Chapter 4 Modern Genetics Thursday, December 10, 2009 Pages

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. Identify what determines sex, and explain why some sex-linked traits are more common in males than in females. Describe how geneticists use pedigrees.

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.

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.

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.

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

Sex Chromosomes 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 Mom

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.

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

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.

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.

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