Mendel started his
experiments by looking at one trait, flower color. The first thing
Mendel did was start out with 2 pure strains of pea plants. He made
sure that each plant that produced purple flowers, did so in each generation
through many generations. He also noticed that there were white flowers
and ensured that the plants that produced white flowers, did so in each
generation. These then were his pure strains of parents. He
crossed this and called this the "P1" generation. Each
strain was planted in separate areas and when they flowered, Mendel removed
the anthers from each purple flower and placed it in a container. He
also collected the stamens from the plants with white flowers. This
prevented the flowers from pollinating themselves. He then pollinated
the white plants flowers with the pollen from the purple flowers and vice
versa. The seeds were then collected. These were the offspring
of the parental cross. He called this offspring the F1
generation. F in this instance stands for filial. When the seeds
were planted, and they matured, they all had purple flowers. The white
flower did not show up in the F1.
Mendel then let the offspring reproduce on their own. The F1
has now become the P2, the second parental cross. When
these individuals produced seeds, these were planted and an amazing
thing happened. The white flower had returned. The parents were
all purple and now the white flower is back, but in much reduced numbers.
In fact the ratio of purple flowers to white was 3:1. Mendel then
decided, after some time of course and many other crosses similar to this,
that each parent MUST have 2 factors and then each of the parents gives one
factor to the offspring. The fact that a parent gives one of the
factors to its offspring must mean that the factors, things that we now call
alleles, must separate and "move" into its own gamete. Today we know
this is a fact and it is known as Mendel's First Law, The Law of Segregation.
In genetics, we can define the kind and type of genes a person has by
using some interesting vocabulary. The genes that an individual holds
is called a genotype. A genotype defines what kind of alleles a person
has. A person may have two alleles that are dominant or two alleles
that are recessive. This genotype is called a homozygous genotype.
Homo comes from a Latin term which translates to mean "same." There is
a difference though. Mendel found that some traits showed up more
often. This type of allele is called a dominant allele while the other
is called a recessive allele. A homozygous dominant genotype is one in
which the individual has two alleles that are dominant. A homozygous
recessive individual is one in which the individual has two recessive
alleles. The F1 offspring of Mendel's first cross showed
the dominant trait. But is the Law of segregation is correct at least
one of those alleles had to be recessive. This is in fact true.
A genotype that has two different alleles is heterozygous. Hetero is
taken fro the Latin which is translated as "different" referring to two
different alleles.
The term phenotype is the outward expression of the gene. In
other words, the phenotype of the first parents were purple and white.
The purple parents genotype was homozygous dominant and the genotype of the
white parent is homozygous recessive.
The following is a series of punnett squares that are used to predict the
possible offspring genotypes and phenotypes.