for each trait. When both forms of a gene are the same either both dom inant or both recessive you are said to be homozygous for that trait. If you have one dom inant gene and one recessive gene, you are said to be heterozygous for that trait.

One final thing before you begin the activit y. As you will see in the activity, when you receive the dominant form of a gene whether homozygous or heterozygous, you will express the dominant form of the gene. Y ou will only express the recessive form of the gene if you receive the recessive form from both of your parents, thus being homozygous for the recessive form. Finally, this information should provide you with the basics of how appearance is determined by DNA.

If you are a bit confused, follow the steps of the activity and many concepts above will be seen. By performing the activity, you will be able to see exactly what is meant by some of the terms me ntioned above.

Good Luck creating your offspring! This activity requires the use of sharp scissors to cut out the chromosomes. Use caution when using scissors. Ask an adult to help you if necessary. After this activity, you should be able to understand how DNA determines your appearance. Remember DNA is condensed into chromosomes. You have 23 pairs of chromosomes, 23 from your mother and 23 from your father. Although these scientists' observations connected genes to chromosomes, they still didn't use the word "gene" to represent what Mendel called "elementen" or what Darwin called "gemmules.

Scientists now knew how chromosomes behaved during both mitosis and meiosis, but they still hadn't linked Mendel's ideas of heredity with these observations. Some thirty-five years after Mendel's work, however, American researcher Walter Sutton Figure 4 proposed a connection between trait inheritance and the path that chromosomes travel during meiotic cell division and gamete formation. In particular, when observing meiotic cells in the testes of the lubber grasshopper Brachystola magna , Sutton noted that it was possible to distinguish and track the individual chromosomes in these cells.

He also noticed that these chromosomes existed in pairs that could be distinguished from other pairs by their size, and that upon the union of two gametes during fertilization, the chromosomes in the newly fertilized cell maintained their original forms. Sutton therefore proposed that all chromosomes have a stable structure, or "individuality," that is maintained between generations. Bringing the idea full circle, Sutton also concluded that the association of paternal and maternal chromosomes in pairs after gamete fusion, and their subsequent separation during the reducing division of meiosis, "may constitute the physical basis of the Mendelian law of heredity.

Though Sutton believed he had described evidence for the physical basis of Mendel's principles of inheritance, definitive proof was still lacking. Scientists thus needed an experimental system in which the inheritance of genetic traits could be linked directly to the movement of chromosomes.

Such an opportunity presented itself soon thereafter, with a distinct mutation in the fruit fly Drosophila melanogaster. During the early years of the twentieth century, fruit flies were the model organism of choice for many genetic researchers, including those who worked in Thomas Hunt Morgan's famous "fly room" laboratory at Columbia University in New York City.

Why fruit flies? For one, fruit flies breed quickly, so they are efficient organisms for scientists who want to follow traits in offspring through several generations. Also, the fruit fly has only four pairs of chromosomes, so these chromosomes can be easily recognized and tracked from one generation to the next. The Morgan lab therefore set out to examine patterns of heredity through multiple series of breeding experiments with fruit flies, and in doing so, they hoped to discover exactly how heredity was or was not related to chromosomes.

Eventually, the answer to this question became clear-all because of the appearance of a lone fly with unusually colored eyes. Fruit flies normally have brilliant, red-colored eyes, although occasionally, male flies with white eyes would appear in Morgan's laboratory Figure 5. Intrigued by these white-eyed males, Morgan's research team decided to follow this trait through multiple breeding cycles of white eyed males and red-eyed females.

In doing so, the researchers noticed that the white-eyed trait was only passed onto other male flies. In fact, after the researchers conducted multiple rounds of breeding white-eyed males and red-eyed females without identifying a single white-eyed female, they began to suspect that white eye color was inherited along with the sex of the fly. This observation confirmed the chromosome theory proposed by Sutton.

According to this theory, male flies should always inherit male characteristics by virtue of inheriting the "male" chromosome denoted Y ; likewise, female flies should always inherit "female" chromosomes denoted X , which means that these flies should not display male characteristics. Thousands of matings had convinced the Morgan lab that white eyes were clearly a characteristic associated with only the Y chromosome.

One day, however, the researchers in Morgan's lab encountered an unusual fly that challenged their conclusions regarding the relationship between sex and eye color. This exceptional fly was a white-eyed female that had resulted from a cross between two parents with red eyes. Where did this female's white-eye trait come from? How could this trait be explained?

And did this fly disprove the basic premise of the chromosome theory? In the Morgan lab's search to make sense of the white-eyed female, Lilian Vaughn Morgan Thomas Morgan's wife suggested that this exceptional fly might have an unusual chromosome composition.

The research team seized upon this suggestion, and they soon examined some of the white-eyed female's cells under the microscope.

In doing so, the scientists realized that Mrs. Morgan was right - the fly's cells did indeed appear to contain an extra chromosome. Specifically, these cells contained two X chromosomes as well as a single Y chromosome. The extra chromosome was determined to be the result of a defect during meiosis that caused a high frequency of nondisjunction.

Nondisjunction is the failure of two sister chromatids to separate during the second meiotic division. Thus, when an egg containing two nondisjoined X chromosomes, each of which carried the mutant white gene, was fertilized by a sperm cell containing the Y chromosome, the product was an XXY female with white eyes.

Rather than disproving the chromosome theory, this "exceptional" female actually provided strong experimental support that genes were in fact located on chromosomes. Morgan's lab also found that the trait for white eyes could appear even if a fly's father didn't have white eyes. This showed that flies could carry the white-eye trait even if they didn't show it themselves. The trait could vanish and reappear only in certain exceptional moments.

This concept forms the basis of our modern understanding of the hereditary substance that exists on chromosomes but is not always apparent in the outward physical traits of an organism.