Primer of Mendelian Genetics
    The outward appearance of an organism (phenotype) is influenced by its hereditary makeup (genotype).  Many Individual characters (morphological, behavioral, biochemical, molecular, etc.) of organisms may be determined more or less directly by individual hereditary elements called genes (which are made out of DNA). Genes are located on chromosomes, each at a particular physical location called a locus (plural, loci).

     Genetics is the business of analyzing phenotypes to infer the nature of their underlying genotypes. It is distinct from molecular biology, which analyzes genotypes (as written in the DNA) to predict phenotypes (which are frequently proteins). For this reason, molecular biology is sometimes called "reverse genetics."

1. Alternative forms of genes are called alleles; every individual possesses two alleles for each gene.
    An individual with two identical alleles is a homozygote and is described as homozygous;
        an individual with two dissimilar alleles is a heterozygote and is described as heterozygous.
    For example, some people can taste the chemical phenylthiocarbimide (PTC).
        The character "PTC sensitivity" may be due to a gene with two alleles,
        one for "taster" and one for "non-taster".

2. Some alleles (called dominant) mask the phenotypic expression of other alleles (called recessive).
    Dominant alleles are symbolized with a capital letter (A) &
        recessive alleles with a lower-case letter (a).
    For example, the "taster" allele (T) may mask the "non-taster" allele (t):
        a heterozygous Tt individual will show the "T" phenotype ("taster").
    Molecular phenotypes are typically the result of the expression of both alleles:
        such alleles are called codominant.

3. The two alleles separate (segregate) during the formation of gametes (eggs & sperm) in meiosis;
     half of the germs cells carry one allele & half carry the other [Mendel's Law of Segregation].

4. The random union of gametes produces zygotes that develop into new individuals.
    The zygotic genotypes will occur in characteristic ratios, according to the genotypes of the parents.
    For example, a monohybrid cross between two heterozygotes ( Aa x Aa )
        produces an expected genotypic ratio of 1:2:1 among AA, Aa, & aa genotypes.

5. The genotypic ratios produce characteristic phenotypic ratios,
        according to the dominance relationships of the alleles involved.
        For example, if A is dominant to a, the cross between heterozygotes produces
            an expected phenotypic ratio of 3:1 among "A" and "a" phenotypes.
       Co-dominant molecular alleles typically produce a 1:2:1 ratio of "A", "Aa", & "a" phenotypes.

6.  Alleles at separate loci are inherited independently [Mendel's Law of Independent Assortment]
       This produces charactertistic genotypic and phenotypic ratios.
           For example, a dihybrid cross between two "double heterozygotes" ( AaBb x AaBb )
                The genotypic ratios are 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1
                and the phenotypic ratios are 9 "AB" : 3 "Ab" : 3 "aB" : 1 "ab"

7.  Loci that occur on the same chromosome may not show independent assortment.
         Such loci are said to be linked: the ratios will vary according to the distance between them
                  The ratios can be used to construct physical maps of chromosomes
         Some loci are linked on the sex chromosomes and are described as sex-linked
                 Female mammals are XX: they have two alleles for any sex-linked locus
                         Male mammals are XY: they have a single allele for any sex -linked locus
                  X-linked genes are inherited in acharacteristic pattern that differs between sexes
                         Y-linked genes are expressed only in males
           

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All text material ©2006 by Steven M. Carr

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