Life Sciences, Part 10

Question 1: Define and discuss mutation at the DNA level.

Answer 1: A DNA mutation occurs when the normal gene sequence is altered. Mutations can happen when DNA is damaged as a result of environmental factors, such as chemicals, radiation, or ultraviolet rays from the sun. It can also happen when errors are made during DNA replication. The phosphate-sugar side rail of DNA can be damaged if the bonds between oxygen and phosphate groups are disassociated. Translocation happens when the broken bonds attempt to bond with other DNA. This repair can cause a mutation. The nucleotide itself can be altered. A C, for example, might look like a T. During replication, the damaged C is replicated as a T and paired with a G, which is incorrect base pairing. Another way mutations can occur is if an error is made by the DNA polymerase while replicating a base. This happens about once for every 100,000,000 bases. A repair protein proofreads the code, however, so the mistake is usually repaired.

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Question 2: Define and discuss gene, genotype, phenotype, and allele.

Answer 2: A gene is a portion of DNA that identifies how traits are expressed and passed on in an organism. A gene is part of the genetic code. Collectively, all genes form the genotype of an individual. The genotype includes genes that may not be expressed, such as recessive genes. The phenotype is the physical, visual manifestation of genes. It is determined by the basic genetic information and how genes have been affected by their environment. An allele is a variation of a gene. Also known as a trait, it determines the manifestation of a gene. This manifestation results in a specific physical appearance of some facet of an organism, such as eye color or height. For example the genetic information for eye color is a gene. The gene variations responsible for blue, green, brown, or black eyes are called alleles. Locus (pl. loci) refers to the location of a gene or alleles.

Question 3: Discuss the concept of dominant and recessive.

Answer 3: Gene traits are represented in pairs with an upper case letter for the dominant trait (A) and a lower case letter for the recessive trait (a). Genes occur in pairs (AA, Aa, or aa). There is one gene on each chromosome half supplied by each parent organism. Since half the genetic material is from each parent, the offspring’s traits are represented as a combination of these. A dominant trait only requires one gene of a gene pair for it to be expressed in a phenotype, whereas a recessive requires both genes in order to be manifested. For example, if the mother’s genotype is Dd and the father’s is dd, the possible combinations are Dd and dd. The dominant trait will be manifested if the genotype is DD or Dd. The recessive trait will be manifested if the genotype is dd. Both DD and dd are homozygous pairs. Dd is heterozygous.

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