ACT Reading Practice Test 24

NATURAL SCIENCE: Heredity and Gene-linkage: A Possible Relationship

The ability of every organism on earth to reproduce is the hallmark of life. Reproduction can be either asexual, involving a single parent, or sexual, involving two parents. Sexual reproduction begets offspring that

Line 5 inherit half of their genes from each parent. This transmission of genes from one generation to the next is called heredity. Each hereditary unit, the gene, contains specific encoded information that translates into an organism’s

10 inherited traits. Inherited traits range from hair color, to height to susceptibility to disease. Genes are actually segments of the DNA molecule, and it is the precise replication of DNA that produces copies of genes that can be passed from parents to offspring.

15 DNA is subdivided into chromosomes that each include hundreds or thousands of genes. The specific traits or characteristics of each offspring depend on the arrangement and combination of the chromosomes supplied by both parents.

20 Genes located on the same chromosome tend to be inherited together. Transmission of these so-called linked genes can affect the inheritance of two different characteristics. Thomas Hunt Morgan was the first biologist to associate specific genes with specific

25 chromosomes. In the early 20th century, Morgan selected a species of fruit fly, Drosophila melanogaster, on which to study his genetic theory. The fruit fly is a prolific breeder, producing hundreds of offspring in a single mating. In addition, the fruit fly has only four

30 pairs of easily distinguishable chromosomes, making it the ideal experimental organism. Soon after Morgan commenced working with Drosophila, he began to notice variations in certain traits. For example, Morgan noticed that the natural

35 characteristics of Drosophila included gray bodies and normal wings. However, mutant examples of these characteristics sometimes appeared; these flies had black bodies, and much smaller, vestigial wings. Morgan crossed female flies that appeared normal, but

40 carried the mutant genes, with males that exhibited the mutations. He expected the offspring to include equal numbers of gray flies with normal wings, black flies with vestigial wings, gray flies with vestigial wings, and black flies with normal wings. What he found was
45 a disproportionate number of gray flies with normal wings and black flies with vestigial wings, which suggested to him that the genes for body color and wing size are transmitted together from parents to offspring because they are located on the same chromosome and
50 must be somehow linked. Additional research conducted by Morgan on D. melanogaster demonstrated that many, often spontaneous mutations occur across generations. These observations, together with the results of experiments

55 carried out to test his theory on linked genes, led Morgan to postulate that the location of the genes on the chromosomes contributes to the likelihood of any given gene being transmitted from parent to offspring. This theory of linear arrangement, along with Morgan’s

60 other important contributions to the field of genetics, led to his being awarded the Nobel Prize in Physiology or Medicine in 1933. Current research exploring the significance of linked genes reveals that many factors affect the trans-

65 mission of certain traits from parents to offspring. The location of genes on a particular chromosome is but one of a multitude of determinants involved in whether or not a characteristic will be inherited.