Genetics Cheat Sheet

The core ideas of Genetics distilled into a single, scannable reference — perfect for review or quick lookup.

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Quick Reference

DNA Structure

DNA (deoxyribonucleic acid) is a double-stranded helical molecule composed of nucleotides, each consisting of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (adenine, thymine, cytosine, guanine). The two strands run antiparallel and are held together by hydrogen bonds between complementary base pairs, with adenine pairing with thymine and cytosine pairing with guanine.

Genes

A gene is a discrete segment of DNA that contains the instructions for producing a functional product, typically a protein or a functional RNA molecule. Genes are the fundamental units of heredity, and the human genome contains approximately 20,000 to 25,000 protein-coding genes, though many additional non-coding sequences play regulatory roles.

Alleles

Alleles are alternative forms of a gene that arise through mutation and occupy the same locus on homologous chromosomes. An organism may be homozygous (carrying two identical alleles) or heterozygous (carrying two different alleles) at a given locus, and the combination of alleles influences the organism's phenotype.

Mendelian Inheritance

Mendelian inheritance describes the patterns of heredity first elucidated by Gregor Mendel, governed by the Law of Segregation (each organism carries two alleles per gene, which separate during gamete formation) and the Law of Independent Assortment (alleles of different genes assort independently during meiosis). These principles predict the ratios of genotypes and phenotypes observed in genetic crosses.

Dominant and Recessive Traits

A dominant allele expresses its phenotypic effect in both the homozygous and heterozygous conditions, while a recessive allele only manifests its phenotype when present in the homozygous state. Dominance is a relationship between alleles at a single locus and does not imply that the dominant allele is more common or more advantageous in a population.

Genotype and Phenotype

The genotype is the specific set of alleles carried by an organism at one or more genetic loci, while the phenotype is the observable expression of those alleles as physical traits, biochemical characteristics, or behaviors. The relationship between genotype and phenotype is influenced by dominance patterns, gene interactions, and environmental factors.

Mutations

A mutation is a permanent alteration in the nucleotide sequence of DNA that can occur spontaneously during replication or be induced by mutagens such as ultraviolet radiation or certain chemicals. Mutations range from single-nucleotide changes (point mutations) to large-scale chromosomal rearrangements and serve as the ultimate source of all genetic variation.

Genetic Engineering

Genetic engineering encompasses a set of technologies used to directly manipulate an organism's genome by adding, deleting, or modifying specific DNA sequences. Modern tools such as CRISPR-Cas9 allow researchers to make precise, targeted edits to the genomes of bacteria, plants, animals, and human cells with unprecedented efficiency and accuracy.

Epigenetics

Epigenetics is the study of heritable changes in gene expression that occur without alterations to the underlying DNA sequence. These changes are mediated by mechanisms such as DNA methylation, histone modification, and non-coding RNA activity, which can activate or silence genes in response to environmental signals and developmental cues.

Genomics

Genomics is the interdisciplinary field devoted to mapping, sequencing, and analyzing entire genomes, encompassing all of an organism's genetic material rather than individual genes. Genomics integrates high-throughput sequencing technologies with bioinformatics to study genome structure, function, evolution, and the complex relationships between genes and traits.

Key Terms at a Glance

Allele:One of two or more variant forms of a gene that occupy the same locus on homologous chromosomes and can produce different phenotypic effects.

Autosome:Any chromosome that is not a sex chromosome. Humans have 22 pairs of autosomes and one pair of sex chromosomes (XX or XY).

Carrier:An individual who is heterozygous for a recessive allele associated with a genetic disorder, showing no symptoms but capable of passing the allele to offspring.

Codon:A sequence of three consecutive nucleotides in mRNA that specifies a particular amino acid or signals the termination of translation during protein synthesis.

Crossing Over:The reciprocal exchange of genetic material between homologous chromosomes during prophase I of meiosis, resulting in recombinant chromosomes with new allele combinations.

Diploid:A cell or organism containing two complete sets of chromosomes, one inherited from each parent, designated as 2n. In humans, the diploid number is 46.

DNA Replication:The molecular process by which a double-stranded DNA molecule is copied to produce two identical daughter molecules, each containing one original and one newly synthesized strand (semiconservative replication).

Dominant:Describes an allele that expresses its phenotypic effect in both the homozygous and heterozygous states, masking the expression of the recessive allele.

Epigenome:The complete set of chemical modifications to DNA and histone proteins in a cell that regulate gene expression without altering the DNA sequence itself.

Gene Expression:The process by which information encoded in a gene is used to synthesize a functional product, typically through transcription of DNA into mRNA followed by translation into protein.

Genetic Code:The set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of a protein, using three-letter codons that specify each of the 20 amino acids and three stop signals.

Genome:The complete set of genetic material present in an organism, including all of its genes as well as non-coding DNA sequences.

Genotype:The genetic constitution of an organism at one or more loci, represented by the specific alleles present on its chromosomes.

Haploid:A cell or organism containing a single set of unpaired chromosomes, designated as n. In humans, gametes (sperm and egg) are haploid with 23 chromosomes.

Heritability:A statistical measure estimating the proportion of phenotypic variation in a population that is attributable to genetic differences among individuals, ranging from 0 to 1.

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