• Section 9
• Gregor Mendel was an Austrian monk who
is known as the father of genetics due to
his work with garden peas. Since he was
into math, he wanted to know if there was
a way to calculate the offspring’s traits.
• Mendel proved that if you cross a tall plant
with a short plant, all of the offspring would
be tall. Then if you crossed those tall
plants, 75% of them would be tall and 25%
would be short.
• A gene carries encoded information about
specific traits. Each gene has a location
on a chromosome called a “locus.”
• Alleles are various molecular forms of a
gene for the same trait. An allele usually
refers to a single letter, “T” for example.
• Homozygous = Same letters; AA or aa for
• Heterozygous – Different capitalization; Aa
• Homozygous dominant = AA
• Homozygous recessive = aa
• Heterozygous = Aa
• Genotype is the sum of the genes (letters)
• Phenotype is how the genes are
• Mendel’s Law of Dominance
• Some traits are dominant and others are
• A Punnett Square is a diagram used to
predict the outcome of a cross between
• Mendel’s Law of Segregation
• Half of your traits come from one parent
and the other half come from the other
• In rabbits brown fur is dominant to white fur.
Predict the outcome between a homozygous
dominant brown rabbit and a white one.
• In flowers, red is dominant to white. Predict the
outcome between a heterozygous flower and a
homozygous dominant flower.
• In bears, long fur is dominant to short fur.
Predict the outcome between a short furred
bear and one that is heterozygous.
• In some plants, tall is dominant to short.
Predict the outcome between two heterozygous
• An organism is heterozygous or
homozygous dominant they look identical.
To determine which is which, you perform
a test cross.
• If you cross your unknown (either
heterozygous or homozygous dominant)
with homozygous recessive and ANY
ONE of the offspring is recessive, then
your unknown organism is heterozygous.
• A monohybrid cross involved only 1 trait at
a time. A dihybrid cross involves 2 traits.
• Mendel’s Law of Independent Assortment
• Traits are not linked to one another and
you have and equal chance of getting
each gene independently of the others.
• A dihybrid cross between two
heterozygous organisms for two
contrasting traits results in a 9:3:3:1 ratio.
• In some species of plants, red flowers are
dominant to white flowers and tall plants
are dominant to short plants. Predict the
outcome between 2 heterozygous
• In incomplete dominance, the
heterozygous results in a blending of the
• RR = Red
• WW = White
• RW = Pink
• In codominance the traits don’t blend but
both show up. The heterozygous results
in spotted or speckled off spring.
= Black feathers
= White feathers
= Speckled feathers
• Where there are more than 2 alternative
forms of a trait, it is said to have multiple
• Blood type is a common example of
• Type A = IA
• Type B = IB
• Type AB = IA
• Type O = ii
• Some traits actually ARE linked by sex.
These include baldness, color blindness,
• Males are represented by XY. The X from
the mother and the Y from the father.
Sperm can carry either an X OR a Y
• Females are represented by XX. They
inherit and X from both parents. All eggs
carry an X chromosome.
– Normal female
– Normal female (but a carrier)
– Affected female
Y – Normal male
Y = Affected male
• A pedigree shows the history of a trait in a
• Circles represent females.
• Squares represent males
• Horizontal lines represents a mating
• Vertical lines represent an offspring
• A triangle represents an abortion of
• A diamond indicates unknown sex.
• Shaded in means affected
• A diagonal lines means dead.
• All pedigrees must have a title, each
generation must be numbered with a
roman numeral, and each member must
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