Lec10 Genetics Recap

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