HiSET · Science · 6 min read
Genetics, Inheritance, and Natural Selection
- Explain the roles of DNA, genes, chromosomes, and mutations in providing instructions for proteins and generating heritable variation.
- Differentiate between genotypes and phenotypes, and calculate offspring probabilities using Punnett squares under complete dominance.
- Describe how natural selection, environmental pressures, and adaptations drive evolutionary change in populations over generations.
The Molecular Basis of Heredity: DNA, Genes, and Chromosomes
In eukaryotic cells, most genetic instructions are stored in the nucleus, where DNA is packaged with proteins into chromosomes; mitochondria and, in plants and algae, chloroplasts also contain DNA. Genes are specific segments of this DNA that provide instructions for making proteins or functional RNA molecules. The language of DNA consists of sequences of four nitrogenous bases: adenine, thymine, cytosine, and guanine. In a protein-coding gene, the specific sequence of these DNA bases specifies the amino acid sequence of a protein. Because protein structure depends on this code, altering a gene's DNA sequence can change the structure and biological function of the resulting protein.
A mutation is a permanent change in the nucleotide sequence of an organism's DNA. Mutations are critically important in biology because they are the ultimate source of new genetic variation within a species. A mutation is not automatically negative; depending on how it alters protein function and affects the organism, a mutation can have a beneficial, harmful, or neutral effect on an organism's survival.
In sexually reproducing animals, the location of a mutation determines whether it can be inherited. Body cells, known as somatic cells, make up the tissues of an individual. Mutations in somatic cells generally affect only that individual and are not inherited through sperm or eggs. In contrast, gametes, such as sperm and egg cells, contain half the chromosome count of regular body cells. In sexually reproducing animals, mutations that occur in gametes or the cells that produce them can be passed to offspring, allowing new variations to enter future generations.
Common mistakes
- Believing that acquired traits can be inherited: Physical changes developed through use, injury, or habit, such as scars, calluses, muscle mass built from exercise, or speaking a specific language, are acquired characteristics and are not directly passed to offspring through genes.
- Thinking that individual organisms evolve: Natural selection acts on the phenotypes of individual organisms, but evolutionary change occurs at the population level over generations. An individual organism does not evolve genetically during its own lifetime.
- Confusing genotypic and phenotypic ratios: In a monohybrid cross between two heterozygous parents, the genotypic ratio is 1:2:1, but the phenotypic ratio is 3:1 because the dominant allele masks the recessive allele in heterozygous individuals.
- Assuming natural selection creates mutations when needed: Mutations arise without foresight, and natural selection does not produce mutations to meet a need; it can also favor beneficial variants that arise after an environmental change.
Key points
- Eukaryotic genes occur on nuclear chromosomes and, in some cells, in mitochondria or chloroplasts; protein-coding genes specify proteins, while other genes produce functional RNA molecules.
- Mutations are changes in DNA sequence that generate genetic variation; in sexually reproducing animals, mutations must occur in gametes or the cells that produce them to be passed to offspring.
- An organism's genotype is its inherited combination of alleles, while its phenotype represents observable traits shaped by interactions between genotype and the environment.
- Under complete dominance, crossing two heterozygous parents produces an expected genotypic ratio of 1 homozygous dominant to 2 heterozygous to 1 homozygous recessive, and an expected phenotypic ratio of 3 dominant to 1 recessive.
- Inherited traits like eye color and blood type are transmitted via DNA, whereas acquired characteristics like scars and muscle built from exercise are not passed to offspring.
- Natural selection occurs when environmental pressures lead to higher reproductive success (biological fitness) for individuals with advantageous adaptations, increasing favorable allele frequencies in populations over generations.
- Geographic separation can lead to independent adaptations and new species, while populations unable to adapt quickly enough to environmental changes face extinction.
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