Mendelian Genetics:
Laws of Dominance, Segregation & Independent Assortment

In Principle:
Systematic study of the laws of inheritance requires analysis of crosses
    These were first undertaken by Gregor Mendel in the 1860's [IG1 ResBrief 15.1, pp. 292-293]
       Mendel worked with seven phenotypic characters in Garden Peas (Pisum sativum) [HOMEWORK]
       Mendel arranged controlled crosses, analyzed the results numerically,
            and correctly inferred the rules governing their outcomes
    Rediscovery of Mendel's Laws in 1900 signalled the start of modern genetics

Phenotype (outward appearance) is influenced by genotype (hereditary makeup)
    or, individual characters are influenced by particular genes
    or, individual genes are expressed in such a way that they influence characters
    BTW: Genes are made of DNA located in chromosomes,
                  each at a particular physical location (a locus: plural, loci)
               Genes are typically expressed as proteins

Alternative forms of genes are called alleles;
    Genes exist in numerous allelic variants
    Any one (diploid) 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.

    Ex.: 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".
    Ex.: Pea seeds have alternative phenotypes green / yellow, or round / wrinkled

Some alleles (called dominant) mask the phenotypic expression of other alleles (called recessive)
    [Mendel's Law of Dominance] (IG1 Research Briefing 15.1, pp. 292-293)
    That is, the phenotype of the heterozygote resembles that of the dominant homozygote

    Dominant alleles are symbolized with capital letters (A)
        Recessive alleles with lower-case letters (a)
    Genotypes are described by giving both alleles: AA or Aa or aa
        Phenotypes are described by the letter of the expressed allele: "A" or "a"

    Ex.: the "taster" allele (T) is dominant to the "non-taster" allele (t) [HOMEWORK]:
               Homozygous TT or heterozygous Tt individuals show the "T" phenotype ("taster"):
                    only the homozygous tt individual shows the  "t" phenotype ("non-taster")

    Ex.: the "yellow" allele (Y) masks the "green" allele (y)
                the "round" allele (R) masks the "wrinkled" allele (r)
             Yy and Rr peas are yellow and round, respectively
              yy and rr peas are green and wrinkled, respectively
                    [Alternatively, yellow peas are GG or Gg,  round peas are W-  or Ww
                        and green & wrinkled peas are ggww
    Do not confuse inheritance of a genotype and expression of a phenotype

Mendel showed experimentally:
Alleles separate (segregate) during the formation of gametes (eggs & sperm) in meiosis

        half carry one allele & half carry the other [Mendel's Law of Segregation].
         [Mendel did not know about chromosomes, meiosis / mitosis, or DNA]

The random union of gametes produces zygotes that develop into new individuals.

       The zygotic genotypes occur in characteristic ratios, according to the parental genotypes
       Ex.: a monohybrid cross between two heterozygotes ( Aa x Aa )
           produces an expected genotypic ratio of 1 : 2 : 1 among AA, Aa, & aa genotypes.

The genotypic ratios produce characteristic phenotypic ratios,

        according to the dominance relationship of the alleles involved.
        Ex.: if A is dominant to a, the cross between heterozygotes produces
               an expected phenotypic ratio of 3 : 1 between "A" and "a" phenotypes.

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

    [Note: This law does not hold if the loci are physically adjacent ("linked") on the same chromosome
       Linkage alters the characteristic ratios: Mendel did not observe linkage

: Some online practice problems


All text material ©2012 by Steven M. Carr