Primer of Mendelian Genetics

    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 principles were first described by Gregor Mendel in 1867. 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, 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.

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).

    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"

*
Mendel was unaware that genes reside on chromosomes
      Genes that occur on the same chromosome are said to be linked
      Gene loci located near each other on a single chromosome will not assort independently.
             The characteristic ratios will be modified, according to how close they are.
             The modified ratios can be used to create a genetic map of the chromosome

    For example, sex in humans is determined by genes on sex chromosomes (X and Y)
           females are XX
have two alleles (one on each X)
           males are XY and have only one allele on the single X (hemizygous)
    Characters on the X (or Y) chromosomes are sex-linked


All text material ©2012 by Steven M. Carr