MENDEL’S LEGACY

GENETIC’S is the field of Biology devoted to understanding how characteristics are transmitted from Parents to Offspring. Genetics was founded with the work of Gregor Mendel an Austrian monk who worked with garden peas, Pisum sativum.

OJECTIVES: Describe the steps involved in Mendel’s experiments on garden peas. Distinguish between dominant and recessive traits. State TWO Laws of Heredity that were developed from Mendel’s work. Explain the difference between an ALLELE and a GENE. Describe how Mendel’s results can be explain by scientific knowledge of genes and chromosomes.

Gregor Mendel
1. Mendel was an Austrian monk.
2. Mendel formulated two fundamental laws of heredity in the early 1860's.
3. He had previously studied science and mathematics (including Statistics) at the University of Vienna. Mendel’s knowledge of statistics later proved valuable in his research on Heredity – the transmission of characteristics from Parent to Offspring.

Blending Concept of Inheritance, Before Mendel
1. This theory stated that offspring would possess traits intermediate between those of different parents.
2. Red and white flowers produce pink; a later return to red or white was considered instability in genetic material.
3. Charles Darwin wanted to develop a theory of evolution based on hereditary principles; blending theory was of no help.
    a. The blending theory did not account for variation and could not explain species diversity.
    b. The theory of inheritance proposed by Mendel can account for presence of differences among members of a population generation after generation.
    c. Mendel's work was unrecognized until 1900.

Mendel's Garden Peas, Pisum sativum

1. Mendel did a statistical study (he had a mathematical background).

2. He prepared his experiments carefully and conducted preliminary studies.

3. He chose the garden pea, Pisum sativum, because peas were easy to cultivate, had a short generation time, and could be cross-pollinated.

4. From many varieties, Mendel chose 22 true-breading varieties for his experiments.

5. True-breeding varieties had all offspring like the parents and like each other.

6. Mendel studied Seven Traits or Characteristics. Each characteristic occurred in Two Contrasting Traits:
    a. Flower Color – purple or white
    b. Flower Position along Stem – axial or terminal
    c. Seed or Pea Color - yellow or green
    d. Seed Texture or Shape – round (smooth) or wrinkled
    e. Pod Color – green or yellow
    f. Pod Appearance or Shape – inflated or constricted
    g. Plant Height - long or short stems

7. Mendel traced inheritance of individual traits and kept careful records of numbers.

8. He used his knowledge of Statistics to analyze his observations of the Seven Characteristics.

Mendel’s Methods

1. Mendel was able to document the traits of each generation’s parents
by carefully controlling how the pea plants were Pollinated.

2. Pollination occurs when pollen grains produced in the Male Reproductive parts of a flower, the Anthers, are transferred to the Female Reproductive part of the flower, the Stigma.

3. Self-pollination occurs when pollen is transferred from the Anthers of a flower to the Stigma of either the same flower or a flower on the same plant.

4. Cross-pollination involves flowers of Two Separate Plants.

5. Pea plants normally reproduce by Self-Pollination.

6. Mendel was able to interrupt the Self-pollination and perform Cross-pollination by removing the Anthers from a flower and manually transferring the Anther, or actually the Pollen, of a flower on one plant to the Stigma of a flower on another plant.

7. Mendel was able to protect his flowers from receiving any other Pollen that might be
transferred by wind, insects, giving him more control over the pollination of his pea plants.

MENDEL’S EXPERIMENTS

1. A hybrid is the product of parent organisms that are True-Breeding or PURE-BREEDING for different forms of One Trait.

2. Plants that are PURE for a trait Always produce offspring with that Trait. The term STRAIN denotes plants that are Pure for a Specific Trait.

3. Mendel produced strains by allowing the plants to Self-Pollinate for several generations.

4. He eventually obtained 14 strains, one for each of the 14 traits he observed. He called each
strain a Parental Generation or  P1 generation. This is the parental generation in a breeding experiment.

5. Mendel used Cross-pollination between two strains and tracked each trait through two
generations. (Monohybrid Cross)

6. F1 generation is the first-generation offspring in a breeding experiment. The First Filial Generation.

7. F2 generation is the second-generation offspring in a breeding experiment. The Second Filial Generation.

8. He also performed reciprocal crosses of pollen on stigmas (e.g. tall-with-short and short-with tall). 

Mendel's Results and Conclusions

1. His results were contrary to those predicted by a blending theory of inheritance.

2. Whenever Mendel crossed strains, one of P1 Traits failed to appear in the F1 plants.
In every case, that trait reappeared in a ratio of 3:1 in the F2 generation.

3. He found that the F1 plants resembled only one of the parents.

4. Characteristic of other parent reappeared in about 1/4 of F2 plants; 3/4 of offspring resembled the F1 plants.

5. These 3:1 results were possible if :

   a. Mendel hypothesized that the trait in the F1 generation contained two factors for each trait;

   b. One Dominant Factor because it masked, or dominated the other factor for a specific trait;

   c. and one Recessive Factor, the Trait that did not appear in the F1 generation but reappeared in the F2 generation in the ratio of 3:1.

   d. factors separated when gametes were formed; a gamete carried on copy of each factor;

   e. Random fusion of all possible gametes occurred upon fertilization.

   f. Thus, a trait controlled by a Recessive Factor had No observable effect on an organism’s Appearance when it was paired with a trait controlled by a Dominant Factor.

6. Results of his experiments led Mendel to develop his First Law of Inheritance:

7. Mendel's Law of Segregation: Each organism contains two factors for each trait; factors segregate, or separate, in formation of gametes, During Meiosis; each gamete contains one factor for each trait. When Two Gametes combine during Fertilization, the Offspring have two factors controlling a specific trait.

8. Mendel's law of segregation is consistent with the theory of inheritance because many
individual factors are passed on from generation to generation.

9. Reshuffling of factors explains variations and why offspring differ from their parents.

10. Mendel also crossed plants that differed in two characteristics (Dihybrid Crosses)
such as flower color and seed color.

11. The data from these more complex crosses (Dihybrid Crosses) showed that traits produced by Dominant Factors Do Not Necessarily appear together. Mendel concluded that the Factors for different characteristics are NOT Connected. This led Mendel to develop his Second Law of Inheritance:  

12. Mendel's Law of Independent Assortment states that factors for different characteristics are distributed to gametes independently.   

As Viewed By Modern Genetics

1. Most of Mendel’s findings agree with what now know about Molecular Genetics - the
Study of the Structure and Function of Chromosomes and Genes.

2. A Gene is a segment of DNA on a Chromosome that controls a particular Hereditary Trait. Because Chromosomes occur in Pairs, Genes Also occur in Pairs.

3. Each of several Alternative forms of a Gene is called an ALLELE. Mendel’s Factors are now called alleles.

4. Letters are used to represent Alleles:

   a. Capital Letters refer to a Dominant Allele that masks or hides expression of a recessive allele.

   b. Lowercase Letters refer to a Recessive Allele its expression is masked by a dominant allele.

GENETIC CROSSES

Today Geneticists rely on Mendel’s work to Predict the likely outcome of Genetic Crosses.

OBJECTIVES: Explain how probability is used to predict the results of genetic crosses.  Use a Punnett square to predict the results of monohybrid and dihybrid crosses.  Explain how a testcross is used to show the genotype of an individual whose phenotype is dominant.  Differentiate a monohybrid cross from dihybrid cross.

GENOTYPE AND PHENOTYPE

1. The Genetic Makeup of an organism is its GENOTYPE. The Genotype consists of the alleles that the organism inherits from its parents.

2. Pea plant flower color could be PP, Pp or pp. The P allele is the dominant allele and represents purple flowers (PP or Pp). The p allele is the Recessive allele. Two Recessive alleles (pp) give white flowers.

3. The Appearance of an organism as a result of its genotype is called PHENOTYPE.

4. The Phenotype of a PP or Pp pea plant is Purple Flowers.

5. The Phenotype of a pp pea plant is White Flowers.

6. When Both Alleles of a Pair are ALIKE, the organism is said to be HOMOZYGOUS
for that characteristic. An organism may be Homozygous Dominant (genotype PP)
or Homozygous Recessive (genotype pp).

7. When the Two Alleles in the Pair are DIFFERENT, the organism is HETEROZYGOUS
(genotype Pp) for that characteristic.

PROBABILITY

1. Probability is the likelihood that a specific event will occur or is the likely outcome
a given event will occur from random chance.

2. A Probability may be expressed as a Decimal (0.75), a Percentage (75%), or a Fraction (3/4).

3. Probability is determined by the following Equation:

PROBABILITY =    Number of times an event is expected to happen
                                  Number of opportunities for an event to happen

4. The results predicted by probability are more likely to occur when there are many trails.

5. With each coin flip there is a 50% chance of heads and 50% chance of tails.

6. Chance of inheriting one of two alleles from a parent is also 50%.

PREDICTICTING RESULTS OF A MONOHYBRID CROSSES

1. A cross between individuals that involves One Pair of Contrasting Traits is called a
MONHYBRID CROSS.

2. A cross between a pea plant that is pure for producing purple flowers (genotype PP)
and one that is pure for producing white flowers (genotype pp) is an example of a Monohybrid Cross.

3. Biologist use a Diagram called a PUNNETT SQUARE to aid in predicting the
probability that certain traits will be inherited by offspring.

4. This tool provides a simple method to calculate probable results of a genetic cross.

5. In a Punnett square, all possible types of sperm alleles are lined up vertical, all
possible egg alleles are lined up horizontally; every possible combination is placed in squares.

Example 1: HOMOZYGOUS X HOMOZYGOUS

P = Dominant Purple         p = Recessive White

1. Genotype PP X Genotype pp 

2. The combination of Alleles in the Punnett Square indicate all the possible genotypes that can result from the cross. The predicted genotype is Pp every case.

3. There is a 100% probability that the offspring will have the Genotype Pp (Heterozygous Dominant)
and the Phenotype purple flower color.

Example 2: HOMOZYGOUS X HETEROZYGOUS

B = Dominant Black   b = Recessive Brown

1. Genotype BB X Genotype Bb

2. The combination of Alleles in the Punnett Square indicates all the possible genotypes that can result from the cross. The predicted genotype BB is 2/4 or 50 % and the genotype Bb is 2/4 or 50 %.

3. There is a 50% probability that the offspring will have the Genotype BB (Homozygous Dominant) and the Phenotype Black.

4. There is a 50% probability that the offspring will have the Genotype Bb (Heterozygous dominant) and the Phenotype Black.

5. The probability of the Phenotype of Black coat in every case is 4/4 or 100%.

Example 3: HETEROZYGOUS X HETEROZYGOUS

B = Dominant Black   b = Recessive Brown

1. Genotype Bb X Genotype Bb

2. The combination of Alleles in the Punnett Square indicates all the possible genotypes that can result from the cross. The predicted genotype BB is 1/4 or 25 % and the genotype Bb is 2/4 (1/2) or 50 % and genotype bb is 1/4 or 25%.

3. 3/4 or 75% of the offspring from this cross are predicted to have a Black Coat and 1/4 or 25% of the offspring are predicted to have a Brown Coat.

4. The Ratio of the Genotypes that appear in offspring is called the GENOTYPIC RATIO. FIG. 9-7 = 2:1:1

5. The Ratio of the Phenotypes that appear in offspring is called the PHENOTYPIC RATIO. FIG. 9-7 = 3:1

Example 4: TESTCROSS

1. TESTCROSS are used to determine the genotype of an unknown individual.

2. You perform a Testcross in which an individual of unknown genotype is crossed with a Homozygous Recessive (bb) individual.

3. A Testcross can determine the Genotype of any individual hose Phenotype is dominant. Genotypes BB or Bb.

4. If the unknown genotype is homozygous black (BB), all offspring will be black.

5. If the genotype is heterozygous (Bb), about 1/2 the offspring will be black.

6. If the cross produces one brown offspring in a litter of eight, the genotype of the black coated parent is likely to be heterozygous, Bb.

Example 5: INCOMPLETE DOMINANCE

1. When one allele has complete dominance over another, a relationship called COMPLETE DOMINANCE.

2. In Complete Dominance, heterozygous plants (PP) and dominant homozygous plants (Pp) are indistinguishable in Phenotype. Both pea plants have purple flowers.

3. Sometimes, the F1 offspring will have a Phenotype in between that of the Parents, a relationship called INCOMPLETE DOMINANCE.

4. Incomplete dominance occurs when Two or More Alleles Influence the Phenotype, resulting in a Phenotype Intermediate between the Dominant Trait and Recessive Trait.

5. In four o’clocks both the Allele for Red Flowers (R) and the Allele for White Flowers (r) influence the Phenotype.

6. Neither Allele is completely dominant over the other Alleele.

7. When four o’clocks self-pollinate, red flowering plants produce only red flowering offspring, and white flowering plants only produce white flowering offspring.

8. However, when red four o’clocks are crossed with white four o’clocks the F1 offspring all have pink flowers. 100% of the offspring of this cross have the Rr Genotype, which results in the Pink Phenotype.

Example 6: CODOMINANCE

1. CODOMINANCE occurs when Both Alleles for a gene are Expressed in a Heterozygous offspring.

2. In Codominance neither Allele is Dominant or Recessive, nor do alleles blend in the phenotype.

3. A Capital Letter is used for one allele (R), the other allele is a Capital Letter and apostrophe (R’). Given the Genotype RR’.

PREDICTING RESULTS OF DIHYBRID CROSSES

1. A DIHYBRID CROSS is a cross between individuals that involves TWO Pairs of Contrasting Traits.

2. Predicting the results of a Dihybrid Cross is more complicated that predicting the results of a Monohybrid cross because there are more possible combinations.

Example 1: HOMOZYGOUS X HOMOZYGOUS

R = Dominant Round         r = Recessive Wrinkled
Y = Dominant Yellow       y = Recessive Green

1. Suppose you wanted to predict the results of a cross between a pea plant that is homozygous for round, yellow seeds (RRYY), and one the is homozygous for wrinkled, green seeds (rryy).
 

2. The Punnett square is used to predict the results of the cross, and contains 16 boxes.

3. The independently assorted alleles from one parent – RY, RY, RY, RY, listed along the left side of the Punnett Square.

3. The independently assorted alleles from one parent – ry, ry, ry, ry, listed along the top of the Punnett Square.

4. The Genotype for all the offspring of this cross will be Heterozygous for both Traits, RrYy, and the Genotype of all the offspring will have Round and Yellow Seeds.

Example 2: HETEROZYGOUS X HETEROZYGOUS

R = Dominant Round         r = Recessive Wrinkled
Y = Dominant Yellow       y = Recessive Green

1. Crossing two Pea Plants Heterozygous for Round and Yellow Seeds, Genotype RrYy.

2. The Gametes for Both Parents will be RY, Ry, rY, ry.

3. The offspring of this Dihybrid Cross will have nine (9) different Genotypes.
4. These nine Genotypes will result in pea plants with the following four (4) Phenotypes:

9/16 with round, yellow seeds (Genotypes RRYY, RRYy, RrYY, and RrYy)

3/16 with round green seeds (Genotypes RRyy, Rryy)

3/16 with wrinkled, yellow seeds (Genotypes rrYY and rrYy)

1/16 with wrinkled, green seeds (Genotype rryy)

5. A phenotypic ratio of 9:3:3:1 is expected when heterozygous for two traits are crossed.