Unit A Reproduction

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3.2 Sexual Reproduction and the Diversity of Life Meiosis is only the first stage of sexual reproduction. It produces gametes. To reproduce sexually, however, the gametes that are produced by one organism have to join with the gametes that are produced by another organism of the same species. In this section, you will take a closer look at sexual reproduction and learn why it is so important. LEARNING TIP Adjust your reading pace. Pause after reading each paragraph on this page. Fertilization Produces Diploid Cells The joining or fusing of two gametes is called fertilization (Figure 1). Recall that meiosis produces haploid daughter cells (n), called gametes, which contain half the genetic material of the parent cell. When two haploid gametes fuse during fertilization, the resulting diploid cell has a complete set of chromosomes (2n). The first diploid cell of the offspring is called the zygote. sperm (n) egg (n) zygote (2n) The zygote receives half of its chromosomes from each parent. For example, human gametes have 23 chromosomes. After fertilization, the resulting zygote has 46 chromosomes or 23 pairs of chromosomes. This first cell of the new organism will undergo cell division to increase in size. Each new cell will contain the same genome as the zygote. sperm (n) egg (n) fertilization zygote (2n) Figure 1 Fertilization is the fusion of haploid gametes (sperm and egg) to produce a diploid zygote. Diversity Among Offspring There is a vast amount of variation, or diversity, among the different species. For example, mosses look very different from butterflies or elephants. Within a species, there is also a vast amount of diversity. The dogs pictured in the Preview, on pages 72 and 73, look slightly different from one another. Look around your classroom and observe some of the obvious differences among individual humans. Some of your classmates are taller or shorter than you are and have different-coloured hair, skin, and eyes. Even though you may resemble members of your immediate family, there are differences. Sexual reproduction accounts for all this variation. Each gamete contributes one half of each pair of homologous chromosomes in the zygote. Having homologous chromosomes produces the variation among individual members of the species. Even though each homologous NEL 3.2 Sexual Reproduction and the Diversity of Life 79
allele for straight hairline homologous chromosomes gene for hairline allele for widow’s peak Figure 2 The different forms of a gene are called alleles. In any one individual, there are only two alleles for every gene—one allele on each homologous chromosome. chromosome has a corresponding gene, these genes may be slightly different from one another. The different forms of the same gene are called alleles (Figure 2). In humans, for example, homologous chromosomes may contain two different alleles for the gene that controls the shape of the hairline: one chromosome has the allele for a widow’s peak and the other chromosome has the allele for a straight hairline (Figure 3). The allele for a widow’s peak is dominant. A dominant allele will express its trait (the trait will show up in a person’s appearance) if it is present. The allele for a straight hairline is recessive. A recessive allele will only be expressed if both chromosomes contain the recessive allele. If an individual has one of each allele, the dominant trait is expressed. (a) (b) Figure 3 Humans can have either a widow’s peak (a) or a straight hairline (b). Figure 4 These horses are both roans. They have two colours of hairs in their coats. One has a mix of black hairs and white hairs. The other has a mix of brown hairs and white hairs. In carnations, when a plant has one allele for red flowers and one allele for white flowers, the result is pink flowers. This is incomplete dominance: two different alleles produce a mixture of the traits. Codominance occurs when both traits are expressed in one individual. An animal born with an allele for white hair and an allele for red hair will have a roan-coloured coat, with a mix of white hairs and red hairs (Figure 4). In both cases, each parent has contributed only one of its two alleles in its sperm cell or egg cell. Some characteristics are controlled by more than one gene or there are more than two alleles of the gene. In these cases, the combination of the two alleles from both parents can produce an entirely different trait in the offspring. Eye colour in humans is one example. The amount of pigment and the pattern of pigment in the eye are controlled by several genes. The incredible process of sexual reproduction produces the different combinations that account for the vast diversity among living things. Comparing Sexual and Asexual Reproduction Sexual reproduction is much more complicated than asexual reproduction. Does this complexity mean that sexual reproduction is better than asexual reproduction? Both methods of reproduction have existed for millions of years, and both methods have advantages and disadvantages. 80 Unit A Reproduction NEL
Page 1 and 2: 32 NEL
Page 3 and 4: CHAPTER 2 Cell Growth and Reproduct
Page 5 and 6: 2.1 The Importance of Cell Division
Page 7 and 8: 2.1 CHECK YOUR Understanding 1. Giv
Page 9 and 10: Figure 2 Plant cell structures cyto
Page 11 and 12: 2.3 From DNA to Proteins You have l
Page 13 and 14: Did You KNOW? The Human Genome The
Page 15 and 16: LEARNING TIP Think about the sectio
Page 17 and 18: USING MITOCHONDRIAL DNA TO SOLVE MY
Page 19 and 20: centrioles nucleus chromosome spind
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Page 23 and 24: LEARNING TIP Make connections to yo
Page 27 and 28: Binary fission allows for rapid pop
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Page 33 and 34: 2A Investigation Observing Cell Div
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Page 37 and 38: CHAPTER 2 Review Cell Growth and Re
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Page 41 and 42: CHAPTER 3 Sexual Reproduction KEY I
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Page 45 and 46: Interphase Prophase Metaphase Anaph
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4.6 Prenatal Procedures (a) (b) Fig
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4.6 CHECK YOUR Understanding 1. Nam
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4.7 Reproductive Technologies LEARN
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4.7 CHECK YOUR Understanding 1. At
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Sample Stakeholder Perspectives •
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CHAPTER 4 Review Human Reproduction
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Review Key Ideas and Vocabulary 1.
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UNIT A Review Reproduction Unit Sum
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15. Complete the following table to
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Unit A Reproduction

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