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
       Mendel worked with seven phenotypic characters in Garden Peas (Pisum sativum) [HOMEWORK]
       Mendel arranged controlled crosses, analyzed the results numerically,
            and correctly inferred the laws governing their outcomes
    Rediscovery of Mendel's Laws in 1900 signaled 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 often [but not always] expressed as proteins

Alternative forms of genes are called alleles;
    Most genes exist in multiple allelic variants
    Any 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 mask the phenotypic expression of other alleles
            We call the former dominant, and the latter 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 can be described by the letter of the expressed allele: "A" or "a"

    Ex.: the "taster" allele (T) is dominant to the "non-taster" allele (t) :
               Individuals that are either homozygous TT or heterozygous Tt express the "T" phenotype ("taster"):
                    only the homozygous tt individual express the  "t" phenotype ("non-taster")

           Or, TT homozygotes and Tt heterozygotes show the taster phenotype, tt homozygotes are non-tasters

    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 gg ww
]

    Do not confuse inheritance of a genotype and expression of a phenotype
            Dominance is a relationship between alleles, not between phenotypes
            Yellow does not dominate Green

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


Homework
: Some online practice problems

              

All text material 2013 by Steven M. Carr