Modern Genetics Name Date Class Modern Genetics Guided Reading and Study Human Inheritance This section explains some patterns of inheritance in humans. It also describes the functions of the sex chromosomes and the relationship between genes and the environment. Use Target Reading Skills As you read, write the main idea the biggest or most important idea in the graphic organizer below. Then write three supporting details that further explain the main idea. Main Idea Human traits are controlled by single genes with two alleles, single genes with... Detail Detail Detail a. b. c. Patterns of Human Inheritance 1. The probability that two heterozygous parents for widow s peak will have a child with a straight hairline is percent.
Name Date Class Modern Genetics Guided Reading and Study Human Inheritance (continued) 2. A gene with three or more alleles for a single trait has. 3. Is the following sentence true or false? Even though a gene has multiple alleles, a person can carry only two of those alleles. 4. Complete the table by writing all possible combinations of alleles for each blood type. Blood Types Blood Type Combination of Alleles A B AB O 5. Why do some human traits, such as height and skin color, show a large number of phenotypes? 6. Is the following sentence true or false? Skin color is controlled by more than one gene.
Modern Genetics Name Date Class Modern Genetics Guided Reading and Study The Sex Chromosomes 7. Is the following sentence true or false? Genes on chromosomes determine whether a baby is a boy or a girl. 8. Females have two chromosomes. Males have one chromosome and one chromosome. 9. Circle the letter of each sentence that is true about human sex chromosomes. a. All eggs carry one X chromosome. b. Half of a male s sperm cells have an X chromosome. c. None of a male s sperm cells have a Y chromosome. d. The egg determines the sex of the child. 10. Genes on the X and Y chromosomes are called. 11. Why are males more likely than females to have a sex-linked trait that is recessive? 12. Is the following question true or false? A carrier for colorblindness is colorblind. 13. Why is a son who receives the allele for colorblindness from his mother always colorblind? The Effect of Environment 14. The effects of genes are often altered by the. 15. List one environmental factor that can affect a person s height.
Name Date Class Modern Genetics Section Summary Human Inheritance Key Concepts What are some patterns of inheritance in humans? What are the functions of the sex chromosomes? What is the relationship between genes and the environment? Many human traits are controlled by a single gene with one dominant allele and one recessive allele. As with tall and short pea plants, these human traits have two distinctly different phenotypes, or physical appearances. For example, the allele for a widow s peak, which is a hairline that comes to a point in the middle of the forehead, is dominant over the allele for a straight hairline. Some human traits are controlled by single genes with two alleles, and others by single genes with multiple alleles. Still other traits are controlled by many genes that act together. Height and skin color are both examples of human traits controlled by many genes. When more than one gene controls a trait, there are many possible combinations of genes and alleles. There is an enormous variety of phenotypes for height, for example, and human skin color ranges from almost white to nearly black, with many shades in between. Some human traits are controlled by a single gene that has more than two alleles. Such a gene is said to have multiple alleles three or more forms of a gene that code for a single trait. An example of a human trait that is controlled by a gene with multiple alleles is blood type. There are four main blood types A, B, AB, and O controlled by three alleles. The sex chromosomes are one of 23 pairs of chromosomes in each body cell. The sex chromosomes carry genes that determine whether a person is male or female. They also carry genes that determine other traits. If you are female, you have two X chromosomes. If you are male, you have an X and a Y chromosome. Whether you inherited an X or Y chromosome from your father determines your sex. Genes on the X and Y chromosomes are often called sex-linked genes. Traits controlled by sex-linked genes are called sex-linked traits. Because males have only one X chromosome, males are more likely than females to have a sex-linked trait that is controlled by a recessive allele. One example of a sex-linked trait that is controlled by a recessive allele is red-green colorblindness. A carrier is a person who has one recessive allele for a trait and one dominant allele. Although a carrier does not have the trait, the carrier can pass the recessive allele on to his or her offspring. In the case of sex-linked traits, only females can be carriers. The effects of genes are often altered by the environment the organism s surroundings. Many of a person s characteristics are determined by an interaction between genes and the environment. Several genes determine human height. However, environment also influences people s heights. People s diets can affect their height. A poor diet can prevent a person from growing as tall as might be possible.
Name Date Class Modern Genetics Review and Reinforce Human Inheritance Understanding Main Ideas Fill in the Punnett squares for (A) dimples, a trait controlled by a dominant allele, and (B) colorblindness, a trait controlled by a recessive sex-linked allele. Then answer the questions that follow. (The father s alleles are written across the top of the Punnett square. The mother s alleles are written on the left side.) d A: Dimples D 1. 2. d 5. Does either the mother or the father in A have dimples? d 3. 4. 6. What percentage of children are likely to have dimples? X C X c B: Colorblindness X c Y 7. 8. 9. 10. 11. Is either the mother or father in B colorblind? 12. What percentage of female children are likely to be colorblind? 13. What percentage of male children are likely to be colorblind? Building Vocabulary Fill in the blank to complete each statement. 14. Three or more forms of a gene that code for a single trait are called. 15. have alleles that are passed from parent to child on a sex chromosome. 16. A(n) is a person who has one recessive allele for a trait and one dominant allele for the same trait. 17. carry genes that determine whether a person is male or female.
Modern Genetics Name Date Class Modern Genetics Enrich Sex-Linked Genes One example of a sex-linked trait is the blood disorder hemophilia A. Normally when a blood vessel is ruptured, such as when a wound in the skin occurs, the wound bleeds, but then the blood clots and a scab is formed. People who have hemophilia A are unable to make certain proteins necessary for blood to clot properly. When cuts and other injuries are obtained, bleeding is difficult to stop without medical treatment. Hemophilia A is a recessive trait and the allele is carried on the X chromosome. This means that a female can be a carrier of the trait without actually suffering from the disorder. A male who receives the trait on the X chromosome will exhibit the disorder. Answer the following questions on a separate sheet of paper. 1. Complete the Punnett square shown here where X h represents an X chromosome carrying the allele for hemophilia A. X h X X Y a. What percentage of the children of these parents will probably have hemophilia A? What gender will they be? b. What percentage of the children of these parents will be carriers of hemophilia A? What gender will they be? 2. Complete the Punnett square shown here where X h represents an X chromosome carrying the allele for hemophilia A. X h X X h Y a. What percentage of the children of these parents will probably have hemophilia A? b. What percentage of the children of these parents will be carriers of hemophilia A? 3. A husband and wife have three children: two boys and one girl. The father has hemophilia A. Neither of the boys has hemophilia A. The girl is a carrier of hemophilia A. What are the possible genotypes of the parents?