Heredity and DNA Science Study Guide
Organisms exhibit characteristics that define them. For example, an elephant has a trunk, an oak tree has green leaves and makes acorns, and humans have large brains. All these characteristics were inherited from parent organisms that looked and acted similarly. These heritable characteristics are transmitted on structures we call genes and chromosomes. In sexual reproduction, each parent contributes half of his or her genes to the offspring. The scientific study of heritable traits is called genetics, and Gregor Mendel is considered to be the father of genetics from his work with pea plants.
Genes and Chromosomes
Chromosomes are tiny structures within the cell nucleus that are the physical basis of heredity. Certain regions of the chromosomes are designated as genes. Each gene contains the information necessary to produce a single trait in an organism, and each gene is different from any other. For any trait, we inherit one gene from our father and one from our mother. Sometimes, even the genes in these pairs will be slightly different from each other.
Alternate forms of the same gene are called alleles. When the alleles are identical, we say that the individual is homozygous for that trait (a child may have blue eyes because he or she inherited two identical blue eye color genes from each parent). When the alleles are different, we say the individual is heterozygous for that trait (a child may have brown eyes because he or she inherited different eye color genes from each parent).
When genes exist in a heterozygous pairing, usually one is expressed over the other, and we say that it is dominant. The unexpressed gene is called recessive. With brown versus blue eyes, the allele for brown eyes is dominant over the one for blue eyes. Thus, the heterozygous child with blue and brown genes will have brown eyes. This general principal has many variations and exceptions.
Patterns of Inheritance
Because of the way gametes are formed, we can get some interesting distributions of characteristics in offspring. Biologists refer to the genetic makeup of an organism as its genotype. However, the collection of physical characteristics that result from the action of genes is called an organism's phenotype. Patterns of inheritance may yield surprising results because the genotype determines the phenotype, but the phenotype may hide some of the unexpressed alleles.
For example, if two blue-eyed parents reproduce, their offspring will have blue eyes because only blue alleles exist within the parents' gametes. However, two brown-eyed parents may actually be able to produce a blue-eyed child. This happens because the blue allele is hidden in the parents. Remember that a brown-eyed person can be that way because both his or her alleles are for brown eyes or because one allele is for brown and the other is for blue. When their gametes form, each parent may produce only gametes with alleles for brown eyes or they might produce gametes that contain the blue allele. If the latter occurs in both parents, then their offspring can have blue eyes even though both have brown eyes.
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