II. Mendel's Principle of Segregation and Recombination
* when gametes are formed during meiosis (DIPLOID to MONOPLOID) the homologous chromosomes (which contain the many different genes) separate randomly
* this is called SEGREGATION

* if the organism has a homozygous (the same) genotype for a given trait (TT or tt), then all gametes will also have that trait

TT <----diploid----> tt
meiosis
T or T <----monoploid----> t or t

* if the organism has a heterozygous (not the same) genotype for a given trait (Tt), then half of the gametes will have one trait, and the other half will have the other trait

Tt <----diploid
meiosis
T or t <----monoploid

* when the monoploid gametes fuse during fertilization, the diploid number of chromosomes is restored
* this can result in a new combination of genes
* this is called RECOMBINATION

Let's look back at Mendel's first few pea plant crosses.

P1 (pure parent generation) cross was: TT X tt

F1 (first filial generation) cross was Tt X Tt

F2 (second filial generation) offspring had genotypes of TT, Tt, tT, and tt

Now, compare the different phenotypes (physical appearance) and genotypes (genetic makeup).

For the F2 generation what is the PHENOTYPIC RATIO?

3 tall : 1 short

For the F2 generation, what is the GENOTYPIC RATIO?

1 homozygous dominant : 2 heterozygous : 1 homozygous recessive

PHENOTYPE DOES NOT ALWAYS EQUAL GENOTYPE!

* to show segregation (separation of genes/chromosomes) and recombination (during fertilization), we use Punnett Squares