Heredity and DNA Science Study Guide (page 2)
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.
Molecular Basis of Heredity
Like the rest of the body, heredity is dependent on the functioning of biological molecules. The molecule at the basis of heredity is the long, chain polymer we call deoxyribonucleic acid or, more familiarly, DNA. A gene consists of DNA molecules, which are known as polymers. A polymer is a very large molecule made up of many similarly repeating units (called nucleotides in DNA). The shape of the DNA molecule is a double spiral or helix, sort of like a winding staircase with a handrail on each side.
In what is known as the Central Dogma of biology, DNA contains hereditary information, which is transferred (or transcribed) into another molecule called ribonucleic acid (RNA), which is then transformed (or translated) into proteins. It is these protein molecules that are responsible for the expression of inherited traits.
Four special molecules called nucleotides are contained within the structure of the larger DNA molecule. These nucleotides are known as adenine, cytosine, guanine, and thymine. They are commonly abbreviated by their first letters to A, C, G, and T. The arrangement of these nucleotides forms a code that can contain hereditary information. Special enzymes in the cell nucleus read this code and transcribe it into an RNA molecule, specifically known as messenger RNA, because it takes the coded message from the nucleus into the cell's cytoplasm. It is here that the small organelles called ribosomes translate the coded messenger RNA into protein molecules. These protein molecules are either destined to become structural components (such as in the muscles) or enzymes where they will regulate metabolic reactions. These proteins give us our inherited traits.
Human beings have been manipulating this process for thousands of years. We perform this process when we breed crop animals or plants. In the past, we were clever but not very sophisticated because we had to find the best animal or plant and breed it until we achieved the desired crop. The corn we eat today is very different than the plant called maize from which it was bred. The more sophisticated and technically challenging manipulation of an organism's DNA that we perform today is called genetic engineering and is part of the biotechnology industry.
Organisms exhibit characteristics that define them, such as an elephant having a trunk, an oak tree having green leaves, and humans having 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. 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. The molecule at the basis of heredity is the long, chain polymer we call deoxyribonucleic acid or DNA. A gene consists of molecules of DNA. In what is known as the Central Dogma of biology, DNA contains hereditary information, which is transferred (or transcribed) into another molecule called ribonucleic acid (RNA), which is then transformed (or translated) into proteins. These protein molecules are responsible for the expression of inherited traits.
Practice problems of this concept can be found at: Heredity and DNA Science Practice Questions
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