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Chapter 4 Modern Genetics

Chapter 4 Modern Genetics. Thursday, December 10, 2009 Pages 117 --124. Section 1. Human Inheritance. Objectives. Explain what multiple alleles are. Explain why some human traits show a large variety of phenotypes. Explain how environmental factors can alter the effects of a gene.

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Chapter 4 Modern Genetics

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  1. Chapter 4Modern Genetics Thursday, December 10, 2009 Pages 117 --124

  2. Section 1 Human Inheritance

  3. Objectives • Explain what multiple alleles are. • Explain why some human traits show a large variety of phenotypes. • Explain how environmental factors can alter the effects of a gene. • Identify what determines sex, and explain why some sex-linked traits are more common in males than in females. • Describe how geneticists use pedigrees.

  4. Traits Controlled by Single Genes • Often one allele is dominant, while the other is recessive. • These resulting traits have 2 distinctly different phenotypes, or physical appearance.

  5. Multiple Alleles • Human traits that are controlled by a single gene that has more than two alleles. • 3 or more forms of a gene that code for a single trait. • A person can only carry 2 since chromosomes exist in pairs. Blood type is determined by a single gene with 3 alleles.

  6. Traits Controlled by Many Genes • Some human traits show a large number of phenotypes because the traits are controlled by many genes. • The genes act together as a group to produce a single trait. • Examples include height and skin color.

  7. Male or Female? • The sex of a baby is determined by genes on chromosomes. • Among the 23 pairs of chromosomes in each body cell is a single pair of chromosomes called the sex chromosomes. • This determines whether a person is male or female. Male Female

  8. Sex Chromosomes Mom • Sex chromosomes are the only pair of chromosomes that do not always match. • Females have two X chromosomes. • Males have one X and one Y chromosome. • The Y chromosome is much smaller. • It is the sperm that determines the sex of a child. Dad

  9. Sex-Linked Genes • Genes on the X and Y chromosomes are called sex-linked genes. • Alleles are passed from parent to child on a sex chromosome.

  10. Sex-Linked Traits • Sex-linked genes can have dominant and recessive alleles. • 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. • Example: red-green colorblindness

  11. Carrier • A carrier is a person who has one recessive allele for a trait and one dominant allele. • The carrier does not have the trait but can pass the recessive allele on to his or her offspring. • With sex-linked traits, only females can be carriers.

  12. Pedigrees • A tool geneticists use to trace the inheritance of traits in humans. • A chart or family tree that tracks which members of a family have a particular trait.

  13. Homework • Workbook 4.1 (due 12/14) • Vocabulary quiz (12/15)

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