Genes and Heredity

Heredity is the passing on of traits, or characteristics, from parents to offspring. Genes are the tiny coded messengers responsible for passing on the traits. Each gene is composed of one of many possible combinations and sequences of DNA, which signals an organism to produce different types of proteins. These proteins control every detail of how an organism will grow and develop. The most complicated creatures as well as the simplest one celled bacteria are all directed by a coded set of instructions written in its DNA.

Every gene occupies a certain location on a chromosome strand, and every organism has a unique number and order of genes on its chromosomes. The more closely related two organisms are, the more alike their chromosomes. All organisms that use sexual reproduction have two strands of chromosomes in the nucleus of each cell. Humans, for example, have 23 pairs of chromosomes, for a total of 46, in every cell.

Which of the following organisms likely have the most similar chromosomes?

• humans and frogs

• birds and lions

• humans and apes

• lions and alligators

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In the 1940s scientists discovered that DNA is made up of molecules called nucleotides. Each nucleotide consists of three different materials: a base, a sugar, and a phosphate. The key to understanding how DNA worked seemed to lie among the base materials.

A chemist named Rosalind Franklin then discovered that DNA strands seemed to be in a spiral shape, but again, no one knew why.

Watson and Crick described the shape they had found as a double helix. A double helix looks like a twisted ladder that spirals around and around. Each rung of the ladder is made of one of the four nucleotide bases. The secret of DNA, or what Crick called "the secret of life" is that every A molecule on one side of the rung always connects to a T on the other side, while every G molecule connects to a C.

The key to understanding how DNA works is

• to understand that each base only connects to another specific base.

• to understand more about the sugar in each nucleotide.

• to understand that it looks like a twisted ladder that spirals around.

• to understand the role of the phosphate in each nucleotide.

In higher mammals like humans, the only time that two organisms naturally share the exact same DNA is when they are identical twins. Identical twins are rare, only occurring if a single fertilized egg splits apart right when it begins to grow. Cloning is a technique in which scientists artificially replicate this, producing offspring identical to the parent or another organism.

It's a difficult process, but cloning is one of several ways that scientists are now using our knowledge of DNA to engineer new characteristics into organisms, by adding to or changing DNA strands. Through this genetic engineering, scientists have produced plants that create their own insecticides and body cells that glow in the dark to help locate diseased tissue. By altering the sequence and types of DNA molecules in a fertilized cell, it may someday be possible to grow new body parts to replace those that have been damaged in an accident or by cancer.

According to the passage, which of the following best describes genetic engineering?

• useless

• destructive

• impossible

• innovative

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Gregor Mendel, a Christian monk, performed an experiment by putting pollen from a pea plant with purple flowers on the pistil of a plant with white flowers. The resulting second generation of offspring had all purple flowers. Mendel wondered if all future pea plant flowers would be purple, if the ability to make white flowers had receded or become hidden. He was wondering if white flowers were a recessive trait.

When organisms reproduce by sexual reproduction, each offspring receives two strands of chromosomes - one from each parent. For any given trait, if the genes are not identical, one of the two traits will be dominant and the other will be recessive. The dominant, or stronger, gene is the one that the offspring develops. However, the organism still retains a gene for the recessive trait on one of its chromosomes, so it may pass that trait down to its offspring one day.

Which of these provides evidence that brown eyes are dominant and blue eyes are recessive?

• Everyone in your extended family has green eyes.

• Your mom has blue eyes, your dad has brown eyes, and you have brown eyes.

• Your parents have blue eyes, your siblings have blue eyes, and so do you.

• People with blond hair often have blue eyes.

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Mendel's pea experiments explain why different members of a family might have different traits such as brown eyes, curly hair, dark skin, tallness, or even big feet. It explains why some traits may skip a generation, like why you have your grandfather's hands. It helped us take advantage of existing traits to breed organisms a certain, more desirable way. Through selective breeding, people realized that they could improve the chances of offspring having a desired trait - cows that produce more milk, thoroughbred horses that run faster, plants that produce bigger fruit, vegetables that taste better, grains that are more productive.

According to the passage, an advantage of selective breeding is

• the ability to produce offspring with more desirable traits.

• traits can skip a generation or two.

• children can have curly hair.

• family members have different traits.

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When a cell divides to grow into two new cells, the double helix splits into two single strands of chromosomes, each of which can then replicate a mirror image of itself. This process, called mitosis, happens every time a cell divides, except for sex cells preparing for sexual reproduction. The result is two new cells, each with a double helix of DNA strands identical to that of the original cell.

Though rare, sometimes an error occurs in this process, resulting in a mutation. Radiation or the appearance of other chemicals can increase their frequency. Sometimes an extra base is inserted, substituted, deleted or damaged. Usually these mistakes repair themselves, but sometimes they remain and, as a result, can create a change in the coding for a specific trait. If the new trait helps the organism survive, it may be duplicated and passed on to offspring, which, in time, may lead to an evolution in the organism.

According to the passage, as a result of a mutation, an organism

• will most likely die.

• will receive chemical radiation.

• may evolve for better survival.

• may be duplicated.

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The passing of traits from parents to offspring is called

• heredity.

• deoxyribonucleic acid, or DNA.

• cloning.

• precipitation.

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The traits of an organism are passed to its offspring on small coded strands of DNA material called

• meiosis.

• recessive traits.

• genes.

• heredity.

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James Watson and Francis Crick are the scientists who discovered

• mutated cells

• the double helix structure of DNA

• that there are dominant and recessive genes

• evolutionary traits in human beings

During sexual reproduction, each parent contributes

• a double helix of chromosomes

• one strand of chromosomes, or half of the new organism's DNA

• a dominant gene and a recessive gene

• multiple mutated cells

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