Gene LinkageGene Linkage

Inheritance Patterns for Inheritance Patterns for

---Two Genes on the Same Chromosome---Two Genes on the Same Chromosome

---Genes on the Sex Chromosomes ---Genes on the Sex Chromosomes

Sex DeterminationSex Determination

Sex ChromosomesSex Chromosomes: homologous : homologous chromosomes that differ in size and chromosomes that differ in size and genetic composition between males and genetic composition between males and femalesfemales

Human Human

ChromosomeChromosome

XX YY

SizeSize LargerLarger SmallerSmaller

Genetic Genetic

CompositionComposition

Multiple Multiple

genesgenes

unrelated unrelated

to genderto gender

Very few genesVery few genes

SRY region determines SRY region determines maleness maleness

Sex DeterminationSex Determination

Autosome:Autosome: any chromosome that is any chromosome that is not a sex chromosomenot a sex chromosome

Humans have Humans have 22 autosome pairs 22 autosome pairs and 1 pair of sex and 1 pair of sex chromosomeschromosomes

Sex Determination Sex Determination

Female Male Female Male XX x XYXX x XY

XX

YY

X XX X

XXXX XXXX

XYXY XYXY

Phenotypic Ratio of OffspringPhenotypic Ratio of Offspring ½ Female + ½ Male½ Female + ½ Male

Sex Linked Genes Sex Linked Genes Hemophilic Male Non-hemophilic Hemophilic Male Non-hemophilic Female Female (father is hemophilic)(father is hemophilic)

XXhh

YY

XXHH X Xhh

XXHHXXhh XXhhXXhh

XXHHYY XXhhYY

Phenotypic Ratio of OffspringPhenotypic Ratio of Offspring¼ hemophilic males + ¼ non-hemophilic males¼ hemophilic males + ¼ non-hemophilic males

¼ hemophilic females + ¼ non-hemophilic females¼ hemophilic females + ¼ non-hemophilic females

XXhhY x XY x XHHXXhh

Males are Males are hemizygous for hemizygous for traits on thetraits on the X chromosomeX chromosome

Females can beFemales can behomozygous or homozygous or heterozygous for heterozygous for traits on thetraits on the X chromosomeX chromosome

DadDadXXhhYY

XXhh

Patterns of InheritancePatterns of Inheritance

• Autosomal DominantAutosomal Dominant• Autosomal RecessiveAutosomal Recessive• Sex-Linked Dominant (X-linked) Sex-Linked Dominant (X-linked) • Sex-Linked Recessive (X-linked)Sex-Linked Recessive (X-linked)

Pedigree for An Autosomal Dominant TraitPedigree for An Autosomal Dominant Trait

Pedigree for Albinism: Pedigree for Albinism: An Autosomal Recessive TraitAn Autosomal Recessive Trait

Pedigree forPedigree for Hemophilia, Hemophilia, An X-linked Recessive Trait An X-linked Recessive Trait

Pedigree forPedigree for Colorblindness, Colorblindness, An X-linked Recessive Trait An X-linked Recessive Trait

Patterns of InheritancePatterns of Inheritance

• Autosomal DominantAutosomal Dominant

• Autosomal RecessiveAutosomal Recessive

• Sex-Linked Dominant (X-linked) Sex-Linked Dominant (X-linked)

• Sex-Linked Recessive (X-linked)Sex-Linked Recessive (X-linked)

• Mitochondrial InheritanceMitochondrial Inheritance

Linked GenesLinked Genes

Mendel’s Mendel’s experimentsexperiments

Linked GenesLinked Genes

Gene Gene locationlocation

Genes on separate Genes on separate chromosomes or far chromosomes or far apart on the same apart on the same chromosomechromosome

Genes on the same Genes on the same chromosomechromosome

Gamete Gamete typestypes

Equal numbers of all Equal numbers of all possible allele possible allele combinationscombinations

More parental More parental combinations than combinations than recombinant recombinant combinationscombinations

How Do We Study How Do We Study Gene Linkage?Gene Linkage?

Example from the Fruit FlyExample from the Fruit Fly

Red eyes, x Pink eyesRed eyes, x Pink eyes

Beige body Ebony bodyBeige body Ebony body

RRBBRRBB rrbbrrbb

F1: Red eyes, Beige bodyF1: Red eyes, Beige body RrBb RrBb

Testcross: cross to individual of Testcross: cross to individual of known genotype known genotype

F1: Red eyes X Pink eyesF1: Red eyes X Pink eyes

Beige body Ebony bodyBeige body Ebony body

RrBbRrBb rrbbrrbb

How Do We Study How Do We Study Gene Linkage? Gene Linkage?

How Do We Study How Do We Study Gene Linkage? Gene Linkage?

F2 phenotypeF2 phenotype Number of Number of OffspringOffspring

Expected for Expected for Unlinked GenesUnlinked Genes

Red eyesRed eyes

Beige bodyBeige body

398398 250250

Pink eyesPink eyes

Ebony bodyEbony body

382382 250250

Red eyesRed eyes

Ebony bodyEbony body

108108 250250

Pink eyesPink eyes

Beige bodyBeige body

112112 250250

How Do We Study How Do We Study

Gene Linkage?Gene Linkage?

If genes are linked: If genes are linked:

Red eyes, x Pink eyesRed eyes, x Pink eyes

Beige body Ebony bodyBeige body Ebony body

r br b

R BR B

F1: Red eyes, Beige bodyF1: Red eyes, Beige body

R BR B

R BR B

r br b

r br b

F1: Red eyes, Beige bodyF1: Red eyes, Beige body

How Do We Study How Do We Study

Gene Linkage?Gene Linkage?

R BR B

r br bXX

R BR B

r br b r Br B

R bR b

Four types of gametes are producedFour types of gametes are produced Parental Parental Recombinant Recombinant

Alleles are inAlleles are inCis ConfigurationCis Configuration

How Do We Study How Do We Study Gene Linkage?Gene Linkage?

F1:Red eyes X Pink eyesF1:Red eyes X Pink eyes

Beige body Ebony bodyBeige body Ebony body

R BR B

r br b

r br b R bR b r Br B

R BR B

r br b

r br b

r br b

R BR B

r br b

r br b

r br b

R bR b

r br b

r Br B

r br b

How Do We Study How Do We Study Gene Linkage?Gene Linkage?

F2 phenotypeF2 phenotype Number of Number of OffspringOffspring

Chromosome Chromosome arrangementarrangement

Red eyesRed eyes

Beige bodyBeige body

398398 RB//rbRB//rb

ParentalParental

Pink eyesPink eyes

Ebony bodyEbony body

382382 rb//rbrb//rb

ParentalParental

Red eyesRed eyes

Ebony bodyEbony body

108108 Rb//rbRb//rb

RecombinantRecombinant

Pink eyesPink eyes

Beige bodyBeige body

112112 rB//rbrB//rb

RecombinantRecombinant

Genetic Map UnitsGenetic Map Units

1% recombination = 1 map unit1% recombination = 1 map unit 108 + 112108 + 112 x 100 = 22% x 100 = 22% 10001000 These genes are located 22 map units These genes are located 22 map units

apart on the same chromosome.apart on the same chromosome.Map is drawn as: Map is drawn as:

R BR B

I 22 map units II 22 map units I

Knowing the gene distance allows us to predict the expected percentages of each type Knowing the gene distance allows us to predict the expected percentages of each type of gamete. of gamete.

How Do We Study How Do We Study

Gene Linkage?Gene Linkage?

ParentalParental Recombinant Recombinant

XXr Br B

R bR b

r Br B

R bR b

39%39%

39%39%78%78%

R BR B

r br b 11%11%

11%11%22%22%

Alleles are inAlleles are inTrans ConfigurationTrans Configuration

Gene Linkage SummaryGene Linkage SummaryA.A. Linked genes tend to be inherited together in Linked genes tend to be inherited together in

their original, parental arrangement.their original, parental arrangement.

B. Alleles of linked genes can be separated by B. Alleles of linked genes can be separated by crossing over (recombination) during meiosis.crossing over (recombination) during meiosis.

C.C. Distance between genes is calculated as 1% Distance between genes is calculated as 1% recombination = 1 map unit.recombination = 1 map unit.

D.D. Gene distances can be calculated for genes that Gene distances can be calculated for genes that are less than 50 map units apart. At 50 map units are less than 50 map units apart. At 50 map units and beyond, we cannot distinguish between and beyond, we cannot distinguish between genes far apart on the same chromosome and genes far apart on the same chromosome and genes on separate chromosomes.genes on separate chromosomes.