Genetics Notes
Chapter 3 - Basic Principles of Heredity

These notes are provided to help direct your study from the textbook. They are not designed to explain all aspects of the material in great detail; that is what class time and the textbook is for. If you were to study only these notes, you would not learn enough genetics to do well in the course.

Mendel's Approach
  1. he started with 34 varieties of garden peas that differed in a number of distinct traits, e.g. wrinkled peas vs. smooth peas or tall plants vs. short plants
  2. did two years worth of tests to determine the purity (true-breeders or not) of the varieties
  3. he then performed reciprocal crosses, which test for differences due to sex, for all varieties

The parental generation (P) of a wrinkled-seeded female and a round-seeded male was crossed, and also a round-seeded female and a wrinkled-seeded male was crossed, produced, in both cases, a first filial generation (F1) of all round offspring (figure 3.3).

Reciprocal crosses gave the same results regardless of the sex of the round parent.

  1. Mendel referred to the offspring as hybrids
  2. this cross is called a monohybrid cross because they are hybrids for only one character
  3. because all of the offspring in the F1 were round, he referred to round as the dominant trait and wrinkled as the recessive trait

Next, two individuals from the F1 generation were crossed to yield the second filial F2 generation of 5474 round and 1850 wrinkled.

Round seeds from the F2 (figure 3.3)
  1. some seeds only produce round progeny. They bred true.
  2. other plants when selfed, produce round and wrinkled in a 3:1 ratio

Wrinkled seeds from the F2 (figure 3.3)
  1. bred true and only produced wrinkled seeds

genotype - genetic constitution, that is the suite of genes that an organism possesses

phenotype - observable attributes of the organism, that is its appearance

Mendel found that of the round-seeded plants produced in the second generation, 1/3 produced only round seeds, and 2/3 produced round seeded- and wrinkled-seeded plants. Combining these observations with the wrinkled-seeded plants, he concluded that the second filial generation F2 is composed of 1/4 pure breeding round individuals, 1/2 segregating round individuals, and 1/4 pure breeding wrinkled individuals.

Mendel concluded that....
  1. each parent donated one hereditary unit (allele) to each offspring
  2. as each parent donates one allele (via the gametes) all the offspring must possess 2 units (alleles)
  3. in the F1 generation, all individuals possess one allele for round seeds and one allele for wrinkled seeds with round being dominant to wrinkled
  4. at the gene (locus) for seed shape, there are two forms (alleles).
Mendel did seven such monohybrid crosses. All gave similar results and led to the same conclusions.

homozygous - if an individual has identical alleles at a locus, then the individual is homozygous.

heterozygous - if an individual has non-identical alleles at a locus, then the individual is heterozygous.

The explanation of the passage of these alleles is referred to as Mendel's first principle....

The Rule of Segregation
- a gamete receives only one allele from the pair of alleles possessed by an organism; fertilization reestablishes the double number
OR IN OTHER WORDS

- two members of a gene pair (alleles) segregate from each other during the formation of gametes. As a result, half the gametes carry one allele and the other half carry the other allele.
One of the easiest ways to visualize a breeding experiment is to diagram the cross using a Punnett square. (figure 3.6)

Mendel continued to test his rule of segregation by carrying out breeding experiments through the F6 generation. In each case, he could predict the phenotypic ratios in the offspring. He also used what is called a test cross.

Like any good scientist, Mendel wanted a different kind of cross to test his rule of segregation. The test cross gave him such a test. By crossing a homozygous recessive individual with a heterozygous individual, the rule of segregation would predict a 1:1 phenotypic ratio among the offspring. When Mendel did such a cross, he did observe a 1:1 ratio. This confirmed his prediction and supported the theory (rule of segregation) upon which the prediction was based.

Today, because the rule of segregation is so strongly supported, we use test crosses to determine the genotype of an individual with the dominant phenotype assuming. of course, that the rule of segregation is still valid (tables 3.2 and 3.3).

multiple alleles - a gene can have 2 or more alleles segregating in a population. However, a single individual cannot have more than 2 alleles. The ABO blood group in humans is an example of such a locus (figure 5.6, page 107). If 3 alleles are segregating at a locus, then there are 3+2+1 = 6 possible genotypes among the individuals in the population. If n alleles are segregating at a locus, then there are the sum of i from i = 1 to n possible genotypes.

Rule of Independent Assortment

Mendel extended his experiments to dihybrid crosses. From these data, he postulated the Rule of Independent Assortment. The parental generation consisted of a round yellow pea (RRYY) and a wrinkled green pea (rryy). The F1 generation was round and yellow (RrYy). The F1 generation was selfed to yield the F2 generation.

The F2 generation consisted of... (figures 3.11 and 3.12)
9 round, yellow
3 round, green
3 wrinkled, yellow
1 wrinkled, green

Mendel's second principle states ...
that, during gamete formation, the alleles at one locus segregate into the gametes independently of the pair of alleles found at a different locus.

If we set up a Punnett square, we can see why this is so (figure 3.11). The genotypes of the two F1s are given below.

RrYy x RrYy


Given independent assortment of the alleles in an F1 plant, we would expect 4 types of gametes to be produced by each of the F1s.

- RY, rY, Ry, ry


Each gamete receives one allele for seed color and one allele for seed shape (figure 3.11).
	Genotypic ratio			Phenotypic ratio
 	  RRYY	    1            	  
	  RrYY      2			  round, yellow      9
	  RRYy      2
	  RrYy      4

    	  RRyy      1                     round, green       3
	  Rryy      2

	  rrYY      1			  wrinkled, yellow   3
	  rrYy      2		

	  rryy      1			  wrinkled, green    1
Extending these results to more than 2 loci yields the following table.
monohybrid dihybrid trihybrid n-hybrid
F1 gametic genotypes 2 4 8 2n
proportion of homozygous recessives in F2 1/4 1/16 1/64 (1/2n)2
number of different F2 phenotypes given complete dominance 2 4 8 2n
number of different genotypes 3 9 27 3n


In the crosses that we have examined so far, the alleles have either been dominant or recessive. For example, yellow peas are dominant to green peas, tall plants are dominant to short plants However, alleles can also be codominant in that the alleles are expressed equally and completely in the phenotype (table 5.1, page 103, as in the heterozygote who expresses both M and N antigens (for example, MN blood group in humans).
         1) there are two alleles   Lm, Ln
         2)       genotypes                   phenotypes
                       LmLm                        M blood
                       LnLn                        N blood
                       LmLn                        MN blood

   e.g. sickle cell anemia
         1) two alleles Hba (normal), Hbs
         2)      genotypes                    phenotypes
                   HbaHba      normal
                   HbaHbs      normal
                   HbsHbs      sickle cell anemia
At the molecular level, the traits are codominant as both types of hemoglobin are produced in equal amounts by a heterozygous person. However, at the organismal level, Hba is dominant to Hbs, HbaHba is normal, HbaHbs is essentially normal, and HbsHbs is sickle cell anemia. Incomplete dominance means one allele is incompletely dominant over another allele (figure 5.2, page 103).
  In snapdragons,
           P               RR (red) x rr (white)

           F1                   Rr x Rr (pink)

           F2            RR        Rr           rr
                        red       pink         white
                         1         2             1
  1. at the organismal level, the red allele is incompletely dominant over the white allele
  2. However, at the molecular level the alleles are codominant in that pink is caused by 1 allele (red) coding for a functional protein that produces red pigment and by 1 allele (white) coding for a non-functional protein that does not produce any red pigment. Thus only half the amount of red pigment is produced and a pink flower is the result.
Thus dominance and recessiveness are due to the relative expression of the alleles at the organismal level, but at the molecular level most allelic pairs are likely to be codominant.

Another important point: Mendel also demonstrated that the alleles are unchanged in the passage from one generation to the next. At the time of Mendel's discoveries, the general feeling was that traits were blended in the offspring and the modified allele (blended alleles) were then passed to subsequent generations. Mendel showed that the traits are passed as discrete particles (round or wrinkled) and are not changed when passed from generation to the next. This particulate nature of inheritance also supported Darwin's theory of natural selection in that now selection could operate to change the frequency of alleles while not changing the allele itself.

Epistasis Epistasis means to stand on. The expression of one allele prevents or interferes with the expression of alleles at another locus (chapter 5, pages 108 - 112).

Recessive Epistasis:
In mice, as in many mammals, there are actually 5 loci that control coat color. We will look at two of these loci.
  1. agouti pattern (gray) results from the banding pattern of pigments deposited in each hair
    two alleles
    A is dominant to a and causes banding
    AA, Aa are agouti
    aa produces no bands and the individual is black
  2. another locus controls the expression of the black pigment
    two alleles
    C is dominant to c and causes production of the black pigment
    CC, Cc produces black pigment
    cc produces no black pigment and is white (albino)
P       homozygous black  CCaa x ccAA  homozygous white

F1                      CcAa    all agouti
                                                                   
                     dihybrid cross of F1 x F1

F2                  9 agouti: 3 black: 4 albino
This phenotypic ratio is similar to the 9:3:3:1 ratio expected for the F2 generation in a dihybrid cross with alleles exhibiting complete dominance, except that the last two categories (3+1) have been combined.
  1. one locus whose dominant allele (C) is necessary for the development of color and
  2. another locus whose dominant allele (A) is necessary for the banding pattern to produce the agouti color precursor -----> black pigment ------> agouti pattern        (allele C)                  (allele A)
The recessive genotype of the color gene (cc) is epistatic to, or it interferes with, the dominant allele of the agouti pattern.

Duplicate recessive epistasis
Epistasis in corn
P    homozygous purple  AABB x aabb  homozygous white

F1                   AaBb  all purple

                dihybrid cross of F1 x F1

F2                  9 purple:7 white
                     9:3+3+1 = 9:7
            genotype                phenotype
            A-, B-                   purple           9
            A-, bb                   white            3
            aa, B-                   white            3
            aa, bb                   white            3
                 "-" equals a wild card

The biochemical pathway is . . .
  precursor (colorless)----->intermediate (colorless)----->purple pigment
                     (allele A)                     (allele B)
Thus aa is epistatic to B and bb is epistatic to A.
Review table 5.2 (page 112) to get a picture of the variety of ratios that can be produced depending upon the type of loci and alleles involved. The key to recognizing epistasis is to recognize that the F2 phenotypic ration will be some permutation of 9:3:3:1.

Links on the Web
1) http://anthro.palomar.edu/mendel_1.html

2) Variation and Mendel's Laws -- Discussion of dominance, co-dominance, and multiple alleles.

3) Diverse gene expression patterns are possible -- The notes on extensions to Mendel's principles from a faculty member at Washington State University.


Last updated on 23 January 2007
Provide comments to Dwight Moore at mooredwi@emporia.edu
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