​We all know that offspring resemble their parents more than they do unrelated individuals. If you examine the family members shown in Figure 10.1, you can pick out some similar features among them. 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 of their siblings. 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. But what are the biological mechanisms leading to the hereditary similarity and variation that we call a “family resemblance”? A detailed answer to this question eluded biologists until the advance of genetics in the 20th century.

​Variations on a Theme

Genetics is the scientific study of heredity and inherited variation. In this unit, you’ll learn about genetics at multiple levels, from organisms to cells to molecules. We begin by examining how chromosomes pass from parents to offspring in sexually reproducing organisms. The processes of meiosis (a special type of cell division) and fertilization (the fusion of sperm and egg, as seen in the micrograph) maintain a species’ chromosome count during the sexual life cycle. We’ll describe the cellular mechanics of meiosis and explain how this process differs from mitosis. Finally, we’ll consider how both meiosis and fertilization contribute to genetic variation, such as the variation obvious in the family shown in Figure 10.1 .

Overview: Variations on a Theme

• Offspring resemble their parents more than they do unrelated individuals

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• Heredity is the transmission of traits from one generation to the next

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• Variation is demonstrated by the differences in appearance that offspring show from parents and siblings

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• Genetics is the scientific study of heredity and variation

​A sperm fertilizing an egg.

​​Concept 10.1: Offspring acquire genes from parents by inheriting chromosomes

Friends may tell you that you have your mother’s nose or your father’s eyes. Of course, parents do not, in any literal sense, give their children a nose, eyes, hair, or any other traits. What, then, is actually inherited?

​Inheritance of 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 specific traits that emerge as we develop from fertilized eggs into adults.

​The genetic program is written in the language of DNA, the polymer of four different nucleotides (see Concepts 1.1 and 3.6 ). 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 translate 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

​The transmission of hereditary traits has its molecular basis in the 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, passing on genes of both parents to their offspring

Concept 10.1: Offspring Acquire Genes from Parents by Inheriting Chromosomes

•Genes are the units of heredity and are made up of segments of DNA

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• Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs)

​Except for small amounts of DNA in mitochondria and chloroplasts, the DNA of a eukaryotic cell is packaged into chromosomes within the nucleus. Every species has a characteristic number of chromosomes. For example, humans have 46 chromosomes in their somatic cells —all the cells of the body except the gametes and their precursors. 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 precise sequence of nucleotides along the DNA molecule. A gene’s specific location along the length of a chromosome is called the gene’s locus (plural, loci; from the Latin, meaning “place”). Our genetic endowment (our genome) consists of the genes and other DNA that make up the chromosomes we inherited from our parents.

​​Comparison of Asexual and Sexual Reproduction

Only organisms that reproduce asexually have offspring that are exact genetic copies of themselves. In asexual reproduction , a single individual (like a yeast cell or an amoeba; see Figure 9.2a ) is the sole parent and passes copies of all its genes to its 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. Some multicellular organisms are also capable of reproducing asexually (Figure 10.2 ). Because the cells of the offspring arise via mitosis in the parent, the offspring is usually genetically identical to its parent. An individual that reproduces asexually gives rise to a clone , an individual or group of individuals that are genetically identical to the parent. Genetic differences occasionally arise in asexually reproducing organisms as a result of changes in the DNA called mutations, which we will discuss in Concept 14.5.

​Figure 10.2 Asexual reproduction in two multicellular organisms.

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​(a) Hydra

​(b) Redwoods

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(a) This relatively simple animal, a hydra, reproduces by budding. The bud, a localized mass of mitotically dividing cells, develops into a small hydra, which detaches from the parent (LM).

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(b) All the trees in this circle of redwoods arose asexually from a single parent tree, whose stump is in the center of the circle.

​In sexual reproduction , two parents give rise to offspring that have unique combinations of genes inherited from the two 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. Genetic variation like that shown in Figure 10.1 is an important consequence of sexual reproduction. What mechanisms generate this genetic variation? The key is the behavior of chromosomes during the sexual life cycle.

Inheritance of Genes

•Most DNA is packaged into chromosomes

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• For example, humans have 46 chromosomes in their somatic cells, the cells of the body except for gametes and their precursors

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• Each gene has a specific position, or locus, on a certain chromosome

​10.1 Offspring acquire genes from parents by inheriting chromosomes

​10.1 Offspring acquire genes from parents by inheriting chromosomes

1. Inheritance of Genes

   1. Genes: a DNA sequence/heredity unit that program traits

   2. Gametes: reproductive cells, transmit genes to the next generation

   3. Somatic cells: all body cells except for gametes and their precursors

                                                              i.      Humans have 46 chromosomes in body cells

                                                            ii.      One chromosome contains hundreds to thousands of genes

1. Locus: a gene’s specific location on a chromosome

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Comparison of Asexual and Sexual Reproduction

• In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes

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• A clone is a group of genetically identical individuals from the same parent, produced asexually

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• In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents

​Comparison of Asexual and Sexual Reproduction

1. 1. Asexual reproduction: offspring are exact genetic copies of themselves (clones)

                                                              i.      Single celled eukaryotes use mitosis for asexual reproduction

                                                            ii.      Multicellular eukaryotes that can reproduce by budding

2. Sexual Reproduction: two parent give rise to offspring that have a unique combination of genes inherited from the two parents