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MULTIPLE ALLELES AND POLYGENIC INHERITANCEBiology 3Kent Kawashima
•In genetics, the basic unit of inheritanceGene•Variations or different versions of a geneAllele•A feature, quality or character (iris color)Character•Attribute of a feature or character(iris color: brown)Trait
•Genetic constitution or what alleles you have in your chromosomes
Genotype
•Outcome of your genetic makeup, iris color for example
Phenotype
Character: eye colorTrait: blue
Genotype: AAAATATTATATATCCCCGTAGCTACGATCTATCTATCATCTATCTACTATCTATCTATCTATCTATCTACTATCTATCCGGCGTATCATCGGGGGGGGGGGTACTAGCTAGCTAGCTATCTACGTAGTCTACTGATCTAGCTAGTCATGCTGATCATCGTAGCTGAGCTACTAGTCGATCGTAGCTAGTCATCTAGCTGAGCTAGCTGATCGATCTAGCTAGCTGATCGTAGCGATCGTAGCTAGCTAGTCGATGCTAGCTGATCGTAGCATGCTAGCTGATGCATGCTAGCTAGCTAGCTGATCA…
Phenotype: Megan Fox’s blue eyes
Human have 23 pairs of chromosomes.
one set comes from the mother
one set comes from the father
A gene occurs in pairs in diploid cells.
The combination of what alleles you have in your gene pair is what is called your genotype.
AA, BB, Cc, RrQq, IAIA… These letters are just representations for your genotype. They also simply represent your alleles.
eye color gene pair
Homologous chromosomes
gene for eye color
blue green brown
…can contain any of the versions for that gene.These versions are called your alleles
The eye color gene located in chromosome 19…
blu
e v
ers
ion, gre
en v
ers
ion,
bro
wn v
ers
ion
…AAAATATTATATATCCCCGTAGCTACGATCTATCTATCATCTATCTACTATCTATCTATCTATCTATCTACTATCTATCCGGCGTATCATCGGGGGGGGGGGTACTAGCTAGCTAGCTATCTACGTAGTCTACTGATCTAGCTAGTCATGCTGATCATCGTAGCTGAGCTACTAGTCGATCGTAGCTAGTCATCTAGCTGAGCTAGCTGATCGATCTAGCTAGCTGATCGTAGCGATCGTAGCTAGCTAGTCGATGCTAGCTGATCGTAGCATGCTAGCTGATGCATGCTAGCTAGCTAGCT…
…AAATTATTATATATCCCCGTAGCTACGATCTATCTATCATCTATCTACTATCTATCTATCTATCTCTCTACTATCTATCCGGCGTATCATCGGGGGGGGGGGTACTAGCTAGCTAGCTATCTACGTAGTCTACTGATCTAGCTAGTCATGCTGATCATCGTAGCTGAGCTACTAGTCGATCGTAGCTAGTCATCTAGCTGAGCTAGCAGATCGATCTAGCTAGCTGATCGTAGCGATCGTAGCTAGCTAGTTGATGCTAGCTGATCGTAGCATGCTAGCTGATGCATGCTAGCTAGCTAGCT…
…AAACTATTATATATCCCCGTAGCTACGATCTATCTATCATCTATCTACTATCTATCTATCTATCTGTCTACTATCTATCCGGCGTATCATCGGGGGGGGGGGTACTAGCTAGCTAGCTATCTACGTAGTCTACTGATCTAGCTAGTCATGCTGATCATCGTAGCTGAGCTACTAGTCGATCGTAGCTAGTCATCTAGCTGAGCTAGCCGATCGATCTAGCTAGCTGATCGTAGCGATCGTAGCTAGCTAGTAGATGCTAGCTGATCGTAGCATGCTAGCTGATGCATGCTAGCTAGCTAGCT…
“blue” allele “green” allele “brown” allele
versions of a gene occur because of small changes in the DNA (in pink)
green
Genotype: Homozygous for the green allele
Since your eye color gene occurs as a pair, you may have two similar alleles
green
blue green
…or you may have two different alleles
Genotype: Heterozygous
green
…or you may have two different alleles
Genotype: Heterozygous
brown
…or you may have two different alleles
Genotype: Heterozygous
brownblue
gene for eye color
blue green brown
•Only one gene controls the nature of the character…•More than two versions of the gene exists…
This is called “Multiple Allele Inheritance”
gene for eye color
What if many genes have an effect to one character?
gene for eye color
gene for eye color gene for eye color
This is called “Polygenic Inheritance”
gene for eye color
gene for eye color
gene for eye color
gene for eye color
In Polygenic Inheritance…
many genes
+
0
add more of something!
no effect.
alleles that either contribute to a trait or not
gene for eye color
gene for eye color
gene for eye color
gene for eye color
Example.
+
+ 0
+ 0
+
For each + allele you have, it tells your cells to add more pigment to your iris.
The 0 alleles however do not add nor subtract pigment, they do not contribute to your phenotype.
+
+
+ 0
+ allele is called a contributory allele
0 allele is called a non-contributory allele
+
+
+
+
+
+
+
+
+
+
+
+
+
+
0
0
+
+
+
+ 0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
+
+
Compare.
6 +, 2 0 2 +, 6 0 4 +, 4 0 8 + 8 0
+
+
+
+
+
++
+
+
+
+
+
+
+
0
0
+
+
+
+ 0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
+
+
Compare.
graydark browngreenbluelight brown
+
+
+
+
+
++
+
+
+
+
+
+
+
0
0
+
+
+
+ 0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
+
+
Arranged from least to most melanin produced
gray dark browngreenblue light brown
In polygenic inheritance…
• Many genes contribute to affect a singular character
• There is no dominance, an allele either contributes to a phenotype or not
• There is an equal effect for each contributory allele
In polygenic inheritance…
• Polygenes have two types of alleles, a contributory allele that adds to the phenotype, and a non-contributory allele that has no effect on the phenotype
A (BIG LETTER) - contibutory allelea (small letter) - non-contributory allele
In polygenic inheritance…
• Phenotypes show a great number of variations
• Polygenic inheritance is usually the mode of inheritance for traits that are continuous – weight, height and skin tone
So…Is iris color inherited through multiple alleles or polygenes?
Number of gene pairs
Number of genotype
(regardless of position in a gene pair)
Number of phenotypes in
F2
Fraction of F2
similar to a parent
n 3n 2n +1 1/4n
1 3 3 1/4
2 9 5 1/16
3 27 7 1/64
Shortcuts for Polygenic Inheritance Genetics
If you cross…
AA x aa AABB x aabb
Aa x Aa AaBb x AaBb
1 AA2 Aa, aA1 AA
1 AABB
4 AaBB, aABB, AABb, AAbB
6 AaBb, aAbB, AabB, aABb, aaBB, AAbb
4 aAbb, Aabb, aabB, aaBb
1 aabb
rela
tive
phenot
ype
por
tion
s ex
pect
ed
all possible genotypes
AA x aa
Aa x Aa
1 AA2 Aa, aA1 aa
If you cross…
P
F1
F2
Number of gene pairs: n = 1
Number of phenotypes in F2: 21 + 1 = 3
Number for genotypes regardless of position in the gene pair: 31 = 3 (two big A, one big A, no big A)
If you cross…
P
F1
F2
AABB x aabb
AaBb x AaBb
1 AABB
4 AaBB, aABB, AABb, AAbB
6 AaBb, aAbB, AabB, aABb, aaBB, AAbb
4 aAbb, Aabb, aabB, aaBb
1 aabb
Number of gene pairs: n = 2
Number of phenotypes in F2: 22 + 1 = 5
Number for genotypes regardless of position in the gene pair: 32 = 92 big A, 2 big B2 big A, 1 big B2 big A, 0 big B1 big A, 2 big B1 big A, 1 big B1big A, 0 big B0 big A, 2 big B0 big A, 1 big B0 big A, 0 big B
regardless of position in the gene pair
(A + a)2n
11 1
1 2 11 3 3 11 4 6 4 1
1 5 10 10 5 11 6 15 20 15 6 1
1 7 21 35 35 21 7 11 8 28 56 70 56 28 8 1
binomial equation
Pascal’s triangleUse Pascal’s Triangle to get the coeefficients of the binomial expansion
(A + a)2 = 1A2 + 2Aa + 1a2for 1 gene pair, n=1
11 1
1 2 11 3 3 11 4 6 4 1
1 5 10 10 5 11 6 15 20 15 6 1
1 7 21 35 35 21 7 11 8 28 56 70 56 28 8 1
Use Pascal’s triangle for coefficients
(A + a)4 = 1A4 + 4A3a + 6A2a2 + 4Aa3 + 1a4for 2 gene pairs, n=2
11 1
1 2 11 3 3 11 4 6 4 1
1 5 10 10 5 11 6 15 20 15 6 1
1 7 21 35 35 21 7 11 8 28 56 70 56 28 8 1
Use Pascal’s triangle for coefficients
(A + a)6 = 1A6 + 6A5a + 15A4a2 + 30A3a3 + 15A2a4 + 6Aa5 + 1a6
for 3 gene pair, n=3
11 1
1 2 11 3 3 11 4 6 4 1
1 5 10 10 5 11 6 15 20 15 6 1
1 7 21 35 35 21 7 11 8 28 56 70 56 28 8 1
Use Pascal’s triangle for coefficients
(A + a)2n = (2nC0)A2na0 + (2nC1)A2n-1a1 + (2nC2)A2n-2a2 + … + (2nC2n)A0a2n
for n number of gene pairs,
11 1
1 2 11 3 3 11 4 6 4 1
1 5 10 10 5 11 6 15 20 15 6 1
1 7 21 35 35 21 7 11 8 28 56 70 56 28 8 1
Use Pa
scal’s
triangle
for coefficie
ntsOR
2n C x, x=0 → 2n
combination
(A + a)4 = 1A4 + 4A3a + 6A2a2 + 4Aa3 + 1a4
for 2 gene pairs, n=2
number of contributory alleles
number of non-contributory alleles
Coefficients show the relative portion of the generation showing that polygenic genotype
means 1/16 has 4 contributory alleles
means 4/16 has just 3 contributory alleles
