7

7.1 Mendel and the Garden Pea

7.2 What Mendel Found

7.3 Mendel Proposes a Theory

7.4 Analyzing Mendel's Results

7.5 Mendel's Laws

7.9 Chromosomes Are the Vehicles of Mendelian Inheritance

7.10 Human Chromosomes

7.6 Epistasis

7.7 Multiple Alleles

7.8 Other Modifications of the Genotype

7.11 The Role of Mutations in Human Heredity

7.12 Hemophilia

7.13 Sickle-Cell Anemia

7.14 Other Disorders

7.15 Genetic Counseling and Therapy

Constructing a Genetic Map
This exercise explores the recombination of three alleles in a dihybrid cross. You can either move the position of genes on a chromosome or enter recombination frequencies and see what genetic map you get.

Gene Segregation Within Families
This interactive exercise allows you to explore the potential makeup of families. Specifically, you can assess the likelihood that a family of a given size will have all girls (or boys) or that offspring will be homozygous for a recessive trait.

Heredity in Families
In this exercise, you can explore a bank of pedigrees and analyze the dominance/recessiveness and X-linkage versus autosomal location of alleles within families.

Gene Therapy. Scientists have long sought a way to introduce "healthy" genes into humans that lack them. Such a therapy was actually achieved in 1990 for a rare blood disorder, but it has been difficult to advance the research further, as the adenovirus used to carry the healthy version of the gene proved inappropriate (it's a cold virus, and most people have antibodies directed against it). New virus vectors avoid these problems, and offer hope of cures for many hereditary disorders. Cystic fibrosis, for example, results from a defect in a single gene encoding a chloride ion transport protein, and gene therapy may finally offer a way to cure it.

1. Improvements in gene vectors renew hope for gene therapy.
2. Altering ANDi: Inserting DNA into a primate.
3. Gene therapy may allow us to combat debilitating Parkinson's disease.

Mendel and Genetics
Few scientists have made as great an impact on biology as Gregor Mendel, a Bavarian monk who over a hundred years ago worked out the basic pattern of heredity. He carried out experimental crosses with common pea plants in his monastery garden, and was able to explain the pattern of their inheritance with a simple "model" in which factors are contributed by each parent to an offspring. We now know Mendel's factors to be chromosomes.

Human Hereditary Disorders
Many inherited disorders reflect a mutation in a specific gene. Cystic fibrosis, for example, is a fatal disorder caused by a defective chloride channel protein--overly thick mucus blocks the passages of lungs and pancreas. The Cf mutation is carried by 1 in 20 Caucasians. Sickle-cell anemia is caused by a mutation that alters one amino acid in the protein hemoglobin. Unfortunately, the change causes hemoglobin molecules to stick together, clumping up uselessly. The disorder is common in Central Africa because it confers resistance to malaria in heterozygous individuals.

How Regulatory Genes Direct Vertebrate Development
Most genetic disorders are the result of chemical modification of DNA. While many mutations have little or no effect on an organism, the mutations causing genetic disorders often have profound effects. Consider the activity of the anterior pituitary gland in humans. The anterior pituitary releases hormones from specialized cells that regulate growth, metabolism, lactation, reproduction, and response to stress. Early in development, the cells of the anterior pituitary differentiate into many specialized hormone-producing cell types, triggered by the activation of specific transcription factors. Many pituitary transcription factors have been identified, and mutations in these transcription factor genes are sometimes associated with inherited pituitary disorders. For example, mutations in the LHX3 gene, encoding a pituitary transcription factor, have been identified in patients with retarded growth, pituitary hormone deficiency, and spinal deformities. Simon Rhodes, along with colleagues at Indiana University and Purdue University, is investigating the way in which mutations in the LHX3 gene alter pituitary function to produce these profound effects.