More Variations to More Variations to Mendel’s LawsMendel’s Laws

Mitochondrial GenesMitochondrial Genes

Mitochondrial Inheritance Mitochondrial Inheritance PatternPattern

• Mitochondrial genes are passed from Mitochondrial genes are passed from mothers to offfspring.mothers to offfspring.

• Only females pass on the genesOnly females pass on the genes

The 37 Mitochondrial GenesThe 37 Mitochondrial Genes

• 24 encode proteins important for 24 encode proteins important for protein synthesisprotein synthesis– Mutations can have devastating effectsMutations can have devastating effects

• 13 encode proteins needed for 13 encode proteins needed for energy productionenergy production– Mutations often affect skeletal muscle Mutations often affect skeletal muscle

and cause fatigueand cause fatigue

HeteroplasmyHeteroplasmy

• A mutation can occur in one mitochondrial DNA A mutation can occur in one mitochondrial DNA ring and not another.ring and not another.

• When the mitochondria divide, different When the mitochondria divide, different batches of daughter mitochondria are produced batches of daughter mitochondria are produced (some with the mutation, some without)(some with the mutation, some without)

• It is therefore possible to have mutant It is therefore possible to have mutant mitochondrial DNA in some tissues but not mitochondrial DNA in some tissues but not othersothers

• Causes variation is expressivity of a Causes variation is expressivity of a mitochondrial disease depending on which mitochondrial disease depending on which tissues/organs have cells with mutated tissues/organs have cells with mutated mitochondrial DNAmitochondrial DNA

LinkageLinkage

• Two genes on the same chromosome Two genes on the same chromosome may stick togethermay stick together

• Example: Dihybrid cross of pea Example: Dihybrid cross of pea plants with purple flowers (Pp) and plants with purple flowers (Pp) and long pollen grains (Ll)long pollen grains (Ll)

Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross Self-cross

p

L l

P

L

p

l

Figure 5.10

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross

F1

Self-cross

p

L l

P

L

p

l

Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross

F1

Self-cross

p

L l

P

L

p

l

Female gametesPL Pl pL pl

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

Malegametes

PL

pl

Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross

F1

Self-cross

p

L l

P

L

p

l

Female gametesPL Pl pL pl

PPLL PPLl PpLL PpLl

PPLl PPll PpLl Ppll

PpLL PpLl ppLL ppLl

PpLl Ppll ppLl ppll

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

PPLL PpLl

PpLl ppll

Malegametes

PL

pl

Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross

F1

Phenotypic ratio 3:

Self-cross

p

L l

P

L

p

l

Female gametesPL Pl pL pl

PPLL PPLl PpLL PpLl

PPLl PPll PpLl Ppll

PpLL PpLl ppLL ppLl

PpLl Ppll ppLl ppll

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

PPLL PpLl

PpLl ppll

Malegametes

PL

pl

Phenotypic ratio 9:3

Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross

F1

Phenotypic ratio 3:

Self-cross

p

L l

P

L

p

l

Female gametesPL Pl pL pl

PPLL PPLl PpLL PpLl

PPLl PPll PpLl Ppll

PpLL PpLl ppLL ppLl

PpLl Ppll ppLl ppll

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

PPLL PpLl

PpLl ppll

Malegametes

PL

pl

Phenotypic ratio 9:3:3

Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross

F1

Phenotypic ratio 3:1

Self-cross

p

L l

P

L

p

l

Female gametesPL Pl pL pl

PPLL PPLl PpLL PpLl

PPLl PPll PpLl Ppll

PpLL PpLl ppLL ppLl

PpLl Ppll ppLl ppll

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

PPLL PpLl

PpLl ppll

Malegametes

PL

pl

Phenotypic ratio 9:3:3:1

Crossing Over May Disrupt Crossing Over May Disrupt LinkageLinkage

Linkage MapsLinkage Maps

• The frequency of The frequency of recombination between recombination between two genes is two genes is proportional to the proportional to the distance between the distance between the genesgenes– i.e.i.e. The farther apart 2 The farther apart 2

genes are, the more likely genes are, the more likely their linkage will be their linkage will be disrupted during crossing disrupted during crossing overover

– Therefore, % recombination Therefore, % recombination tells us the relative location tells us the relative location of the genesof the genes

Linkage MapsLinkage Maps

Sex ChromosomesSex Chromosomes

Sex ChromosomesSex Chromosomes

• Autosome =a Autosome =a chromosome that does chromosome that does NOT contain a gene that NOT contain a gene that determines sexdetermines sex– i.e.i.e. any chromosome that any chromosome that

is not a sex chromosomeis not a sex chromosome

• Humans have 22 autosome pairs and one pair of sex chromosomes

The Sex ChromosomesThe Sex Chromosomes

• Heterogametic -Males have an X and a Y chromosome (XY)

• Homogametic -Females have 2 X chromosomes (XX)

• In other species sex can be determined in different ways – For example, in birds and

snakes• males are homogametic

ZZ

• females are heterogameticZW

Sex DeterminationSex Determination

The Y ChromosomeThe Y Chromosome

• Has 231 protein-Has 231 protein-encoding genesencoding genes– The X chromosome has The X chromosome has

>1500 genes>1500 genes

• Contains ampliconsContains amplicons– Palindrome-ridden Palindrome-ridden

regionsregions– GACATACAGGACATACAG

The SRY GeneThe SRY Gene

• Sex-determining region of Y (SRY)Sex-determining region of Y (SRY)• Encodes a transcription factorEncodes a transcription factor

– A type of protein that controls the A type of protein that controls the expression of other genesexpression of other genes

• Leads to:Leads to:– Development of Wolffian ductsDevelopment of Wolffian ducts– Break down of MBreak down of Müüllerian ducts llerian ducts – Secretion of testosteroneSecretion of testosterone

In early embryo (week In early embryo (week 6)6)

X-linked and Y-linked TraitsX-linked and Y-linked Traits

• Genes carried on the sex chromosomesGenes carried on the sex chromosomes• X-linked traitsX-linked traits

– In females, an X-linked trait is passed on just like an In females, an X-linked trait is passed on just like an autosomal trait because there are a pair of X autosomal trait because there are a pair of X chromosomeschromosomes• 2 copies required for expression of a recessive trait2 copies required for expression of a recessive trait• Females get one X from mom and one X from dadFemales get one X from mom and one X from dad

– In males, only one copy of a recessive allele are neededIn males, only one copy of a recessive allele are needed• Males are “hemizygous” because the genes on the X Males are “hemizygous” because the genes on the X

chromosome have no match on the Y chromosomechromosome have no match on the Y chromosome• Males get their X from momMales get their X from mom

• Y-linked traitsY-linked traits– Very rareVery rare– Transmitted from father to sonTransmitted from father to son

X-linked TraitsX-linked Traits

Possible genotypesPossible genotypes

XX++Y Y Hemizygous Hemizygous wild type malewild type male

XXmmYY Hemizygous mutant male Hemizygous mutant male

XX++XX+ + Homozyogus wild femaleHomozyogus wild female

XX++XXm m Heterozygous female carrierHeterozygous female carrier

XXmmXXm m Homozygous mutant femaleHomozygous mutant female

X-linked Recessive Traits

• Always expressed in hemizygous males

• Female homozygotes show the trait but female heterozygotes do not

• Affected males: Inherited from affected or heterozygous mother

• Affected females: affected fathers and affected or heterozygous mothers