Mendelian Genetics

Pea Plant Traits

Mendel only tracked characteristics coming in two distinct forms, examples of this would be the color of the plant, to the shape it takes.

True breeding is a term that describes having the same allele 100% of the time, which produces offspring that have the same characteristic over generations. Mendel classified this as true breeding.

Generations 

This can be broken down to 3 sub levels, with P generation, F1 generation, and F2 generation.
The P generation represents the true breeding parental generation as described earlier. These consists of fully homozygous dominant or recessive genes (100% of the true breeding as implied).
The F1 generation represents the first fillial hybrid offspring of the P generation. In simpler terms, this represents the generation following the P generation which may accumulate recessive traits despite having a homozygous dominant parent alleles.
The F2 generation represents the second fillial offspring of the F1 generation. Similar to the F1 this is following the F1 generation and can have a complex assortments of traits through a variety of combinations.

This is further explained through the idea of "Punnet Squares".

What are Punnett Squares?

They are diagrams used to predict allele combinations of offspring on a cross. This is based on known genetic compositions, examples include having prior knowledge of parental genetic information.

In these simple diagrams, we denote capital letters as dominant traits, and denote lowercase letters as recessive traits.

Dominant alleles as the name suggests, will always dominate over recessive. This means even if I have a "Bb" gene for example for my genotype, my phenotype the trait expressed on me physically will be represented as "B" not "b". This is because dominant alleles take priority in our physical representation. To sum it up, dominant traits are ones that are actively seen and contribute to our phenotypes, while recessive traits remain in our genotype they have no active play. 

Although we touch on Punnett squares briefly, they are quite simple to create, and can give reliable approximations to the expected offspring genotypes.  We encourage you guys to create punnet squares and play around with these diagrams, come up with ratios, and learn patterns between generations.

Principles of Heredity

1. The law of segregation

2. The law of independent assortment

As seen with previous topics, one crucial distinction between meiosis and mitosis, is meiosis's ability to undergo random assortment. Well for these two laws we can think back to the principles of probability, and independence.

These two fundamental principles of heredity were developed throughout Mendel's experiments. 

He discovered that crosses between purple and white true breeding pea plants produced only purple F1 offspring. Does this imply the white characteristic to disappear? No, because the white characteristic came back in the F2 generation. How did this happen?

This hypothesized the idea that purple is dominant trait to the white flower being recessive. This created a 3:1 ratio within the F2 generation, and leads to our first law, the law of segregation.

The two alleles for the same trait separate during gamete formation, and end up in different gametes. This means that each gamete for P generation will contain one allele for flower. True breeding plants will have two identical alleles, F1 generation are all hybrids, and F2 generation produces a 3:1 ratio.

The law of independent assortment implies that genes for one trait are NOT inherited with genes of another trait. This law only applies to genes that are located on different chromosomes (not homologous) or genes that are substantially far apart on the same chromosome. This was determined by the utilization of dihybrid crosses. By crossing between plants for true breeding for traits, produced F1 hybrids known as dihybrids.