GED · Science · 6 min read
Genetics, Inheritance, and Evolution
- Explain how inherited alleles determine an organism's genotype and how genotype and environmental factors interact to shape its phenotype.
- Predict the outcomes of genetic crosses using Punnett squares and interpret inheritance patterns on pedigree charts.
- Describe how natural selection, genetic variation, and multiple lines of scientific evidence explain biological evolution and common ancestry.
The Molecular Basis of Heredity: DNA, Genes, and Traits
Every living organism relies on DNA to store the instructions necessary for life. DNA carries genetic information that influences an organism's biological traits, shaping everything from physical appearance to internal biological functions. Within this long molecule are specific functional units called genes. Genes are segments of DNA that contain instructions for making proteins or functional RNA molecules, which carry out essential tasks within cells.
Genes can exist in different versions known as alleles. Alleles are alternative forms or variants of a specific gene. The particular combination of alleles an organism carries for a gene or set of genes associated with a trait is called its genotype. In contrast, an organism's phenotype is the observable physical or physiological expression of its traits. An organism's phenotype is determined by the interaction between its genotype and environmental factors, meaning that inherited instructions and surroundings work together to produce the final outcome.
In many sexually reproducing diploid organisms, individuals carry two alleles for each autosomal gene, usually one inherited from each parent; exceptions include genes on sex chromosomes. An individual is homozygous if it possesses two identical alleles for a specific gene. An individual is heterozygous if it possesses two different alleles for a specific gene. In standard scientific notation, dominant alleles are usually represented by uppercase letters, while recessive alleles are represented by lowercase letters.
| Term | Definition | Example Notation |
|---|---|---|
| Homozygous | Possesses two identical alleles for a specific gene | AA or aa |
| Heterozygous | Possesses two different alleles for a specific gene | Aa |
| Genotype | The combination of alleles an organism carries for a trait | AA, Aa, or aa |
| Phenotype | Observable physical or physiological expression of traits | Expressed physical characteristic |
Common mistakes
- Confusing homologous and analogous structures: Homologous structures share anatomical similarities from a common ancestor even if their functions differ (like human, whale, and bat forelimbs), whereas analogous structures perform similar functions but evolved independently without indicating recent common ancestry.
- Believing that prior births affect future genetic probability: Each fertilization event is an independent probability event completely unaffected by the outcomes of previous offspring.
- Thinking all mutations are passed to children: Somatic mutations occur in non-reproductive body cells and usually affect only the individual; only mutations in sperm, eggs, or the cells that produce them can be passed to offspring in sexually reproducing animals.
- Treating genotype and phenotype as identical: Genotype refers to an organism's genetic makeup, whether at one gene or across many genes, while phenotype is the observable expression of traits shaped by genotype and environmental factors.
Key points
- DNA contains genes that code for proteins or functional RNA, and alternative forms of these genes are called alleles.
- An organism's genotype is its allele combination, while its phenotype is the observable trait shaped by genotype and environmental factors.
- In complete dominance, dominant alleles mask recessive ones, and Punnett squares model the independent probabilities of offspring inheriting specific genotypes.
- Mutations in sperm, eggs, or their precursor cells can be inherited through sexual reproduction in animals; mutations in other cells can be inherited through asexual reproduction, as in some plants.
- Natural selection acts directly on phenotypic variation, causing adaptive traits that increase reproductive success to increase in frequency over generations.
- Common ancestry and biological evolution are supported by the fossil record, comparative anatomy, embryology, biogeography, and molecular homologies.
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