Crossing One Trait

Problems
Multiple alleles
Monohybrid
Mendel
Incomplete
Disorders
Dihybrid
Codominance
Chapter 7main
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.