Chapter 12 Notes
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Gregor Mendel
o Father of Genetics
o Studied how traits are passed from one generation to the next
o Discovered that some traits are dominant over others
 Dominant – a trait that shows up in each generation
 Recessive – a trait that is “hidden” by the dominant trait
o Allele – alternate forms of a gene (straight hair v. curly hair; freckles vs. no freckles)
 Organisms have 2 alleles for each trait – one from mother and one from father
Two laws from Mendel’s research:
o Law of Segregation
o Law of Independent Assortment
Punnett Squares
o Tool used to predict probable offspring of a genetic cross
o Monohybrid Cross – looks at one trait at a time (box has 4 squares)
o Dihybrid Cross – looks at two traits at a time (box has 16 squares)
Genotype
o the genetic makeup of an individual (cannot be seen)
o either homozygous or heterozygous
 Homozygous – both alleles are the same (HH of hh)
 Heterozygous – alleles are different (Hh)
Phenotype – the physical appearance of an individual
Types of Monohybrid Crosses (Complete Dominance)
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Homozygous X Homozygous
o HH X HH
o hh X hh
Homozygous X Heterozygous
o HH X Hh
o hh X Hh
Heterozygous X Heterozygous
o Hh X Hh
Test Cross – crossing an individual with the dominant phenotype with a homozygous recessive individual to determine the
genotype of the dominant individual
o H? X hh
 ? can be H or h
Types of “Special” Crosses
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Incomplete Dominance – neither trait is completely dominant over the other
o Red flowers X White flowers  Pink Flowers
CoDominance – both traits are equally expressed at the same time
o White Hair X Red Hair  Roan Coat (in horses when red and white hair are both visible)
Multiple Alleles – more than 2 alleles exist for a given trait; a type of codominance
o Blood Types – A, B & O
Sex Linked Traits – traits that are located on the X or Y chromosomes
o Hemophilia, Male Pattern Baldness and Color Blindness located on the X chromosome