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Learn how Gregor Mendel established the basics of genetics and explore the different modes of inheritance for genetic disorders in humans. Understand the concept of dominant, recessive, and codominant alleles and their impact on gene expression. Study specific disorders such as Tay-Sachs disease, Huntington's disease, and cystic fibrosis. Use Punnett squares to predict offspring in monohybrid and dihybrid crosses.
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Standards:Gen 2.7 Use Punnett squares to explain Mendel’s three genetic laws and to predict offspring in monohybrid and dihybrid crosses of common genetic traits and disorders. Essential Questions: How did Gregor Mendel establish the basics of genetics?
Nearly all disorders and traits once considered to be caused by single genes are actually influenced y environmental factors and usually by the actions of other genes, too.
Human Alleles • Many human genes have become known through the study of genetic disorders. • Genetic Disorders can be caused by various “Modes of Inheritance” • Autosomal recessive alleles • Autosomal dominant alleles • Codominant alleles • What makes an allele dominant, recessive, or codominant? • It all depends on the nature of a gene’s protein product and its role in the cell.
Tay-Sachs disease Huntington’s disease Galactosemia Albinism Cystic fibrosis Hypercholes- terolemia Phenylketonuria Achondroplasia Concept Map Section 14-1 Some Single Gene Disorders in Humans caused by Autosomal Recessive Autosomal Dominant include include Go to Section:
Figure 14-8 The Cause of Cystic Fibrosis Section 14-1 Chromosome # 7 CFTR gene 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. Normal CFTR is a chloride ion channel in cell membranes. Abnormal CFTR cannot be transported to the cell membrane. The cells in the person’s airways are unable to transport chloride ions. As a result, the airways become clogged with a thick mucus. Go to Section:
Comparing Dominance & Recessiveness • Determining whether an allele is dominant or recessive is critical in medical genetics because it helps predict which individuals are at high risk of inheriting a particular condition (phenotype).
DOMINANT: Can appear in either sex because an autosome carries the gene. If the child has it, then at least one parent has it. Do not skip generations If no offspring inherit the trait in one generation, its transmission stops because the offspring can pass on only the recessive form of the gene. RECESSIVE: Can appear in either sex & can skip generations. Affected individuals have a homozygous recessive genotype, whereas in heterozygotes (carriers) the wild type allele masks expression of the mutant allele. Parents of an affected individual are heterozygous or have the trait. Most occur unexpectedly Incest increases the risk of having a child with an autosomal recessive trait Comparing Dominance & Recessiveness
Active Reading Strategy: • Visit the ARIS website at www.mhhe.com/lewisgenetics8 • Select “Self Study, chapter 4” and “Web Activities” to find the website links needed to complete the following activity: • Go to the website for the National Organization for Rare Disorders. • Identify an autosomal recessive disorder and an autosomal dominant disorder. • Create a family for each one, and describe transmission of the disease over three generations.
