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Biology LibreTexts

4: Inheritance

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    • 4.1: Meiosis
      Most eukaryotes replicate sexually - a cell from one individual joins with a cell from another to create the next generation. For this to be successful, the cells that fuse must contain half the number of chromosomes as in the adult organism. Otherwise, the number of chromosomes would double with each generation! The reduction in chromosome number is achieved by the process of meiosis.
    • 4.2: Mendelian Genetics
      Mendel’s First Law, also called The Law of Equal Segregation, states that during gamete formation, the two alleles at a gene locus segregate from each other; each gamete has an equal probability of containing either allele. More than one allele of a gene can be present in an individual since  most eukaryotic organisms have at least two sets of homologous chromosomes. For organisms that are predominantly diploid, chromosomes exist as pairs, with one homolog inherited from each parent.
    • 4.3: Linkage
      Mendel’s Second Law does not apply to every pair of genes that could be studied. In fact, we now know that alleles of loci that are located close together on the same chromosome tend to be inherited together. This phenomenon is called linkage, and is a major exception to Mendel’s Second Law of Independent Assortment. Researchers use linkage to determine the location of genes along chromosomes in a process called genetic mapping and is important to natural processes of heredity and evolution.
    • 4.4: Exceptions to simple dominance
      The phenotypes described thus far have a nearly perfect correlation with their associated genotypes; in other words an individual with a particular genotype always has the expected phenotype. However, many phenotypes are not determined entirely by genotype alone. They are instead determined by an interaction between genotype and non-genetic, environmental factors.
    • 4.5: Epistasis
      Epistasis (which means “standing upon”) occurs when the phenotype of one locus masks, or prevents, the phenotype of another locus. Thus, following a dihybrid cross fewer than the typical four phenotypic classes will be observed with epistasis. As we have already discussed, in the absence of epistasis, there are four phenotypic classes among the progeny of a dihybrid cross.
    • 4.6: Sex-linkage and sex determination
      In the previous chapter we introduced sex chromosomes and autosomes. For loci on autosomes, the alleles follow the normal Mendelian pattern of inheritance. However, for loci on the sex chromosomes this is mostly not true, because most of the loci on the typical X-chromosome are absent from the Y-chromosome, even though they act as a homologous pair during meiosis. Instead, they will follow a sex-linked pattern of inheritance.