GENETICS
Ch. 12
*Definition: the study of heredity
~heredity: the passing of traits from
parent to offspring.
Gregor Mendel
the “Father of Modern Genetics”
• Austrian monk
• 1860’s
• used garden pea plants  why?
3 reasons:
1. the structure of the pea flowers (usually
self-pollinates).
2. the presence of contrasting traits.
3. rapid reproductive cycle (~90 days).
http://www.flinnsci.com/introgenetics
• *Mendel studied 7 distinct characters (physical
features that are inherited; ex. flower
color)…pg. 268
– *YOU ARE RESPONSIBLE FOR KNOWING THESE!
• Trait: one of several possible forms of a
character; ex. purple or white
• Mendel’s experiments used monohybrid
crosses (a 4 box Punnett square) in order to
study one pair of contrasting traits (ex. purple
vs. white for flower color).
• P generation 
F1
(true-breeding parents)
(1st filial)
 F2
(2nd filial)
-VOCAB• ~purebred: an organism that receives the
same genetic traits from both of its parents
(*self-pollination in plants).
• ~hybrid: an organism that receives
different forms of a genetic trait from each
parent (purple X white).
Mendel and Modern Genetics
12.2
• gene: sections of a chromosome that
code for a trait.
*most organisms have 2 copies
(homologous chromosomes) of every
gene and chromosome…1 from each
parent.
• allele: versions of a gene (ex. tall vs.
short for plant height)…T and t
• dominant allele: the expressed form of a
gene when 2 different alleles are present.
*ex. Tt (T is dominant; the plant is tall)
• recessive allele: a form of a gene that is
not expressed when the dominant allele is
present (masked).
• genotype: the set of alleles an
individual has for a character.
*genetic makeup
-includes both genes in a
homologous pair of chromosomes.
-ex. Rr, rr, RR
• phenotype: the trait that results from
a set of alleles.
– *-ex. tall, short, purple, white.
• Genotype determines phenotype!
• homozygous: 2 alleles in a gene pair are
identical.
• heterozygous: 2 alleles in a gene pair are
different.
• Punnett square: shows the expected
outcomes of a genetic cross.
*shows all possible combinations of
gametes. (ch.12.3)
• Probability: the likelihood that a specific
event will occur.
– can be written as a %, fraction, ratio, or in words
• EXAMPLES 
• Test Cross: used to determine an
organism’s genotype.
-done by breeding the organism
whose genotype is unknown with a
homozygous recessive organism
(known genotype).
-ex. G = green (dominant)
g = yellow (recessive)
-Question: what is the genotype of a
plant with green peas?...to find the
answer, we can perform a test cross.
Mono vs. Dihybrid Crosses
• Monohybrid Cross: predicts the offspring of a cross for one
trait (ex. plant height).
• Dihybrid Cross: predicts the offspring of a cross for two
traits (ex. plant height and seed shape).
EXAMPLES (dihybrid)
• coin toss with 2 pennies the outcome of
1 flip doesn’t influence the outcome of the
other b/c the coins are not attached in any
way (independent of one another).