Terms to get straight

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Terms to get straight Gene vs. Allele A gene
codes for the general trait while the different
alleles for that gene code for specific forms of
that trait (e.g., a gene for eye color, alleles
for blue, brown, green). Heterozygous vs.
Homozygous Heterozygous (diploid) individuals
have two different alleles for a gene while
homozygous individuals have two copies of the
same allele for a gene. Dominant vs. Recessive
In diploid organisms, dominant alleles only need
to be present in one copy to be expressed as a
trait while recessive alleles need to be present
in two copies (i.e., no dominant copy
present). Genotype vs. Phenotype An organisms
genotype is the alleles it actually has (e.g., AA
or Aa or aa) while its phenotype is what it looks
like (e.g., brown eyes or blue eyes). Phenotypes
are the result not only of a genotype, but also
the actions of other genes and the environment.
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A
½ of gametes have A allele, ½ have a allele
Heterozygous, diploid individual in G1 (chromatin)
Beginning of Meiosis
A
a
A
S
a
Principle of Segregation
a
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Aa x aa offspring genotypic ratio 022
(AAAaaa) offspring phenotypic ratio 22
(dominantrecessive)
An Aa individual makes ½ A gametes and ½ a gametes
? Aa
? aa
A
a
aa
a
Aa
a
Aa
aa
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Does allele A have to remain with allele B (and
allele a with b)?
A
B
Beginning of Meiosis
A
a
A
A
A
S
B
b
B
B
B
a
b
Four possible gamete types AB, Ab, aB, ab in
equal proportions
a
Principle of Independent Assortment
b
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B locus
C locus
Combined genotype
¼ x ¼ x ¼ 1/64 AABBCC
¼ x ¼ x ½ 1/32 AABBCc
1/64 AABBcc
1/32 AABbCC
1/16 AABbCc
1/32 AABbcc
1/64 AAbbCC
1/32 AAbbCc
1/64 AAbbcc
1/32 AaBBCC
1/16 AaBBCc
1/32 AaBBcc
1/16 AaBbCC
AaBbCc X AaBbCc
1/8 AaBbCc
1/16 AaBbcc
1/32 AabbCC
1/16 AabbCc
1/32 Aabbcc
1/64 aaBBCC
1/32 aaBBCc
1/64 aaBBcc
1/32 aaBbCC
1/16 aaBbCc
1/32 aaBbcc
1/64 aabbCC
1/32 aabbCc
1/64 aabbcc
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Genotypes
Phenotypes
1/64 AABBCC
1/32 AABBCc
1/32 AABbCC
1/16 AABbCc
Dominant for A, B, C 27/64
1/32 AaBBCC
1/16 AaBBCc
1/16 AaBbCC
1/8 AaBbCc
1/32 AaBBcc
1/64 AABBcc
Dominant for A B, Recessive for C 9/64
1/32 AABbcc
1/16 AaBbcc
1/16 AabbCc
1/32 AabbCC
Dominant for A C, Recessive for B 9/64
1/64 AAbbCC
1/32 AAbbCc
1/64 AAbbcc
Dominant for A, Recessive for B C 3/64
1/32 Aabbcc
1/64 aaBBCC
1/32 aaBBCc
Dominant for B C, Recessive for A 9/64
1/32 aaBbCC
1/16 aaBbCc
1/64 aaBBcc
Dominant for B, Recessive for A C 3/64
1/32 aaBbcc
1/64 aabbCC
Dominant for C, Recessive for A B 3/64
1/32 aabbCc
Recessive for A, B, C 1/64
1/64 aabbcc
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A
a
a
A
a
A
b
B
B
b
In this scenario, the A B loci are on the same
chromosome, which means that the alleles from
maternal origin (A b) might be passed on
together more frequently, which means that the
alleles of paternal origin (a B) will also be
passed on together. However, because of crossing
over, there will still be as many AB and ab
gametes as Ab and aB.
In this scenario, the A B loci are on the same
chromosome and very close together, limiting the
amount of crossing over that could occur between
them. This means that the alleles from maternal
origin (A b) will be passed on together much
more frequently. The vast majority of gametes
will be Ab and aB, very few will be AB and ab.
B
b
In this scenario, because of the principle of
independent assortment, the maternal chromosome
with A has an equal chance of pairing with the
maternal chromosome with b as it does with the
paternal chromosome with B. 50 of the time you
get Ab aB gametes 50 of the time you get AB
ab gametes
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F?
F?
Ff
Ff
F?
F?
Ff
ff
F?
Ff
ff
Ff
normal female
webbed female
normal male
webbed male
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Determining if a trait is dominant or recessive

• Cues that the trait is dominant
• appears in every generation
• unaffected individuals do not have affected
  offspring
• a heterozygote parent has ½ chance of having
  affected offspring
• often disorder is more severe in homozygous

• Cues that the trait is recessive
• appears in alternate generations or skips
  generations
• unaffected individuals can have affected
  offspring (but be careful of new mutations)
• two carrier parents have ¼ chance of having
  affected offspring

However, these are only cues. To identify a
trait as certainly dominant or recessive, you
must look at specific matings where offspring
show different phenotypes than the parents.
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• Is albinism (according to this pedigree) dominant
  or recessive?
• For which individuals can you not determine a
  genotype?

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?

• Is being spotted (according to this pedigree)
  dominant or recessive?
• What is the expected phenotypic ratio from the
  mating marked with a ??
• What is the probability that the mating indicated
  by the arrow produces a homozygous dominant
  offspring?

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Jewell
Tillie
Abe
Maggie
Neeps
Nellie
Lark
Molly
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