X-linked traitsX-linked traits

Modes of InheritanceModes of Inheritance

Autosomal InheritanceAutosomal Inheritance– Dominant vs. RecessiveDominant vs. Recessive

X-linked InheritanceX-linked Inheritance– Genes only on X chromosomeGenes only on X chromosome– Dominant vs. RecessiveDominant vs. Recessive

Codominance or Incomplete Codominance or Incomplete DominanceDominance

Cytoplasmic or Maternal InheritanceCytoplasmic or Maternal Inheritance

Sex DeterminationSex DeterminationPairs of chromosomes matchPairs of chromosomes matchSex chromosomes don’t Sex chromosomes don’t always matchalways match– X and YX and Y– Females: X and X chromosomeFemales: X and X chromosome– Males: X and Y chromosomeMales: X and Y chromosome

Cannot live without X Cannot live without X chromosomechromosome– Y chromosome determines Y chromosome determines

malesmales

Females are XXFemales are XX

Possible gametes: all eggs will have Possible gametes: all eggs will have one X chromosomeone X chromosome

X X

X X

X

X

Males are XYMales are XYHalf of male Half of male gametes will have gametes will have one X one X chromosomechromosome

Half of male Half of male gametes will have gametes will have one Y one Y chromosomechromosome

X Y

X

X

Y

Y

Punnett squares predict sex of Punnett squares predict sex of offspringoffspring

Genotype: 1XX:1XY phenotype: 1 female:1 male

X-linked traitsX-linked traitsX is a sex chromosomeX is a sex chromosomeIt also has hundreds of It also has hundreds of genes genes – Very few have to do with Very few have to do with

gendergender

Y does not have many of Y does not have many of these genesthese genes– Very small chromosomeVery small chromosome– ““sex determining” genesex determining” gene– ““male fertility” genesmale fertility” genes

http://www.uni.uiuc.edu/~dstone/XY.jpg

History of X-linked genesHistory of X-linked genesThomas Hunt Thomas Hunt Morgan @Columbia Morgan @Columbia University in 1910University in 1910– Two students: Two students:

Alfred Strutevant & Alfred Strutevant & Calvin BridgesCalvin Bridges

Found white eye Found white eye male fly(mutant)male fly(mutant)

Normal(red eyes)Normal(red eyes)

They noted something weird…They noted something weird…Mated white eyed Mated white eyed male w/ red eyed male w/ red eyed female(normal)female(normal)

F1 all had red eyesF1 all had red eyes

F2 had 3:1 ratioF2 had 3:1 ratio

Only males had Only males had white eyeswhite eyes

Why?Why?

http://www.nature.com/genomics/images/drosophila_200.jpg

x

Male Female

Male

More researchMore research

Morgan did find a white eyed femaleMorgan did find a white eyed female

Cross with red-eyed femaleCross with red-eyed female

All the male offspring had white All the male offspring had white eyes!!!eyes!!!

What was going on?What was going on?

ExplanationExplanationGene for eye color is found Gene for eye color is found on X chromosomeon X chromosome– Eye color gene is on Eye color gene is on

different chromosomes in different chromosomes in humanshumans

Gene not found on Y Gene not found on Y chromosomechromosome– So males only have allele of So males only have allele of

that genethat gene

These genes called These genes called X-X-linked traits or X-linked linked traits or X-linked genesgenes

Color Blindness: X-linked Color Blindness: X-linked InheritanceInheritance

B: dominant normalB: dominant normal

b:recessive color blind b:recessive color blind alleleallele

Father only has one Father only has one

X chromosome to X chromosome to contributecontribute

Mom gives all normal Mom gives all normal genesgenes

All offspring are All offspring are normalnormal

But all females are But all females are now now carrierscarriers– Seem normal, but carry Seem normal, but carry

disease alleledisease allele– Carry the hidden color Carry the hidden color

blind geneblind gene– Create a punnett with a Create a punnett with a

carrier as a mothercarrier as a mother

X-linked DiseasesX-linked DiseasesCertain genes linked to diseasesCertain genes linked to diseases““+” means wildtype+” means wildtype– Stands for normal geneStands for normal gene– Found in “wild” populationsFound in “wild” populations““M or m” stands for mutation that causes M or m” stands for mutation that causes diseasedisease– M= means mutations is dominant over M= means mutations is dominant over

wildtypewildtype– M= means mutations is recessive to wildtypeM= means mutations is recessive to wildtype

M > + > mM > + > m

Examples of X-linked diseasesExamples of X-linked diseasesDominant X-linked disease (XDominant X-linked disease (XMM))– Rett’s syndromeRett’s syndrome– Vitamind D resistanceVitamind D resistance

Recessive X-linked disease (XRecessive X-linked disease (Xmm))– Infantile spasm syndromeInfantile spasm syndrome– Duchenne Muscular DystrophyDuchenne Muscular Dystrophy– Hemophilia A and hemophila BHemophilia A and hemophila B– Two types of autistic disordersTwo types of autistic disorders– Color blindnessColor blindness

Color BlindnessColor BlindnessLack to see certain colors wellLack to see certain colors wellHard to distinguish them from other colorsHard to distinguish them from other colorsRed/Green Color Deficiency: most commonRed/Green Color Deficiency: most common– 99% of color deficiencies99% of color deficiencies

Blue/Yellow Deficiency: very rareBlue/Yellow Deficiency: very rare– Very rare and way to testVery rare and way to test

Seeing only gray is extremely rareSeeing only gray is extremely rare

In 8-12% of European malesIn 8-12% of European males0.5% of females0.5% of females

2 types of cells in eye2 types of cells in eye

Rod cellsRod cells– Supersensitive in dim lightSupersensitive in dim light– Lets you see at nightLets you see at night– Doesn’t see in colorDoesn’t see in color

ConesCones– Sees in bright lightSees in bright light– Distinguishes colorDistinguishes color

One absorbs red, green, and blueOne absorbs red, green, and blue

Color BlindnessColor Blindness

Red and green cones impaired or Red and green cones impaired or don’t work at alldon’t work at all

Can’t see red or green or see it Can’t see red or green or see it differently than normaldifferently than normal

Ready to see colors?Ready to see colors?

Previous Answer:25Previous Answer:25What about this What about this one?one?

Previous Answer:29Previous Answer:29What about this What about this one?one?

Previous Answer:45Previous Answer:45What about this What about this one?one?

Previous Answer:56Previous Answer:56And this one?And this one?

Previous Answer:6Previous Answer:6This one?This one?

Previous Answer:8Previous Answer:8This one?This one?

That was a hard one.That was a hard one.

Normal:5Normal:5

Color blind may see 2Color blind may see 2