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Using Math to Predict Genetics
Genetics Boot Camp Notes
Probability
Questions 1-3
Which Mathematical Laws?
• Imagine a person inheriting copies of four
genes for two tetrahybrid parents:
AaBbCcDd
• What is the probability that the child is?
– Aabbccdd
– AaBbCcDd
– AaBBCCdd
AABBCCDD
AdBBccDd
Questions 4-5: Terminology
• Dominant
• Recessive
• Homozygous/True
Breeding
• Heterozygous/Hydrid/Carr
ier
• Phenotype
• Genotype
• Gene
• Allele
• Locus
• P,F1,F2
Questions 7 – 8
Incomplete Dominance
• Mix--both genes are
expressed and the
resulting hybrid genotype
is a ‘mix” or blending.
• Notation: Use two
different capital letters to
show dominance
• R=Red, W = white, RW
= Roan
Questions 9 – 10: Codominance
• Both phenotypes are
expressed.
• Represented as
superscripts
• Blood Type example-what is the recessive?
Donating Blood
Questions 11: New twist…Lethal
Alleles
• Sometimes inheriting a
recessive form of the
disease is lethal. That
means that the offspring
won’t survive.
Questions 12-13: Dihybrid Cross
• Test cross: crosses an
unknown with a
homozygous recessive to
determine parental
phenotype.
• Steps in completing an 8
box Punnett Square:
– FOIL for gametes
– Complete Cross
– Analyze outcomes and
compare to expected.
– What pattern does this
show?
Questions
14 – 15:
How are
the
outcomes of
this cross
different
than
expected?
Polygenic Inheritance
• Most visible traits are
based on inheritance
of several genes.
• Eye color-7 genes
• Skin color-3 genes
• What other examples
can you think of?-What makes them
polygenic?
Genes are
linked to
chromosomes
Question 19: Sex-linked notation
• Must show sex
chromosomes in the
Punnett Square
• Show genes as
superscript.
• Most sex-linked
disorders are
recessive.
X-inactivation
• Barr body
• DNA methylation
• Happens during migration of cells in
development. Random!!!
Questions 21-22: What else
causes ratios to deviate from
what is expected?
• Recombination: when
the outcomes of a
dihybrid cross are
different than what is
expected.
• What can you assume
caused the deviation in
the numbers?
Practice
How can Chisquare be used to
analyze these
results?
Recombination due to cross-overs
Calculating Crossover Frequency
• Determine the
crossover frequency in
problem #F1:
– Recombinants/normal
Map Units
• Genes can be “mapped
to a chromosome” by
crossover frequency.
– The greater the
frequency the closer to
the end.
– The order relates to
frequency
Try problem #2. You must
be able to map (order)
genes to their location on
a chromosome.
Recombination frequencies