6.7 Mutations

AP Biology Unit 6.7: Mutations – The Key to Genetic Diversity

Genotype Change = Phenotype Change
A genotype refers to the genetic makeup of an organism, while a phenotype refers to its observable characteristics, such as physical appearance, behavior, and biochemical processes. Changes in an organism’s genotype can lead to changes in its phenotype, affecting its traits. This connection is vital in understanding how mutations influence the world around us.

Gene products, like proteins and RNA, determine the organism’s normal function. Any disruption in these genes can lead to altered phenotypes, which may result in either beneficial or detrimental changes.

For example, cystic fibrosis is caused by mutations in the CFTR gene, which encodes a protein involved in ion transport across cell membranes. Mutations in this gene disrupt the normal function, leading to thick mucus buildup, lung infections, and other symptoms. Another example is adaptive melanism in pocket mice, where a mutation in the MC1R gene results in darker fur, providing better camouflage in dark environments, enhancing survival.

Types of Mutations

Point Mutations and Frameshifts
Mutations are alterations in the DNA sequence, which can impact the type or amount of protein produced, ultimately changing an organism’s phenotype. Point mutations, a change in a single nucleotide, can alter protein structure. For instance, if the mutation occurs in an exon, the protein’s amino acid sequence changes, potentially altering its function.

Insertions or deletions can cause frameshift mutations, shifting the reading frame of the gene, resulting in a non-functional protein. These mutations can significantly alter an organism’s phenotype, often leading to negative consequences.

Effect-Based Mutation Types
Mutations can have varying impacts on phenotypes:

• Positive Mutations: Some mutations lead to a gain of function or increased protein production, which can be advantageous. For example, a mutation in a regulatory region can enhance gene expression.

• Neutral Mutations: These mutations, often found in non-coding regions, do not affect protein function or phenotype.
  
  
  

Environmental Implications
Mutations can arise from DNA replication errors or from exposure to external factors like radiation or chemicals. The outcome of a mutation—whether detrimental, beneficial, or neutral—depends on the environmental context. Mutations that were once harmful can become beneficial under different environmental conditions, driving genetic variation, the foundation of evolution.

Errors in Mitosis and Meiosis = Phenotype Change
Errors during mitosis or meiosis can also lead to changes in phenotype by altering chromosome number.

• Mitosis creates identical daughter cells, while meiosis produces four genetically diverse cells.

• Changes in chromosome numbers often lead to new phenotypes, impacting vigor and fertility. For instance, polyploidy (having more than two sets of chromosomes) can increase vigor in plants but cause sterility in animals.

Human Chromosomal Disorders
Changes in chromosome number can result in disorders such as Trisomy 21 (Down syndrome), where an individual has an extra copy of chromosome 21, causing developmental limitations and characteristic physical features. Turner syndrome, with only one copy of the X chromosome, results in short stature and infertility. Such chromosomal abnormalities can have significant impacts on an individual’s health and development.

Mutations and Genetic Variation

Genetic Variation and Natural Selection
Mutations create genetic variations that drive natural selection. Beneficial mutations can enhance an organism’s survival and reproductive success, allowing it to pass these traits on to future generations. Over time, advantageous variations become more common, shaping the population.

Horizontal Gene Transfer
Horizontal gene transfer increases genetic variation, primarily in prokaryotes. Mechanisms include:

• Transformation: Uptake of naked DNA from the environment.

• Transduction: Viral transmission of genetic information.

• Conjugation: DNA transfer between cells.

• Transposition: Movement of DNA segments within or between molecules.
  
  
  

Examples of Genetic Variation

• Antibiotic Resistance: Bacteria with resistance mutations survive antibiotic exposure, passing resistance to offspring, making them harder to treat.

• Pesticide Resistance: Insects with mutations that confer resistance to pesticides survive and reproduce, leading to resistant populations.

• Sickle Cell Disorder: This genetic disorder results from a single amino acid change in the hemoglobin protein. Individuals with one normal and one sickle cell allele are less susceptible to malaria, providing a survival advantage in malaria-prone environments.

Wrap-Up
Mutations are the primary source of genetic variation, which is essential for natural selection and evolution. Whether through beneficial, detrimental, or neutral effects, mutations shape life on Earth and drive adaptation to changing environments. Genetic variation allows populations to overcome challenges and evolve, highlighting the intricate relationship between genotype and phenotype.

Stay Curious for Unit 7!
We will explore more about how genetic changes impact evolution and natural selection in Unit 7. Remember, understanding mutations helps us grasp the amazing diversity of life and how organisms continue to adapt and survive.