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Chapter 14.

Beyond Mendel’s Lawsof Inheritance

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Extending Mendelian genetics Mendel worked with a simple system

peas are genetically simple most traits are controlled by a single

gene each gene has only 2 alleles, 1 of which

is completely dominant to the other The relationship between genotype &

phenotype is rarely simple

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Incomplete dominance Heterozygotes show an intermediate

phenotype RR = red flowers rr = white flowers Rr = pink flowers

make 50% less color

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Incomplete dominancetrue-breeding

red flowerstrue-breeding white flowers

X

100%100% pink flowers

F1generation(hybrids)

25%white

F2generation

25%red 1:2:1

P

self-pollinate50%pink

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Incomplete dominanceCRCW x CRCW

CR CWmale / sperm

CR

CW

fem

ale

/ egg

s CRCR

CRCW CWCW

CRCW

25%

1:2:1

25%

50%

25%

1:2:1

%genotype

%phenotype

CRCR

CRCW

CRCW

CWCW

25%

50%

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Co-dominance 2 alleles affect the phenotype in

separate, distinguishable ways ABO blood groups 3 alleles = IA, IB, and i

both the IA & IB alleles are dominant to the iallele

IA & IB alleles are codominant to each other determines presences of

oligosaccharides on the surface of redblood cells

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Blood type

universaldonor

universalrecipient

__

__

status

no oligosaccharideson surface of RBC

both type A & type Boligosaccharides onsurface of RBC

type Boligosaccharides onsurface of RBC

type Aoligosaccharides onsurface of RBC

phenotype

type Oi i

type ABIA IB

type BIB IB IB i

type AIA IA IA i

phenotypegenotype

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Blood compatibility Matching compatible blood groups is critical

for blood transfusions A person produces antibodies against

foreign blood factors = oligosaccharides if donor’s blood has an A or B oligosaccharide

that is foreign to the recipient, antibodies inthe recipient’s blood will bind to the foreignmolecules

cause the donated blood cells to clumptogether & can kill the recipient

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Blood donation

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Pleiotropy Most genes are pleiotropic

one gene affects more than onephenotypic character wide-ranging effects due to a single gene: dwarfism (achondroplasia) gigantism (acromegaly)

The genes that we have covered so far affect only one phenotypiccharacter, but most genes are pleiotropic

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Acromegaly: André the Giant

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Pleiotropy It is not surprising that a gene can

affect a number of organism’scharacteristics consider the intricate molecular &

cellular interactions responsible for anorganism’s development cystic fibrosis sickle cell anemia Timothy disease

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Epistasis One gene masks another

coat color in mice =2 genes pigment (C) or

no pigment (c) more pigment (black=B)

or less (brown=b) cc = albino,

no matter B allele

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Polygenic inheritance Some phenotypes determined by

additive effects of 2 or more genes ona single character phenotypes on a continuum human traits

skin color height weight eye color intelligence behaviors

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AlbinismJohnny & Edgar Winter

albinoAfricans

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Nature vs. nurture Phenotype is controlled by both

environment & genes

Color of Hydrangea flowersis influenced by soil pH

Human skin coloris influenced byboth genetics &environmentalconditions

• The relative importance of genes & the environment in influencinghuman characteristics is a very old & hotly contested debate

• a single tree has leaves that vary in size, shape & color, dependingon exposure to wind & sun

• for humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performanceon intelligence tests

• even identical twins — genetic equals — accumulate phenotypicdifferences as a result of their unique experiences

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Chromosome theory of inheritance experimental evidence from improved

microscopy & animal breeding led us toa better understanding of chromosomes& genesbeyond Mendel Drosophila studies

It all started with a fly…

A. H. Sturtevantin the Drosophila

stockroom

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Chromosome theory of inheritance Experimental evidence from improved

microscopy & animal breeding led usto a better understanding ofchromosomes & genes beyond MendelDrosophila studies

A. H. Sturtevantin the Drosophila

stockroom

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Thomas Hunt Morgan Morgan was an embryologist at

Columbia University 1st to associate a specific gene with a

specific chromosomeDrosophila breeding

prolific 2 week generations 4 pairs of chromosomes XX=female, XY=male

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Morgan’s first mutant… Wild type fly = red eyes Morgan discovered a mutant white-eyed

male trace a gene for eye color to a specific

chromosome

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Discovery of sex linkagered eyefemale

white eyemalex

allred eye

offspring

75%red eyefemale

25%white eye

malex

How is this possible?Sex-linked trait!

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Sex-linked traits Although differences between women &

men are many, the chromosomal basis ofsex is rather simple

In humans & other mammals, there are 2sex chromosomes: X & Y 2 X chromosomes develops as

a female: XX redundancy

an X & Y chromosome develops asa male: XY no redundancy

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Sex chromosomes

autosomal

chromosomes

sexchromosomes

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Genes on sex chromosomes Y chromosome

SRY: sex-determining region master regulator for maleness turns on genes for production of male

hormonespleiotropy!

X chromosome other traits beyond sex determination

hemophilia Duchenne muscular dystrophy color-blind

Duchenne muscular dystrophy affects one in 3,500 males born inthe United States.

• Affected individuals rarely live past their early 20s.

• This disorder is due to the absence of an X-linked gene for akey muscle protein, called dystrophin.

• The disease is characterized by a progressive weakening of themuscles and loss of coordination.

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Map of the Y chromosome?

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Sex-linked traits

Hh x HH

XH Ymale / sperm

XH

Xh

fem

ale

/ egg

s XHXH

XHY

XHXh

XHXh

XHY

XhY

XHXhXH

Xh

XHYY

XHXHXH XHY

XHXh XhY

sex-linked recessive

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Sex-linked traits summary X-linked

follow the X chromosomes males get their X from their mother trait is never passed from father to son

Y-linked very few traits only 26 genes trait is only passed from father to son females cannot inherit trait

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Hemophilia is a sex-linked recessive trait defined by the absence ofone or more clotting factors.

• These proteins normally slow and then stop bleeding.Individuals with hemophilia have prolonged bleeding because a firmclot forms slowly.

• Bleeding in muscles and joints can be painful and lead toserious damage.

Individuals can be treated with intravenous injections of the missingprotein.

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X-inactivation Female mammals inherit two X

chromosomes one X becomes inactivated during

embryonic development condenses into compact object = Barr body

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X-inactivation & tortoise shell cat 2 different cell lines in cat

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Male pattern baldness Sex influenced trait

autosomal trait influenced by sex hormones age effect as well: onset after 30 years old

dominant in males & recessive in females B– = bald in males; bb = bald in females

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Mechanisms of inheritance What causes the differences in alleles

of a trait? yellow vs. green color smooth vs. wrinkled seeds dark vs. light skin Tay sachs disease vs. no disease Sickle cell anemia vs. no disease

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Mechanisms of inheritance What causes dominance vs. recessive?

genes code for polypeptides polypeptides are processed into proteins proteins function as…

enzymes structural proteins hormones

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How does dominance work: enzyme= allele coding for

functional enzyme= allele coding for

non-functional enzyme

= 100% non-functional enzyme• normal trait is not exhibited aa

= 50% functional enzyme• sufficient enzyme present• normal trait is exhibited

Aacarrier

= 100% functional enzyme• normal trait is exhibited AA

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AA

How does dominance work: structure= allele coding for

functional structuralprotein

= allele coding fornon-functional structuralprotein

= 100% non-functional structure • normal trait is not exhibited

= 50% functional structure• 50% proteins malformed• normal trait is not exhibited

Aa

= 100% functional structure • normal trait is exhibited aa

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Prevalence of dominance Because an allele is dominant

does not mean… it is better it is more common

Polydactyly:dominant allele

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Polydactylyindividuals are born withextra fingers or toes

dominant to recessiveallele for 5 digits

recessive allele far morecommon than dominant→399 individuals out of 400

have only 5 digits→most people are homozygous

recessive (aa)

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Hound Dog Taylor