Chapter Two The Study of Heredity

Discovering the Mechanisms of Heredity One early attempt at explaining family resemblance was known as the blending theory. If blending took place, however, traits would be irreversibly changed from generation to generation and would not persist. The actual study of human genetics is highly complicated. It is not surprising that an early understanding of genetics came through work with plants.

The Work of Gregor Mendel In his first experiments, Mendel crossed true-breeding plants to create hybrids. When he crossed true-breeding plants with white flowers with true-breeding plants with violet flowers, Mendel discovered that all the hybrids had violet flowers. Mendel then allowed the hybrid plants to self-pollinate. In the next generation, approximately 3/4 of the plants had violet flowers and 1/4 had white. The trait (violet flowers) that is seen in the hybrid is dominant. The trait that cannot be seen (white flowers), yet has the ability to be passed on, is known as recessive.

A Model of Genetic Events Based on his experiments, Mendel formulated two principles of inheritance. The principle of segregation counters the idea of blending inheritance. For every trait, an individual has two hereditary factors, one inherited from each parent. Together, these cells determine the appearance of the trait in the organism. The two factors are discrete, that is, they do not blend.

The Principle of Segregation In the development of sex cells, the hereditary factors separate, forming sex cells that have one or the other factor. Individual sex cells combine at fertilization, producing new arrangements of hereditary units.

Principle of Independent Assortment Mendel’s second principle is the principle of independent assortment. Mendel studied the simultaneous inheritance of more than one trait. From this data, he concluded that inheritance patterns of differing traits are independent of one another. For example, whether a plant is tall or dwarf has no relationship to whether the plant has white or violet flowers.

What Is a Trait? The observable or measurable characteristics of an organism determine the phenotype. A trait is just one aspect of the phenotype. The genotype is the individual’s specific genetic constitution. The phenotype results from the interaction of the genotype and the environment.

Mendelian Inheritance in Humans The hereditary units described by Mendel are called genes. Genes occur in alternate forms termed alleles. If an individual has two of the same alleles, they are said to be homozygous dominant if both alleles are dominant. If both alleles are recessive, they are defined as homozygous recessive. If they have two different alleles, they are heterozygous. There are a number of human characteristics that are inherited in a simple Mendelian manner. An example is the form of the human earlobe, which can be either free-hanging (dominant) or attached (recessive).

Cytogenetics The principles of genetics were outlined by Mendel, but he was unaware of the biological nature of the genetic material. As scientists began to experiment on the genetics of organisms, such as the fruit fly, it became apparent that genetic material is found in the nucleus of the cell.

The Chromosomes When a cell begins to divide, long rope-like structures become visible within the nucleus, known as chromosomes. In humans, both sexes have 22 pairs of non-sex chromosomes, referred to as autosomes, and a 23rd pair of sex chromosomes. The normal female possesses two homologous sex chromosomes (XX) while the normal male has only one X chromosome which pairs with a different type, the Y chromosome.

Cell Division The physical basis of Mendelian genetics becomes clear upon observing the movement of chromosomes during cell division. Mitosis is the process which results in the growth and replacement of body cells. Meiosis is specialized cell division, which results in the production of sex cells, or gametes. In meiosis, the chromosome number is reduced from 46 to 23.

Re-examining Mendelian Genetics The details of cell division help us understand Mendelian genetics. However, it also shows us exceptions to Mendel’s rules. For example, genes on the same chromosome are referred to as linked and may not be inherited independently of one another. Since the X and Y chromosomes are not homologous, traits that are X-linked will have different inheritance patterns for males than for females.

The Molecular Basis of Heredity Understanding the structure of the DNA molecule is also crucial to our comprehension of heredity. DNA consists of long chains of nucleotides lying parallel to one another. Each nucleotide has a backbone of sugar and phosphate linked to a base. There are four complementary bases in DNA: adenine and thymine, which always bond together, as do cytosine and guanine.

The DNA Molecule The DNA molecule is then twisted into a three-dimensional shape known as the double helix.

Replication of DNA

Protein Synthesis Some of the most important molecules of the body are proteins. Proteins are long chains of basic units known as amino acids. The blueprint for a specific protein is located in the DNA. Messenger RNA copies the sequence of bases from the DNA and takes the information to the ribosome. Transfer RNA is found within the ribosome. In tRNA three bases form an anticode for a particular amino acid. The tRNA lines up opposite the appropriate codon on the mRNA molecule. After the amino acids are brought into the proper positions, they link together and form a protein.