4.3.2-4.3.11
Sydney Foil, Jasmine Delaine, Cameron Folmar,
Stephanie Weng
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Alleles: one member of a pair of genes that
codes for distinct traits
Genotypes: the alleles of an organism
Phenotypes: the characteristics of an
organism
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The genotypes of offspring can determined
using a monohybrid cross
◦ A monohybrid cross is a genetic cross using a
single trait with two alleles (both heterozygous)
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Using punnett squares you can find out the
ratio of phenotypes.
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Punnett square worksheet
◦ Work in groups of two or three
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Some genes can have more than just two
alleles.
Although these genes can have multiple
alleles individuals can only have two.
◦ Example: the gene of the A, B, AB, and O blood
types. This gene contains three alleles.
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The alleles of IA and IB are dominant while the i is
recessive.
◦ When both dominant alleles are present they form the blood group AB
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Codominant alleles are pairs of alleles that both
affect the phenotype when present in the
heterozygote.
Phenotype
Genotype
A
IAIA or IAi
AB
IAIB
B
IBIB or IBi
O
ii
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The mother always passes on an X
chromosome to her offspring
The father can pass either an X or Y
chromosome to his offspring
Some genes are present on the X
chromosome and absent from the shorter Y
chromosome in humans
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When the gene controlling the characteristic
is located on the sex chromosome
◦ We associate the characteristic with gender
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Sex-linked genes are carried on the X
chromosome
◦ Because females have two X chromosomes, they
carry two copies of the sex-linked gene while the
males only carry one because they only have one X
chromosome.
◦ Example: Hemophilia and color blindness
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Hemophilia: XH is the allele for normal blood
clotting, this is dominant over the allele Xh
(the recessive gene which causes hemophilia)
If the mother is heterozygous, she is then a
carrier of the disease and passes it on to her
offspring
◦ This means the dominant and recessive alleles are
present.
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Heterozygous: contains two different alleles
◦ Ex: Rr (one recessive, one dominant)
◦ Ex: AB blood type
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Homozygous: having identical alleles for the
same trait
◦ Ex: RR or rr
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Because a female has two X chromosomes
she can carry either both recessive/dominant
genes or she can have one of each (with
respect to sex linked genes).
If female carriers are heterozygous they can
pass it onto their offspring.
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Basketball:
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Six teams of five each
Each team at a lab table
Write answer on white board
Get the question right first, chance to shoot
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"IB Biology Notes - Theoretical Genetics." IB Guides - Free
International Baccalaureate Study Guides, Notes, Videos and
Powerpoints. 2010. Web. 10 Feb. 2012.
<http://www.ibguides.com/biology/notes/theoreticalgenetics>.
"Multiple Alleles." Redirecting... Web. 10 Feb. 2012.
<http://www.hobart.k12.in.us/jkousen/Biology/multalle.htm
>.
Pitocchelli, Jay. "Lecture Notes for Genetics." Saint Anselm
College : Saint Anselm College. 2001. Web. 10 Feb. 2012.
<http://www.anselm.edu/homepage/jpitocch/genbio/geneti
csnot.html>.
"X and Y Chromosomes." Science Photo Library. Web. 10 Feb.
2012. <http://www.sciencephoto.com/media/94963/view>.