13.1 Mendel solved the mystery of heredity.
Early Ideas About Heredity: The Road to Mendel
• Two classical assumptions regarding heredity prior to Mendel were that species were constant and that traits were transmitted directly. (p. 242)
• Early researchers discovered that some inherited characters can disappear and appear again in later generations; some characters can segregate among different offspring; and some forms are more likely to appear than others. (p. 243)
Mendel and the Garden Pea
• Gregor Mendel, an Austrian monk, conducted hybridization studies on garden peas at his monastery. (p. 244)
• Mendel chose garden peas for several reasons: He knew he could produce hybrids by crossing varieties; many varieties were available; and pea plants are easy to grow and have short generation times. In addition, if left alone, the flowers can self-fertilize. (p. 244)
• Mendel conducted his experiment in three stages: (1) He allowed all plants to self-fertilize to produce pure-breeding individuals; (2) he performed crosses between varieties exhibiting alternate character forms; and (3) he allowed hybrid offspring to self-fertilize for several generations. (p. 245)
What Mendel Found
• From his experiments, Mendel derived four conclusions concerning heredity: (1) The plant crosses did not produce progeny of intermediate appearance. (2) For each pair of alternative forms of a character, one alternative was not expressed in the F1 generation, but did reappear in some F2 individuals. (3) Pairs of alternative traits segregated among progeny of a particular cross. (4) Alternative traits were expressed in a 3:1 ratio in the F2 generation, which actually was a 1:2:1 ratio for pure-breeding dominant: non-pure-breeding dominant: pure-breeding recessive. (p. 248)
• Mendel's model of heredity contains five basic elements: (1) Parents do not transmit physiological traits directly to their offspring. (2) Each individual receives two factors that may code for the same trait or two alternative traits. (3) Not all copies of a factor are identical. (4) The alleles from each parent do not influence each other in any way. (5) The presence of a particular allele does not ensure the expression of that trait. (p. 249)
How Mendel Interpreted His Results
• By convention, dominant traits are assigned capital letters and recessive traits lowercase letters, which can easily be put into a Punnett square for analysis. (p. 250)
• Mendel proposed two laws of heredity: the Law of Segregation (alternative alleles segregate independently) and the Law of Independent Assortment (genes on different chromosomes assort independently). (pp. 251-253)
Mendelian Inheritance Is Not Always Easy to Analyze
• Most phenotypes reflect the action of many genes. (p. 255)
• Quantitative traits produce continuous variation. (p. 255)
• Pleiotropic alleles have more than one effect on the phenotype. (p. 255)
• Incomplete dominance occurs when alternative alleles are not fully dominant or recessive, and offspring express both parental phenotypes. (p. 256)
• Environmental effects can affect allele expression to produce varying phenotypes. (p. 256)
• Epistasis can occur when one gene interferes with the expression of another gene. (pp. 257-258)
13.2 Human genetics follows Mendelian principles.
Most Gene Disorders Are Rare
• Most genetic disorders are recessive (e.g., Tay-Sachs), although some are dominant (e.g., Huntington disease). (p. 259)
Multiple Alleles: The ABO Blood Groups
• Some genes, such as those determining ABO blood groups and Rh blood groups, exhibit codominance in which each allele has its own effect. (p. 260)
Patterns of Inheritance Can Be Deduced from Pedigrees
• Family pedigrees can be used to deduce the mode of inheritance of a trait through a family, such as hemophilia in European royal families. (p. 261)
Gene Disorders Can Be Due to Simple Alterations of Proteins
• Sickle cell anemia is caused by a single-nucleotide change in the gene coding for hemoglobin. (p. 262)
Some Defects May Soon Be Curable: Gene Therapy
• Theoretically, some heredity disorders should be able to be cured by inserting a healthy version of a damaged gene into an affected individual. Early attempts have met with varied results. (p. 263)
More Promising Vectors
• New vectors such as AAV may offer greater promise in gene therapy. (p. 264)
13.3 Genes are on chromosomes.
Chromosomes: The Vehicles of Mendelian Inheritance
• Mendelian traits assort independently because the genes determining them are located on chromosomes that assort independently. (p. 266)
Genetic Recombination
• Crossing over creates new genetic combinations. If two different genes are located relatively far apart on a chromosome, crossing over is more likely to occur somewhere between them than if they are located closer together. (p. 267)
• A genetic map can be constructed that measures the distance between genes in terms of the frequency of recombination. (p. 268)
Human Chromosomes
• The twenty-third pair of chromosomes in humans carries the genes that determine sex. The Y chromosome and appears to contain 78 genes. (pp. 270-271)
Human Abnormalities Due to Alterations in Chromosome Number
• Nondisjunction refers to the failure of homologues to separate properly during meiosis and can lead to aneuploidy (incorrect number of chromosomes), which can cause a number of genetic conditions, such as Down syndrome. (p. 272)
Genetic Counseling
• Many genetic abnormalities can be detected early in pregnancy using amniocentesis, chorionic villi sampling, and many newer techniques. (p. 274)