Contents

    Mendel's Experiment
    Essential Terms
    More Key Terms
    Punnett squares
    Incomplete Dominance
    Dihybrid Crosses
    Selective Breeding

Mendel's Experiment

    Gregor Mendel the "Father of Genetics" kept records of every plant that was produced through his pea plant experiment.  He started with 34 varieties of pea seeds in which he noticed 7 opposing characteristics among the plants.
 
Dominant Traits tall  colored seed coats axial flowers green pea pods inflated pea pods yellow peas round peas
Recessive Traits short white seed coats terminal flowers yellow pea pods constricted pea pods green peas wrinkled peas
    The pea plants were self-pollinating plants so Mendel was able to leave them alone after the first generation (0).  With this generation he bred a tall plant with a short plant by manually cross pollinating them.  In result, he noticed that all of the offspring were tall.  The change came in the next generation, though, in which, out of the 1,064 plants obtained, 787 were tall and 277 were short.  This was almost a 3:1 ratio of tall to short pea plants.  In the text that follows, I will explain how genetic diversity takes place in cross breeding.
 

Essential Terms

    Remember that cells have pairs of chromosomes.  Each chromosome in a pair contains genes for the same characteristics.  Thus in Mendel's pea plants each cell in the plants contained two chromosomes which each held a gene to determine whether the plant was tall or short.  The genes on the chromosomes are called alleles.  Each allele or gene is either dominant or recessive.  A dominant allele will be used in transcription instead of the recessive allele.  In other words, if there is a chromosome with a dominant gene and a chromosome with a recessive gene, then the dominant gene will decide the characteristic for the plant.  As in the chart above genes standing for tall plants dominate over genes standing for short plants.  Alleles are denoted with a letter.  We will use "t" for tall for our example.  "T" means that there is a dominant allele and "t" means that there is a recessive allele.

Terms needed to proceed:
 

In Mendel's experiment he crossed a tall pea plant with a short pea plant:  TT    tt

The offspring were plants with a tall phenotype and with a dominant allele and a recessive trait:   Tt     Tt

When these plants self-pollinated themselves they crossed a Tt cell and a Tt cell.  This resulted in plants with the genotype (gene type) of  TT   Tt   tt.
 

More Key Terms

    There are certain labels for the genotypes of organisms with a certain characterisctic.  In other words there are labels for the gene types of organisms with a certain characteristic.
 

Punnett Squares

  Punnett squares are helpful tools when determining the genotype of offspring and the probability of a certain genotype of the offspring of organisms.  For example:
 

    Cross a homozygous dominant pea plant (TT) with a homozygous recessive pea plant (tt).
 

t t
T Tt Tt
T Tt Tt
 One dominant gene from the phenotypically (physical characteristic) tall plant (TT) is crossed with one recessive gene from the phenotypically short plant (tt) to produce plants with the genotype of Tt.  It shows that 100% of the offspring of the cross between TT and tt will have a tall phenotype and Tt will be their genotype.
    If two Tt plants were to be crossed:
 
T t
T TT Tt
t Tt tt
Approximately:
    25% of the offspring will have the genotype of TT.
    50% of the offspring will have the genotype of Tt.
    25% of the offspring will have the genotype of tt.
As a result (remember that the dominant T overrides the recessive t when deciding the characteristic):
    75% of the offspring will have a tall phenotype.
    25% of the offspring will have a short phenotype.

When you look at these percentages, remember the 3:1 ratio of tall to short pea plants obtained in the first few generations of Mendel's experiment.

    Let us change focus from the pea plant to animals.  Pretend that for a certain type of rabbit, having black hair is a dominant trait over white hair.  A rabbit with a heterozygous genotype is crossed with a rabbit with the same genotype.  We will use B for the dominant trait and b for the recessive trait.
 

B b
B BB Bb
b Bb bb

Ratios:

    We could say the percent of each genotype and phenotype in a ratio instead of listing out each of the different situations.
        The genotypic ratio is 1BB: 2Bb: 1bb.  The correct notation for this is 1:2:1.
        The phenotypic ratio is 3B:1b or 3 black to 1 white.  It is written as 3:1.
 
 

Incomplete Dominance

    Incomplete dominance is the term given to the characteristic of certain organisms that have genes that don't dominate the other genes.  In this situation you obtain variations in physical appearance.  For example:  A flower has genes for white petals (w) and red petals (r).  When you mix white flowers with red flowers you get pink flowers.  How is this possible?  The w gene does not dominate over the r gene and vice versa.
 
w w
r wr wr
r wr wr
Both genes express themselves equally so you get a pink color in the petals.  However, by crossing two pink flowers together there is a different outcome.
 
w r
w ww wr
r wr rr
There is a possibility of obtaining one white, two pink, or one red flower.  The phenotypic ratio is 1:2:1 and the genotypic ratio is 1:2:1.
 
 

Dihybrid Cross

    Monohybrid Cross - A monohybrid cross is a cross dealing with only one characteristic (height for example).  This is oppposed to a Dihybrid Cross where two characteristics are taken into consideration (height and color).  In a dihybrid cross on a punnett square you can figure out the ratio and probability that you will get a tall plant with a red bloom as opposed to a short plant with a white bloom.

    I will use a Punnett square to show the outcome of crossing a tall plant with a white bloom with the genotype of TTrr and a tall plant with a red bloom with the genotype of TtRr.  In this situation a red color trait dominates over a white color trait.  There is a greater variety of genotypes that the next generation can have.  The new generation obtains genes with the genotype of Tr from the 1st plant, and obtains genes with the genotype of TR, tR, Tr, or tr from the 2nd plant.
 

Tr Tr
TR TTRr TTRr
tR TtRr  TrRr
Tr TTrr TTrr
tr Ttrr Ttrr
 
 
    Dihybrid crosses can get trickier, though, when the next generation can recieve a combination of TR, Tr, tR, or tr from the first plant and the second plant.
 
 
TR Tr tR tr
TR TTRR TTRr TtRR TtRr
Tr TTRr TTrr TtRr Ttrr
tR TtRR TrRr ttRR ttRr
tr TtRr Ttrr ttRr ttrr
 
This chart gives us information that the odds of a plant in the next generation being tall with a red bloom is 9:7.
 
 

Selective Breeding

    Forever people have wanted the best plants or animals of a species.  One method of obtaining this, is called selective breeding.  Selective breeding is the method of breeding certain organisms together that have desirable traits.  Hopefully their offspring will inherit these desirable traits.  For centuries it has been used to obtain desirable plants and animals.
 
    A recent selective breeding project on plants involved the work of Dr. Jerry Parsons, an Agricultural Specialist of Bexar County, Texas, United States.  He began a project in 1984 to grow red bluebonnets.  After searching, he found a rare patch of pink bluebonnets in Bexar County, Texas.  When these bluebonnets reproduced, he culled out the lighter pink ones, and left the darker reddish ones.  A few generations later he obtained maroon bluebonnets.  (Note:  Dr. Parsons was unable to breed red bluebonnets due to the blue nature of the plant.)  These maroon bluebonnets that he cultivated will be on the market after the 1999 crop matures.  It took 15 years of selective breeding to get a maroon color trait to stand out in enough bluebonnets to sell on the market.  Selective breeding is a slow and tedious process, but it works.
 
A Point of Information
Marroon is one of the colors of Texas A & M University.  Dr. Parsons is affiliated with this school.