MB 13.1

How will we begin our study of genetics?

We will begin our study of genetics in this chapter by examining how chromosomes pass from parents to offspring in sexually reproducing organisms. The process of meiosis (a special type of cell division) and fertilization (the fusion of sperm and egg) maintain a species chromosome count during the sexual life cycle. We will describe the cellular mechanics of meiosis and explain how this process differs from mitosis. Finally, we will consider how both meiosis and fertilization contribute to genetic variation.

What is each chromosome consisted off?

Each chromosome consists of a single long DNA molecule elaborately coiled in association with various proteins. One chromosome includes several hundred to a few thousand genes, each of which is a specific sequence of nucleotides within the DNA molecule. A gene's specific location along the length of a chromosome is called the gene's locus (meaning "place"). Our genetic endowment consists of the genes that are part of the chromosomes we inherited from our parents.

What are somatic cells?

Every species has a characteristic number of chromosomes. For example, humans have 46 chromosomes in their somatic cells - all cells of the body except the gametes and their precursors.

What is genetics?

Genetics is the scientific study of heredity and hereditary variation.

What happens in sexual reproduction?

In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the parents. In contrast to a clone, offspring of sexual reproduction vary genetically from their siblings and both parents: They are variations on a common theme of family resemblance, not exact replicas.

What language is the genetic program written in?

The genetic program is written in the language of DNA, the polymer of four different nucleotides A,C,T,G. Inherited information is passed on in the form of each gene's specific sequence of DNA nucleotides, much as printed information is communicated in the form of meaningful sequences of letters. In both cases, the language is symbolic. Just as your brain translates the word apple into a mental image of the fruit, cells translates genes into freckles and other features. Most genes program cells to synthesize specific enzymes and other proteins, whose cumulative action produces an organism's inherited traits. The programming of these traits in the form of DNA is one of the unifying themes of biology.

What role do gametes play in the reproductive system of plants and animals?

The transmission of hereditary traits has its molecular basis in the precise replication of DNA, which produces copies of genes that can be passed from parents to offspring. In animals and plants, reproductive cells called gametes are the vehicles that transmit genes from one generation to the next. During fertilization, male and female gametes (sperm and eggs) unite, thereby passing on genes of both parents to their offspring.

What is heredity?

The transmission of traits from one generation to the next is called inheritance, or heredity (from the Latin heres, heir). However, sons and daughters are not identical copies of either parent or their siblings.

What are some organisms that reproduce asexually?

A hydra, reproduces by budding. The bud, a localized mass of mitotically dividing cells, develops into a small hydra, which detaches from the parent. All the trees in the circle of redwoods arose asexually from a single parent tree, whose stump is in the center of the circle.

What does "variation" mean in terms of sexual reproduction?

Along with inherited similarity, there is also variation. Farmers have exploited the principles of heredity and variation for thousands of years, breeding plants and animals for desired traits.

What is a clone?

Because the cells of the offspring are derived by mitosis in the parent, the "chip off the old block" is usually genetically identical to its parent. An individual that reproduces asexually gives rise to a clone, a group of genetically identical individuals. Genetic differences occasionally arise in asexually reproducing organisms as a result of changes in the DNA called mutations.

What will you learn about genetics this chapter?

In this unit, you will learn about genetics at multiple levels, from organisms to cells to molecules. On the practical side, you will see how genetics continues to revolutionize medicine and agriculture, and you will be asked to consider some social and ethical questions raised by our ability to manipulate DNA, the genetic material. At the end of the unit, you will be able to stand back and consider the whole genome, an organism's entire complement of DNA. Rapid acquisition and analysis of the genome sequences of many species, including our own, have taught us a great deal about evolution on the molecular level - in other words, evolution of the genome itself. In fact, genetic methods and discoveries are catalyzing progress in all areas of biology, from cell biology to physiology, developmental biology, behavior and even ecology.

What is asexual reproduction?

Only organisms that reproduce asexually have offspring that are genetic copies of themselves. In asexual reproduction, a single individual is the sole parent and passes copies of all its genes to offspring without the fusion of gametes. For example, single-celled eukaryotic organisms can reproduce asexually by mitotic cell division, in which DNA is copied and allocated equally to two daughter cells. The genomes of the offspring are virtually exact copies of the parent's genome.

What are genes?

Parents endow their offspring with coded information in the form of hereditary units called genes.The genes we inherit from our mothers and fathers are our genetic link to our parents, and they account for family resemblances such as shared eye color or freckles. Our genes program the specific traits that emerge as we develop from fertilized eggs into adults.

Explain what causes the traits of parents (such as hair color) to show up in their offspring.

Parents pass genes to their offspring; the genes program cells to make specific enzymes and other proteins, whose cumulative action produces an individual's inherited traits.

A horticulturalist breeds orchids, trying to obtain a plant with a unique combination of desirable traits. After many years, she finally succeeds. To produce more plants like this one, should she crossbreed it with another plant or clone it? Why?

She should clone it. Cross-breeding it with another plant would generate offspring that have additional variation, which she no longer desires now that she has obtained her ideal orchid.

How do asexually reproducing organisms produce offspring that are genetically identical to each other and to their parents?

Such organisms reproduce by mitosis, which generates offspring whose genomes are exact copies of the parent's genome (in the absence of mutation)