Copyright Pearson Prentice Hall Biology Biology Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 14–1 Human Heredity 14-1 Human Heredity Photo credit: Richard Hutchings/Photo Researchers, Inc. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes A picture of chromosomes arranged in pairs is known as a karyotype. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes These human chromosomes have been cut out of a photograph and arranged to form a karyotype. Photo credit: ©CNRI/Science Photo Library/Photo Researchers, Inc. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes Two of the 46 human chromosomes are known as sex chromosomes, because they determine an individual's sex. Females have two copies of an X chromosome. Males have one X chromosome and one Y chromosome. The remaining 44 chromosomes are known as autosomal chromosomes, or autosomes. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes All human egg cells carry a single X chromosome (23,X). Half of all sperm cells carry an X chromosome (23,X) and half carry a Y chromosome (23,Y). About half of the zygotes will be 46,XX (female) and half will be 46,XY (male). Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes Males and females are born in a roughly 50 : 50 ratio because of the way in which sex chromosomes segregate during meiosis. In humans, egg cells contain a single X chromosome. Sperm cells contain either one X chromosome or one Y chromosome. In a population, approximately half of the zygotes are XX (female) and half are XY (male). Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Traits In order to apply Mendelian genetics to humans identify an inherited trait controlled by a single gene. establish that the trait is inherited and not the result of environmental influences. study how the trait is passed from one generation to the next. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Traits A pedigree chart shows the relationships within a family. Genetic counselors analyze pedigree charts to infer the genotypes of family members. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Traits This drawing shows what the symbols in a pedigree represent. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes Blood Group Genes Human blood comes in a variety of genetically determined blood groups. A number of genes are responsible for human blood groups. The best known are the ABO blood groups and the Rh blood groups. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes The Rh blood group is determined by a single gene with two alleles—positive and negative. The positive (Rh+) allele is dominant Rh+/Rh+ or Rh+/Rh - Rh-positive. two Rh- alleles are said to be Rh-negative. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes ABO blood group There are three alleles for this gene, IA, IB, and i. Alleles IA and IB are codominant. i is recessive – ii individuals are type O Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes The presence of a normal, functioning gene is revealed only when an abnormal or nonfunctioning allele affects the phenotype. Many disorders are caused by autosomal recessive alleles. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes This table shows the major symptoms of some well-known genetic disorders. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes Dominant Alleles The effects of a dominant allele are expressed even when the recessive allele is present. Two examples of genetic disorders caused by autosomal dominant alleles are achondroplasia and Huntington disease. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes This table shows the major symptoms of some well-known genetic disorders. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes Codominant Alleles Sickle cell disease is a serious disorder caused by a codominant allele. Sickle cell is found in about 1 out of 500 African Americans. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule From Gene to Molecule How do small changes in DNA cause genetic disorders? Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule In both cystic fibrosis and sickle cell disease, a small change in the DNA of a single gene affects the structure of a protein, causing a serious genetic disorder. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule The most common allele that causes cystic fibrosis is missing 3 DNA bases. As a result, the amino acid phenylalanine is missing from the CFTR protein. Cystic fibrosis is usually caused by the deletion of three bases in the DNA of a single gene. As a result, the body does not produce normal CFTR, a protein needed to transport chloride ions. Cystic fibrosis causes serious digestive and respiratory problems. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule Normal CFTR is a chloride ion channel in cell membranes. Abnormal CFTR cannot be transported to the cell membrane. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule The cells in the person’s airways are unable to transport chloride ions. As a result, the airways become clogged with a thick mucus. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule Sickle Cell Disease Sickle cell disease is a common genetic disorder found in African Americans. It is characterized by the bent and twisted shape of the red blood cells. These red blood cells contain the abnormal hemoglobin characteristic of sickle cell disease. Photo credit: ©Omikron/Photo Researchers, Inc. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule There are three phenotypes associated with the sickle cell gene. An individual with both normal and sickle cell alleles has a different phenotype—resistance to malaria—from someone with only normal alleles. Sickle cell alleles are thought to be codominant. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule Malaria and the Sickle Cell Allele The map on the left shows where malaria is common. The map on the right shows regions where people have the sickle cell allele. Regions where malaria is common Regions where the sickle cell allele is common Copyright Pearson Prentice Hall

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