Inheritance and Mutations

DNA is the genetic material of all organisms on Earth. DNA stores the codes called genes that provide instructions to make proteins that can function in an organism and can determine characteristics of the organisms such as color of hair or flower petals or arms or wings – this is called gene expression. The central dogma states that DNA codes for RNA which codes for proteins that control an organism’s function and appearance.  

DNA is passed onto offspring through both asexual and sexual reproduction. In asexual reproduction, an exact clone of the parent cell is created that has the same DNA unless mutations had occurred. Mutations can occur in genes that can lead to new changes in an organism’s function or appearance. Some of these mutations are beneficial such as a change in an animal’s fur color to help it blend into its environment. Other mutations can lead to various diseases that affect an organism’s health.  

In sexual reproduction two parent cells combine their DNA to create an offspring that has similar characteristics as the parents but is not identical. These are specialized cells that combine called sex cells – one sperm and one egg – that carry half of the genetic information needed to make an offspring. Each offspring will inherit a unique combination of the parents’ genes. This is why people will look similar to their parents and similar to their siblings but not identical.  

DNA is made up of four chemicals known as nucleotides called adenine, thymine, cytosine, and guanine (A, T, C, and G for short). These four chemicals are arranged in different orders that produce different genes that code for different proteins. The genes are the specific arrangement of nucleotides that come in two or more forms. Each form is called an allele that produces different traits such as black or brown hair. Genotype refers to the genes an organism carries. The organism’s physical traits are its phenotype such as black or brown hair.  

In sexual reproduction offspring inherit two alleles for each trait – one from each parent. A person may have one blonde hair allele from one parent and one brown hair allele from another parent. A dominant trait is a trait that is always expressed in the phenotype or appearance of an organism. A recessive trait is expressed only when there are two recessive alleles present with no dominant allele. For example, if brown hair is dominant over blonde hair, and the person has one blonde and one brown hair allele, the person will have brown hair because it is the dominant trait.  

Each pair of genes is a part of an individual’s genotype. The genotype identifies the organisms’ alleles. Genotypes are written as two letters. Dominant alleles are shown as a capital letter and recessive allele is a lowercase letter. If brown hair is dominant it would be shown as a capital B for example and blonde hair would be a lower-case b. Since each parent contributes one allele for each gene and offspring can have three possible genotypes for our hair example: BB, Bb, or bb. Since brown hair is dominant those with the genotype BB or Bb would have the phenotype brown hair. If a person has the genotype bb then they would have no dominant brown hair allele and would have a blonde hair phenotype. This is called Mendelian inheritance.