human biology sylvia s. mader michael windelspecht chapter 20 patterns of genetic inheritance...

Human BiologySylvia S. Mader

Michael Windelspecht

Chapter 20 Patterns of

Genetic Inheritance

Lecture Outline

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Points to Ponder• What is the genotype and the phenotype of an individual?• What are the genotypes for homozygous recessive and

dominant individuals and a heterozygote individual?• Be able to draw a punnett square for any cross (1-trait cross, 2-

trait cross, and a sex-linked cross).• What are Tay-Sachs disease, Huntington disease, sickle-cell

disease, and PKU? • How are each of the above inherited?• What is polygenic inheritance?• What is a multifactorial trait?• What is sex-linked inheritance?• Name 3 X-linked recessive disorders.• What is codominance?• What is incomplete dominance?• What do you think about genetic profiling?

These traits are genetically inherited

Answer these questions about your inheritance:• Do you have a widow’s peak?• Are your earlobes attached or unattached?• Do you have short or long fingers?• Do you have freckles?• Can you roll your tongue?• Do you have Hitchhiker’s thumb?

20.1 Genotype and phenotype

Genotype

Genotype – specific genes for a particular trait written with symbols– Alleles: alternate forms of a specific gene at the same position

(locus) on a gene (e.g., allele for unattached earlobes and attached lobes); alleles occur in pairs

– Dominant gene: will be expressed and will mask a recessive gene (Tt or TT)

– Recessive allele: allele that is only expressed when a gene has two of this type of allele

– Homozygous dominant genotype: 2 dominant alleles (TT or AA)

– Homozygous recessive genotype: 2 recessive alleles (tt or aa)

– Heterozygous genotype: one dominant allele and one recessive allele (Tt or Aa)


Phenotype

Phenotype – the physical or outward expression of the genotype

Genotype Phenotype

EE unattached earlobe

Ee unattached earlobe

ee attached earlobe

What are your genotype and phenotype?


Understanding genotype & phenotype20.1 Genotype and phenotype


sperm

egg

fertilization

growth anddevelopment

unattached earlobeattached earlobeunattached earlobe

Ee

e EE

e eE

eeEE

EeeeEE

Allele Key

E = unattached earlobes e = attached earlobes

e eE

What about your inheritance?20.2 One-and Two-trait inheritance

Crosses• One-trait cross – considers the inheritance of one characteristic

e.g. WW x Ww

• Two-trait cross – considers the inheritance of two characteristicse.g. WWTT x WwTT

• Gametes only carry one allele, so if an individual has the genotype Ww what are the possible gametes that this individual can pass on?Answer: either a W or a w but not both

Another example:

20.2 One-and Two-trait inheritance


gametes

meiosis

Offspring

×Parents

f f

ff

no freckles

no freckles no freckles

ff ff


Punnett squares

• Punnett squares are the use of a grid that diagram crosses between individuals by using the possible parental gametes

• These allow one to figure the probability that an offspring will have a particular genotype and phenotype



31

fF

F

f

KeyF = Frecklesf = No freckles

FrecklesNo freckles

Phenotypic Ratio3:1

FrecklesNo freckles

Offspring

Ff

FfFf

ffFf

FF

×

Parents

Sp

erm

eggs

Practicing punnett squares

• What would a punnett square involving a man (M) with a genotype Ff and a woman (F) with a genotype Ff look like?

F – frecklesf – no freckles


M/F F f

F FF Ff

f Ff ff

eggs

sper

m

Practicing ratios• Genotypic ratio: the number

of offspring with the same genotype

• Phenotypic ratio: the number of offspring with the same outward appearance

• What is the genotypic ratio?1: 2: 1 (1 FF: 2 Ff: 1 ff)

• What is the phenotypic ratio? 3: 1 (3 with freckles and 1 with no freckles)


M/F F f

F FF Ff

f Ff ff

eggs

sper

m

Monohybrid crosses


Monohybrid cross – an experimental cross in which parentsare identically heterozygous at one gene pair (e.g., Aa x Aa)


ww

W

W11

×

ww

W

w

Offspring

Parents

wwWW ×

Parents

Ww ww

Ww

ww

Offspring

ww

Ww

eggseggs

Straight hairlineWidow’s peak

Phenotypic Ratio1:1

Straight hairlineWidow’s peakStraight hairlineWidow’s peak

Key

W =w =

Widow’s peakStraight hairline

Straight hairlineWidow’s peak

W =w =

Key

Phenotypic Ratio

Widow’s peakAllWw

Ww

Ww

Ww

Sp

erm

Sp

erm

a. b.

Possible gametes for two traits20.2 One-and Two-trait inheritance


S S

W W

s s

w wW W

w w

W W

W W W Ww w w w

WW w ww w

s s

s ss

s s s s

s

S S

S

S S S S

SS S

WS ws wS Ws

Cell hastwo pairs ofhomologues.

one pair

one pair

oreither

MEIOSIS I

MEIOSIS II

Allele Key

s = Long fingersS = Short fingersw = Straight hairlineW = Widow’s peak

Dihybrid cross (a type of two-trait cross)

• Dihybrid cross – an experimental cross usually involving parents that are homozygous for different alleles of two genes and results in a 9:3:3:1 genotypic ratio for the offspring



P generation

P gametes

F1generation

WwSs

wwssWWSS

wsWS

×

Practicing a punnett square for a 2-trait cross

• What would the punnett square look like for a dihybrid cross between a male that is WWSS and a female that is wwss?


F1gametes

F1generation

wwSs

ws

wS

Ws

WS

WS Ws

eggs

wS ws

WwSsWwSSWWSsWWSS

WwssWwSsWWssWWSs

WwSS

WwSs

Offspring

Wwss wwSs

wwSS

sper

m

Allele Key

W = Widow’s peakW = Straight hairlineS = Short fingersS = Long fingers

Phenotypic Ratio9:3:3:1

Widow’s peak, short fingersWidow’s peak, long fingersStraight hairline, short fingersStraight hairline, long fingers

9

33

1


Autosomal recessive disorder• Individuals must be homozygous recessive to

have the disorder

20.3 Inheritance of genetic disorders


II

III

IV

Iaa A?

A?

A?A?Aa

A?aaaa

Aa* relatives

Aa

Key

= affected

Aa = carrier (unaffected)AA = unaffectedA? = unaffected

(one allele unknown)

aa

AaA?

Autosomal recessive disorders

• Affected children can have unaffected parents.

• Heterozygotes (Aa) have an unaffected phenotype.

• Two affected parents will always have affected children.

• Affected individuals with homozygous unaffected mates will have unaffected children.

• Close relatives who reproduce are more likely to have affected children.

• Both males and females are affected with equal frequency.

Autosomal dominant disorder• Individuals that are homozygous dominant and

heterozygous will have the disorder



aa = unaffected(one allele unknown)= affected= affected= affectedAA

AaA?Autosomal dominant disorders

• Affected children will usually have an affected parent.• Heterozygotes (Aa) are affected.• Two affected parents can produce an unaffected child.• Two unaffected parents will not have affected children.• Both males and females are affected with equal frequency.

III

II

I Aa Aa

aa Aa

*A? aa aa aa

aaaaaaaaAaAa

Key

Genetic disorders of interest

• Tay-Sachs disease: lack of the enzyme that breaks down lipids in lysosomes resulting in excess and eventually death of a baby

• Cystic fibrosis: Cl- do not pass normally through a cell membrane resulting in thick mucus in lungs and other places often causing infections

• Phenylketonuria (PKU): lack of an enzyme needed to make a certain amino acid and affects nervous system development

• Sickle-Cell disease: red-blood cells are sickle shaped rather than biconcave that clog blood vessels

• Huntington disease: huntington protein has too many glutamine amino acids leading to the progressive degeneration of brain cells


Genetic disorders20.3 Inheritance of genetic disorders


many neurons innormal brain

loss of neurons inhuntington brain


(both brain tissue slides): Courtesy Dr. Hemachandra Reddy, The Neurological Science Institute, Oregon Health & Science University; (woman with Huntington): © Steve Uzzell


thick mucus

defectivechannel

H2O

H2O Cl– Cl– Cl–

H2OClCl

nebulizer

percussionvest

© Pat Pendarvis

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Polygenic inheritance• Polygenic traits - two or more sets of alleles govern one

trait – Each dominant allele codes for a product so these effects are

additive– Results in a continuous variation of phenotypes– Environmental effects cause intervening phenotypes – e.g., skin color ranges from very dark to very light– e.g., height varies among

• Multifactorial trait – a polygenic trait that is particularly influenced by the environment – e.g., skin color is influenced by sun exposure– e.g., height can be affected by nutrition

20.4 Beyond simple inheritance patterns

Polygenic inheritance20.4 Beyond simple inheritance patterns


mostarethis

height

Height in Inchestallshort

64 66 68 70 72 74

few few

Nu

mb

er o

f M

en

Courtesy University of Connecticut/Peter Morenus, photographer

62

Dark

aabb

AABB

Genotypes Phenotypes

AABb or AaBB

AaBb or Aabb or aaBB

Aabb or aaBb

Very dark

Medium brown

Light

Very light


Demonstrating environmental influences on phenotype

• Himalayan rabbit’s coat color influenced by temperature

• There is an allele responsible for melanin production that appears to be active only at lower temperatures

• The extremities have a lower temperature and thus the ears, nose, paws, and tail are dark in color


Incomplete dominance

• Occurs when the heterozygote is intermediate between the 2 homozygotes

• Example: (curly hair) CC x SS (straight hair)

CS (wavy hair)


Codominance

• Occurs when the alleles are equally expressed in a heterozygote

• Example:(Type A blood) AA x BB (Type B blood)

AB (Type AB blood that has characteristics

of both blood types)


Multiple allele inheritance• The gene exists in several allelic forms

• A person only has 2 of the possible alleles

• A good example is the ABO blood system

• A and B are codominant alleles

• The O alleles is recessive to both A and B therefore to have this blood type you must have 2 recessive alleles


Multiple allele inheritance

Based on what you know what type of blood would each of the following individuals have in a cross between Ao and Bo?

possible genotypes: phenotypes: AB Type AB blood Bo Type B blood Ao Type A blood oo Type O blood


Blood type inheritance20.4 Beyond simple inheritance patterns


×

Parents

eggs

Phenotypic Ratio1:1:1:1

Blood type ABlood type BBlood type ABBlood type O

Key

1111

ii

i

i

IBi IAi

IBiIB

IB

IAIB

IAi

sp

erm

Offspring

Sex-linked inheritance

• Traits are controlled by genes on the sex chromosomes X-linked inheritance: the allele is carried on

the X chromosome Y-linked inheritance: the allele is carried on

the Y chromosome Most sex-linked traits are X-linked

20.5 Sex-linked inheritance

X-linked inheritance: Color blindness

Cross:

XBXb x XBY

Possible offspring:XBXB normal vision female

XBXb normal vision female

XBY normal vision male

XbY normal vision male



×

Parents

Key

XB = Normal visionXb = Color-blind

Normal visionColor-blind

Phenotypic Ratio

Females All

Males 1:1 11XbYXBY

XBXbXBXB

XbXB

XB

Y

eggs

XBY XBXb

Offspring

sper

m

X-linked disorders• More often found in males than females because

recessive alleles are always expressed• Most X-linked disorders are recessive:

– Color blindness: most often characterized by red-green color blindness

– Muscular dystrophy: characterized by wasting of muscles and death by age 20

– Fragile X syndrome: most common cause of inherited mental impairment

– Hemophilia: characterized by the absence of particular clotting factors that causes blood to clot very slowly or not at all


X-linked disorders20.5 Sex-linked inheritance

Key

XBXB = Unaffected femaleXBXb = Carrier femaleXbXb = Color-blind femaleXBY = Unaffected maleXbY = Color-blind male

X-linked RecessiveDisorders

• More males than females are affected.

• An affected son can have parents who have the normal phenotype.

• For a female to have the characteristic, her father must also have it. Her mother must have it or be a carrier.

• The characteristic often skips a generation from the grandfather to the grandson.

• If a woman has the characteristic, all of her sons will have it.

grandson

daughter

grandfatherXBXB

XBY XBXb XBY XbXb

XbY

XbYXBY XBXB XBXb

XbY


X-linked disorders: Hemophilia



Unaffected male

Hemophiliac male

Unaffected female

Carrier female

??

??

??

Nicholas IIAlexandra

Olga

All were assassinated

Anastasia

Philip

Harry

Juan Carlos

Elizabeth II

Marie Alexi 9

5

12 13 16 14 15

6

3 4 7 8Mary

Alice1 2 10Louis IV

2. Edward VII3. Irene4. George V6. Margaret

9. Juan

12. Diana

14. Edward

16. Sarah

Leopold Beatrice Helena

Albert

Edward

? ? ?

?

1. Victoria

7. Victoria8. Alfonso XIII

Tatiana

Victoria

Victoria

11

10. Alexandra11. Charles

13. Andrew

15. Anne


(queen): © Stapleton Collection/ Corbis; (prince): © Huton Archive/Getty Images

William

human biology sylvia s. mader michael windelspecht chapter 20 patterns of genetic inheritance...

Documents

genotype ww

aaheterozygous genotype

recessive allele tt

recessive gene tt

particular trait

multifactorial trait

trait inheritancecopyright

recessive alleles tt