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4.2: Mendelian Genetics

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  • Learning Objectives

    • Define and identify examples of: homozygous, heterozygous, allele, gene, locus, dominant, recessive, genotype, phenotype.
    • Construct genetic crosses for Po, F1, and F2 generations and predict the genotypes and phenotypes of offspring.
    • Explain Mendel’s laws of segregation and independent assortment, and how they predict the 3:1 dominant-to-recessive phenotypic ratio among the F2 of a monohybrid cross, or the 9:3:3:1 phenotypic ratio in a dihybrid cross, respectively. Relate the key events of meiosis that explain Mendel’s first and second laws.
    • Be able to draw chromosomes during meiosis with alleles labeled.
    • Interpret phenotypic ratios of progeny in experimental organisms to infer how particular traits are inherited.
    • Predict genotypic and phenotypic ratios or probabilities of outcomes among progeny of single factor and multifactor crosses using simple rules of probability (sum rule and product rule).
    • Cite the most common molecular explanations for dominant and recessive alleles.

    Mendel Studied Visible Character Traits in Pea Plants

    Through careful study of patterns of inheritance, Mendel recognized that a single trait could exist in different versions, or alleles, even within an individual plant or animal. Recalling that genes contain information needed to make proteins, we now understand that alleles are differences in gene sequence. If these differences alter the production, structure, or function of the protein, an observable or measurable change in the organism may occur. For example, Mendel identified two forms of a gene for seed color: one allele gave green seeds and the other gave yellow seeds.

    Figure \(\PageIndex{1}\): Seven traits Mendel studied in peas. (Wikipedia-Mariana Ruiz-PD)

    Representing genes and alleles

    Alleles are forms of genes, if genes are DNA sequences, then alleles are variations in the sequence of a gene.

    In genetics, you may encounter different ways of representing alleles. Traditionally, genes are represented as letters when studying Mendelian genetics. This representation is usually easy to follow in problems involving crosses; however, using letters does not reflect our modern understanding of the genetic differences between alleles, which often involves knowing whether or not a product is functional or how the allele was identified.

    • A plus can be used to indicate that the gene product of an allele is functional.
    • A minus can be used to indicate that the gene product is not functional.
    • If sequence information is known, the nucleotide or amino acid change can be identified.
    • In model organisms, alleles are often given numbers when they are identified as mutants and these numbers can be used to identify different alleles of the gene.

    The table shows some examples of how we might represent genes and alleles.

    Examples of ways genes can be represented

    Examples of representing wild-type alleles

    Examples of representing the mutant alleles

    C representing a hypothetical gene C or C+ c or C - or C1
    white or w gene in Drosophila white or white+ or w+ w - or w1118
    ced-1 gene in C. elegans ced-1 or ced-1(+) or ced-1+ ced-1(-) or ced-1(e1754)

    CFTR gene in humans

    Cftr gene in mouse

    CFTR or CFTR + CFTR(delF508) or CFTR(482G-A)

    Heterozygous and homozygous

    Mendel’s work and discoveries are especially remarkable because he made his observations and conclusions (in 1865) without knowing about the relationships between genes, chromosomes, and DNA. We now know the reason why more than one allele of a gene can be present in an individual: most eukaryotic organisms have at least two sets of homologous chromosomes. For organisms that are predominantly diploid, such as humans or Mendel’s peas, chromosomes exist as pairs, with one homolog inherited from each parent. Diploid cells therefore contain two different alleles of each gene, with one allele on each member of a pair of homologous chromosomes. If both alleles of a particular gene are identical, the individual is said to be homozygous for that gene. On the other hand, if the alleles are different from each other, the genotype is heterozygous.

    Although a typical diploid individual can have at most two different alleles of a particular gene, many more than two different alleles can exist in a population of individuals. In a natural population the most common allelic form is usually called the wild-type allele. However, in many populations there can be multiple variants at the DNA sequence level that are visibly indistinguishable because all produce a normal, wild-type appearance. There can also be various mutant alleles (in wild populations and in lab strains) that vary from wild type in their appearance, each with a different change at the DNA sequence level. Such collections of mutations are known as an allelic series.

    wild type vs wild-type

    The noun wild type (without a hyphen) means the the most common form of a gene, phenotype, or organism under standard conditions.

    • Example: The mutant worms produce fewer offspring than wild type.

    The adjective wild-type (with a hyphen) can be used to describe a gene, allele, organism, phenotype, or trait as what is most commonly present among a population or under standard conditions.

    • Example: A wild-type worm produces about 300 offspring.

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