Primer of Mendelian
        Genetics


Primer of Mendelian Genetics (1865)

    The appearance of an organism (phenotype) is influenced by its heredity (genotype). 
Many individual characters (morphological, behavioral, biochemical, molecular, etc.) of organisms are influenced more or less directly by individual hereditary elements called genes. Genes are located on chromosomes, each at a particular physical location called a locus (plural, loci).

     Genetics is the science of analyzing phenotypes to infer the nature of their underlying genotypes. The basic method is the analysis of crosses. The basic principles were first described by Gregor Mendel in 1865. Genetics operated as a distinct science from the rediscovery of Mendel's work in 1900, without knowledge of the genetic material until 1953. Genetics is distinct from molecular biology, which analyzes genotypes (in a DNA molecule) to predict phenotypes (which are often direct or indirect products of proteins). For this reason, the so-called Central Dogma of molecular biology (DNA » RNA » Protein) is sometimes called "reverse genetics." Terminology here has been updated to reflect 20th & 21st century sage.

1
. Alternative forms of genes are called alleles; every individual possesses two alleles for each gene.
       [Mendel called these units "elementen": the term gene was not used until 1909]

    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.

2
. Some alleles (called dominant) mask the phenotypic expression of other alleles (called recessive).

    Dominance is determined by comparison of the heterozygote phenotype with that of the two homozygotes

    Dominant alleles are symbolized with a capital letter (A);
        recessive alleles with a lower-case letter (a).
        [Mendel's Law of Dominance]

    For example, some people can taste the chemical phenylthiocarbimide (PTC) ("tasters"),
            and some cannot ("non-tasters").
        The character "PTC sensitivity" is influenced by a gene with two alleles,

             one associated with "taster" and one with "non-taster".
        The "taster" allele masks the expression of the  "non-taster" allele in heterozygotes:
             Homozygous TT or heterozygous Tt individuals both show the "T" phenotype ("taster"):
             only a homozygous tt individual show the "t" phenotype ("non-taster").
         Because the phenotype of the Tt individual resembles that of the TT individuals,
              the T allele is described as dominant to the t allele.

3
. The two alleles separate (segregate) during the formation of gametes (eggs & sperm);

     half of the germs cells carry one allele & half carry the other [Mendel's Law of Segregation].

4
. Random union of gametes produces zygotes that develop into new individuals.

    Zygotic genotypes occur in characteristic ratios, according to the genotypes of the parents.
    For example, a 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.

6.  Alleles at separate loci are inherited independently [Mendel's Law of Independent Assortment]
       This produces characteristic genotypic and phenotypic ratios.
           For example, in a dihybrid cross between two "double heterozygotes" (
AaBb x AaBb )
                The genotypic ratios are 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1
                   
for the genotypes AABB  AABb  AAbb  AaBB  AaBb  Aabb aaBB  aaBb  aabb
                and the phenotypic ratios are 9 "AB" : 3 "Ab" :
3 "aB" : 1 "ab"


All text material ©2020 by Steven M. Carr