Unit A Reproduction

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Male Zygotes or Female Zygotes How do females get two X chromosomes and males get one X and one Y chromosome? It all depends on which sperm fertilizes the egg. Figure 3 shows that all the eggs have a single X chromosome, while the sperm can have either one X or one Y chromosome. Figure 4 shows all the possible outcomes of fertilization. LEARNING TIP Are you able to explain how gender is determined? If not, ask yourself, “What do I need to figure out?” Then reread the explanation, and re-examine Figure 4. Parents xy male xx female Gametes Zygotes x y xx female offspring xy male offspring x Figure 4 The sex chromosome in the sperm determines the gender of the offspring. A sperm with an X chromosome will produce a female offspring, and a sperm with a Y chromosome will produce a male offspring. As you can see in Figure 4, because half the sperm produce male offspring and half produce female offspring, there is a 50 percent chance that the child will be male and a 50 percent chance that the child will be female. Gender is determined by the father. GO Sex-Linked Characteristics Have you ever wondered why there are more bald men than bald women? This imbalance occurs because the X and Y chromosomes are not homologous (Figure 2). Unlike the other 22 pairs of chromosomes, the X chromosome and the Y chromosome do not carry matching genes. Female offspring have 23 pairs of homologous chromosomes with corresponding genes. Males, however, have one X chromosome and one Y chromosome. The smaller Y chromosome is missing some of the genetic material that is present in the X chromosome. As a result, certain traits are controlled by the genes on the X chromosome. These are known as sex-linked characteristics and they occur more frequently in males than in females. As well as baldness, red-green colour blindness (Figure 5) is more common in males than in females. Hemophilia is a disease that is more common in males. The symptoms of hemophilia include excessive bleeding due to the absence of some important clotting proteins normally found in blood. Hemophilia is treated with medication that contains the missing clotting factors. If you would like to learn more about how the different genders develop, go to www.science.nelson.com GO Figure 5 Test for red-green colour blindness. More females than males will be able to see the word in the test. NEL 4.4 Gender Determination in Humans 125
4.4 CHECK YOUR Understanding 1. What is a karyotype? 2. During which phase of mitosis are the chromosomes photographed for a karyotype? 3. Describe the steps that are used to produce a karyotype. 4. Explain why karyotypes are important for biologists. 5. How many sex chromosomes do humans have? Name them. 6. Make a drawing of the X and Y chromosomes to show the difference in size. 7. Examine the karyotype in Figure 5. What is the gender of the individual whose karyotype is shown? 8. (a) Describe the sex chromosomes of a female human. (b) Describe the sex chromosomes of a male human. 9. Describe the difference between an egg cell and a sperm cell in terms of the sex chromosomes that may be present. 10. Make a drawing to show the possible zygotes formed after fertilization. Indicate whether each zygote will grow into a female child or a male child. 11. (a) A family has one male child. What is the probability that the next child will be a girl? (b) A family has two female children. What is the probability the next child will be a boy? 12. Name two sex-linked characteristics. 13. Explain why sex-linked characteristics are expressed more often in males than females. 14. Figure 6 shows human fertilization. (a) Indicate the number of chromosomes in each cell. (b) Indicate the sex of cells D and E. (c) Explain how you know the sex of cells D and E. A B C D E 2000X Figure 6 Figure 5 126 <strong>Unit</strong> A <strong>Reproduction</strong> NEL
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CHAPTER 2 Cell Growth and Reproduct
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2.1 The Importance of Cell Division
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2.1 CHECK YOUR Understanding 1. Giv
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Figure 2 Plant cell structures cyto
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2.3 From DNA to Proteins You have l
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Did You KNOW? The Human Genome The
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LEARNING TIP Think about the sectio
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USING MITOCHONDRIAL DNA TO SOLVE MY
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centrioles nucleus chromosome spind
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LEARNING TIP Make connections to yo
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Binary fission allows for rapid pop
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could clone frogs, they were unable
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2A Investigation Observing Cell Div
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2B Investigation Determining the Ra
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CHAPTER 2 Review Cell Growth and Re
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Review Key Ideas and Vocabulary 1.
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CHAPTER 3 Sexual Reproduction KEY I
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Unit A Reproduction

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