12.2A: Genes as the Unit of Heredity
- Describe the structure of a gene and how offspring inherit genes from each parent
Pairs of Unit Factors, or Genes
Mendel proposed that paired unit factors of heredity were transmitted faithfully from generation to generation by the dissociation and reassociation of paired factors during gametogenesis and fertilization, respectively. After he crossed peas with contrasting traits and found that the recessive trait resurfaced in the F 2 generation, Mendel deduced that hereditary factors must be inherited as discrete units. This finding contradicted the belief at that time that parental traits were blended in the offspring.
A gene is made up of short sections of DNA that are contained on a chromosome within the nucleus of a cell. Genes control the development and function of all organs and all working systems in the body. A gene has a certain influence on how the cell works; the same gene in many different cells determines a certain physical or biochemical feature of the whole body (e.g., eye color or reproductive functions). All human cells hold approximately 21,000 different genes.
Genetics is the science of the way traits are passed from parent to offspring. For all forms of life, continuity of the species depends upon the genetic code being passed from parent to offspring. Evolution by natural selection is dependent on traits being heritable. Genetics is very important in human physiology because all attributes of the human body are affected by a person’s genetic code. It can be as simple as eye color, height, or hair color. Or it can be as complex as how well your liver processes toxins, whether you will be prone to heart disease or breast cancer, and whether you will be color blind.
Genetic inheritance begins at the time of conception. You inherited 23 chromosomes from your mother and 23 from your father. Together they form 22 pairs of autosomal chromosomes and a pair of sex chromosomes (either XX if you are female, or XY if you are male). Homologous chromosomes have the same genes in the same positions, but may have different alleles (varieties) of those genes. There can be many alleles of a gene within a population, but an individual within that population only has two copies and can be homozygous (both copies the same) or heterozygous (the two copies are different) for any given gene.
Key Points
- A gene is a stretch of DNA that helps to control the development and function of all organs and working systems in the body.
- Genes are passed from parent to offspring; the combination of these genes affects all aspects of the human body, from eye and hair color to how well the liver can process toxins.
- A human will inherit 23 chromosomes from its mother and 23 from its father; together, these form 23 pairs of chromosomes that direct the inherited characteristics of the individual.
- If the two copies of a gene inherited from each parent are the same, that individual is said to be homozygous for the gene; if the two copies inherited from each parent are different, that individual is said to be heterozygous for the gene.
Key Terms
- gene : a unit of heredity; the functional units of chromosomes that determine specific characteristics by coding for specific proteins
- chromosome : a structure in the cell nucleus that contains DNA, histone protein, and other structural proteins
- genetics : the branch of biology that deals with the transmission and variation of inherited characteristics, in particular chromosomes and DNA