Mendelian Genetics:
Laws of Dominance, Segregation & Independent Assortment


In Principle:
Systematic study of genetic inheritance requires analysis of crosses
    These were first undertaken by Gregor Mendel in 1860's
       Mendel worked with characters in Garden Peas (Pisum sativum) [HOMEWORK]
       Mendel arranged controlled crosses, analyzed results numerically,
                    inferred the laws governing their outcomes
    Rediscovery of Mendel's Laws in 1900 signaled start of modern genetics
            Genetics
was a scientific discipline for 50 years before DNA



Phenotype (external appearance) is influenced by genotype (hereditary makeup)
    or, individual characters are influenced by particular genes
    or, individual genes are expressed in such a way as to influence characters (traits)
        IMPORTANT: A gene "for" a phenotypic trait is almost always an oversimplification
    BTW: Genes are made of DNA located in chromosomes,
                  at a particular physical location (a locus: plural, loci)
               Genes are often [but not always] expressed as proteins

                    Molecular phenotype: a gene "for" an enzyme

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)
           Suppose character "PTC sensitivity" influenced by a gene with two alleles,

                                                                  one for "taster" and one for "non-taster"

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

Mendel's Law of Dominance

Some alleles *mask" the phenotypic expression of other alleles in in heterozygous combination
            Call the former dominant, the latter recessive
(IG1 Research Briefing 15.1, pp. 292-293)
    That is, heterozygote phenotype is identical to that of one of the homozygotes
                
Call allele in that homozygote "dominant", call other "recessive"

    Dominant alleles symbolized with capital letters (A)
        Recessive alleles with lower-case letters (a)
    Genotype described by giving both alleles: AA or Aa or aa
        Phenotype 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 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 WW  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's Law of Segregation

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

        half carry one allele, half carry the other
         [Mendel did not know about chromosomes, meiosis / mitosis, or DNA]

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

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

   
The genotypic ratios produce characteristic phenotypic ratios,

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

Mendel's Law of Independent Assortment
     Alleles at separate loci inherited independently
       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: What genotypic & phenotypic ratios result for a cross AAbb x aaBB ? AABB x aabb ?
                          Calculate the genotypic & phenotypic ratios for a trihybrid cross (AaBbDd x AaBbDd)

      This law may not hold if loci are physically adjacent ("linked") on same chromosome
                Linkage alters characteristic ratios: Mendel did not observe linkage
 
Practice problems in Mendelian Genetics
             

All text material ©2016 by Steven M. Carr