Science Notes
Unit Topic: Genetics and Heredity Part 2
Today’s Topic: Probability and Punnett Squares (p 130-132)
 a branch of mathematics that helps you predict the chance that something will
happen
Probability
*In genetics, probability is determined (figured out) by using a punnett square.
 a tool used to predict the results of Mendelian genetics; used to predict the
chance of a trait being passed from parents to offspring.
* Letters are used to represent dominant and recessive alleles (genes).
--Upper-case letters are used to represent a dominant allele
--Lower-case letters are used to represent a recessive allele
*These letters form a code we call the genotype, which determines the phenotype
*The letters representing the 2 alleles (genes) from 1 parent are written along the top
of the grid…one letter per section.
Punnett Square
*The letters representing the 2 alleles (genes) from the other parent are written along
the side of the grid….one letter per section.
*Fill in grid by donating one allele (gene) from each parent.
*Each cell of the grid represents a 25% chance of future generations exhibiting
(showing) the tested trait.
Genotype
Vs.
Phenotype
Genotype  the genetic makeup for a trait
*the 2 alleles/genes
*the “letters”
Phenotype  the way an organism looks and behaves as a result of its genotype
*the adjective
*the physical appearance
Homozygous
Vs.
Phenotype
Homozygous
 an organism with the same alleles for a trait
*Example: TT and tt
*also known as homogeneous or purebred
Heterozygous
 an organism with 2 different alleles for a trait
*Example: Tt
*also known as heterogeneous or hybrid
*represented by one dominant allele and one recessive allele
5 Genetic Principles
1. Traits, or characteristics, are passed on from one generation of organisms to the
next generation.
2. All traits of an organism are controlled by 2 alleles (2 genes) (2 genes=1 trait)
3. Organisms inherit alleles (genes) in pairs, one allele (gene) from each parent.
4. Some alleles (genes) are dominant, and other alleles (genes) are recessive.
5. Dominant alleles (genes) hide recessive alleles (genes) when both are inherited
by an organism.