Biology and Society: Our Longest-Running Genetic...

Bio 10 -‐ Lecture 15: Inheritance TOC#15

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Biology and Society: Our Longest-Running Genetic

Experiment: Dogs •  People have selected and

mated dogs with preferred traits for more than 15,000 years. –  Over thousands of years, such geneGc Gnkering has led to the incredible variety of body types and behaviors in dogs today.

–  The biological principles underlying geneGcs have only recently been understood.

© 2013 Pearson Education, Inc.

In an Abbey Garden •  Mendel studied

garden peas because they – Are easy to grow – Come in many readily disGnguishable varieGes

– Are easily manipulated

– Can self-‐ferGlize! Very important!

Flower Anatomy Figure 9.2

Carpel (produces eggs)

Stamen (makes sperm- producing pollen)

Petal


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Figure 9.3-3

Removed stamens from purple flower.

Stamens

Transferred pollen from stamens of white flower to carpel of purple flower.

Parents (P)

Carpel

Offspring (F1)

Pollinated carpel matured into pod.

Planted seeds from pod.

4

3

2

1

Figure 9.4

White Purple

Recessive Dominant

Green Yellow

Terminal Axial

Wrinkled Round

Green Yellow

Seed shape

Seed color

Flower position

Flower color

Pod color

Recessive Dominant

Pod shape

Stem length

Inflated Constricted

Tall Dwarf

The 7characters studied by Mendel

•  A character is a heritable feature that varies among individuals.

•  Ex: Hair Color, Eye Color, Fresckles

•  A trait is a variant of a character.

•  Ex: Blonde/Red/Brown/Black, Blue/Brown/Hazel, Present/Absent

•  Each of the characters Mendel studied occurred in two disGnct traits.

In an Abbey Garden



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•  Mendel

–  created purebred varieGes of plants

and

–  crossed two different purebred varieGes.

In an Abbey Garden


•  Hybrids are the offspring of two different purebred varieGes.

–  The parental plants are the P genera4on.

–  Their hybrid offspring are the F1 genera4on.

–  A cross of the F1 plants forms the F2 genera4on.

In an Abbey Garden


Mendel’s Law of Segregation •  Mendel performed many experiments.

•  He tracked the inheritance of characters that occur as two alternaGve traits.

© 2013 Pearson Education, Inc. Figure 9.4

White Purple

Recessive Dominant

Green Yellow

Terminal Axial

Wrinkled Round

Green Yellow

Seed shape

Seed color

Flower position

Flower color

Pod color

Recessive Dominant

Pod shape

Stem length


Tall Dwarf


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Mendel’s Law of Segregation

© 2013 Pearson Education, Inc. Figure 9.4

White Purple

Recessive Dominant

Green Yellow

Terminal Axial

Wrinkled Round

Green Yellow

Seed shape

Seed color

Flower position

Flower color

Pod color

Recessive Dominant

Pod shape

Stem length


Tall Dwarf

Which characters are paired?

Monohybrid Crosses

– A monohybrid cross is a cross between purebred parent plants that differ in only one character.

Blast Anima4on: Single-‐Trait Crosses


p

p

P

P

PP Pp

Pp pp

Figure 9.5-3

Purple flowers

F1 Generation

White flowers

P Generation (purebred parents)

All plants have purple flowers

F2 Generation

Fertilization among F1 plants (F1 × F1)

of plants have purple flowers

of plants have white flowers

3 4

1 4


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4. Gametes carry only one allele for each inherited character. –  The two alleles for a character segregate (separate) from each other during the

producGon of gametes. –  This statement is called the law of segrega4on.

•  Mendel developed four hypotheses from the monohybrid cross, listed here using modern terminology (including “gene” instead of “heritable factor”).

1. The alternaGve versions of genes are called alleles.

Monohybrid Crosses

2. For each inherited character, an organism inherits two alleles, one from each parent.

–  An organism is homozygous for that gene if both alleles are idenGcal. –  An organism is heterozygous for that gene if the alleles are different.

3. If two alleles of an inherited pair differ, –  Then one determines the organism’s appearance and is called the dominant

allele and –  The other has no noGceable effect on the organism’s appearance and is called

the recessive allele.

PP or pp

Pp

P

p

•  Do Mendel’s hypotheses account for the 3:1 raGo he observed in the F2 generaGon?

•  A PunneF square highlights –  The four possible combinaGons of gametes and

–  The four possible offspring in the F2 generaGon.

Monohybrid Crosses


Yes

Figure 9.6

P Generation Genetic makeup (alleles)

Alleles carried by parents

Gametes

Purple flowers PP

White flowers pp

All P All p

p P

F1 Generation (hybrids)


Alleles segregate

Gametes

Purple flowers All Pp

2 1

2 1

Sperm from F1 plant

Eggs from F1 plant

Phenotypic ratio 3 purple : 1 white

Genotypic ratio 1 PP : 2 Pp : 1 pp

p

p

P

P PP Pp

Pp pp

Figure 9.6a

Purple flowers White flowers

P Generation Genetic makeup (alleles)

Gametes

Alleles carried by parents

PP

P

pp

p All All


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Figure 9.6b


All Pp Purple flowers

Gametes

Alleles segregate

P p 2 1

2 1

Figure 9.6c


Sperm from F1 plant

Eggs from F1 plant

Phenotypic ratio 3 purple : 1 white

Genotypic ratio 1 PP : 2 Pp : 1 pp

p

p

P

P

PP Pp

Pp pp

– GeneGcists disGnguish between an organism’s physical appearance and its geneGc makeup.

•  An organism’s physical appearance is its phenotype. •  An organism’s geneGc makeup is its genotype.

Monohybrid Crosses



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Genetic Alleles and Homologous Chromosomes

– A gene locus is a specific locaGon of a gene along a chromosome.

– Homologous chromosomes have alleles (alternate versions) of a gene at the same locus.


Figure 9.7

Homologous chromosomes

P

Genotype:

Gene loci

P

a

aa

b

B

Dominant allele

Recessive allele

Bb PP Homozygous for the dominant allele

Homozygous for the recessive allele

Heterozygous with one dominant and one recessive allele

a

Mendel’s Law of Independent Assortment – A dihybrid cross is the maGng of parental varieGes differing in two characters.

– What would result from a dihybrid cross? Two hypotheses are possible: 1. Dependent assortment or 2. Independent assortment.

Blast Anima4on: Two-‐Trait Crosses



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Figure 9.8

F1 Generation

F2 Generation

RRYY

Predicted results (not actually seen)

P Generation

Gametes RY

rryy

ry

(a) Hypothesis: Dependent assortment

RrYy

RY ry Sperm

Eggs Eggs

RY

ry

Actual results (support hypothesis)

RRYY

RY

rryy

ry

RrYy

(b) Hypothesis: Independent assortment

Gametes

RY ry

Sperm

RY

ry

Ry

rY

Ry rY

RRYY

Rryy RRyy RrYy RRYy

RrYy rrYy Rryy rryy

RrYY

RrYY rrYY RrYy rrYy

RRYy RrYy

Yellow round

Yellow wrinkled Green wrinkled

Green round

16 1

16 3

16 3

16 9

4 1

4 1

4 1

4 1

4 1

4 1

4 1

4 1

2 1

2 1

2 1

2 1

Figure 9.8b

Actual results (support hypothesis)

RY ry Sperm

RY

ry

Ry

rY

Ry rY

RRYY

RRyy RrYy RRYy

RrYy rrYy Rryy rryy

RrYY

RrYY rrYY

RrYy RRYy

Yellow round

Yellow wrinkled Green wrinkled

Green round

RrYy rrYy

Rryy

4 1

16 1

4 1

4 1

4 1

4 1

4 1

4 1

4 1

16 3

16 3

16 9 Eggs

F2 Generation

– Mendel’s dihybrid cross supported the hypothesis that each pair of alleles segregates independently of the other pairs during gamete formaGon.

– Thus, the inheritance of one character has no effect on the inheritance of another.

– This is called Mendel’s law of independent assortment.

–  Independent assortment is also seen in two hereditary characters in Labrador retrievers.

Mendel’s Law of Independent Assortment



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Figure 9.9 Blind dog

Phenotypes

(a) Possible phenotypes of Labrador retrievers

B_N_

Blind dog

Genotypes

(b) A Labrador dihybrid cross

Black coat, normal vision

B_nn

Black coat, blind (PRA)

bbnn

Chocolate coat, blind (PRA)

bbN_

Chocolate coat, normal vision

Mating of double heterozygotes

(black coat, normal vision) BbNn BbNn

Phenotypic ratio of offspring

9 black coat, normal vision

3 black coat, blind (PRA)

3 chocolate coat, normal vision

1 chocolate coat, blind (PRA)

Blind Blind

Figure 9.9a

Blind

Blind

Mating of double hererozygotes (BbNn × BbNn)

9 black coat, normal vision

3 black coat, blind (PRA)

3 chocolate coat, normal vision

1 chocolate coat, blind (PRA)

Using a Testcross to

Determine an Unknown Genotype

•  A testcross is a maGng between

–  an individual of dominant phenotype (but unknown genotype) and

–  a homozygous recessive individual.


Figure 9.10

Two possible genotypes for the black dog:

B_ bb

Testcross

Genotypes

Bb

B b

or BB

B

b Bb Bb bb b

Gametes

Offspring All black 1 black : 1 chocolate


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Figure 9.10

Two possible genotypes for the black dog:

B_ bb

Testcross

Genotypes

Bb

B b

or BB

B

b Bb Bb bb b

Gametes

Offspring All black 1 black : 1 chocolate

The Rules of

Probability


Figure 9.11 F1 Genotypes

F2 Genotypes

Bb female Bb male

Formation of eggs Formation of sperm

Male gametes

Fem

ale

gam

etes

2 1

2 1

2 1

2 1

2 1

2 1

4 1

4 1

4 1

4 1

( × )

B

B B B B

B

b

b

b b b b

•  Mendel’s strong background in mathemaGcs helped him understand paeerns of inheritance.

•  The rule of mul4plica4on states that the probability of a compound event is the product of the separate probabiliGes of the independent events.

Family Pedigrees

– Mendel’s principles apply to the inheritance of many human traits.



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Figure 9.12

DO

MIN

AN

T TR

AIT

S

Free earlobe

REC

ESSI

VE T

RA

ITS

Widow’s peak Freckles

Attached earlobe Straight hairline No freckles

–  Dominant traits are not necessarily •  normal or •  more common.

– Wild-‐type traits are •  those seen most ofen in

nature and •  not necessarily specified

by dominant alleles.

Family Pedigrees


Recessive Disorders

– Most human geneGc disorders are recessive. –  Individuals who have the recessive allele but appear normal are carriers of the disorder.



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Table 9.1

Figure 9.14

Parents

Offspring

Dd Hearing Hearing

Dd

Sperm

Eggs

D

Hearing D Dd

d

Hearing (carrier)

DD

Dd Hearing (carrier)

dd Deaf d

– Cys4c fibrosis is •  the most common lethal geneGc disease in the United

States and •  caused by a recessive allele carried by about one in 31

Americans. •  Prolonged geographic isolaGon of certain populaGons can lead to

inbreeding, the maGng of close relaGves. –  Inbreeding increases the chance of offspring that are homozygous for a

harmful recessive trait.

Recessive Disorders



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Dominant Disorders •  Some human geneGc

disorders are dominant. –  Achondroplasia is a form of dwarfism.

•  The homozygous dominant genotype causes death of the embryo.

•  Thus, only heterozygotes have this disorder.

–  Hun4ngton’s disease, which leads to degeneraGon of the nervous system, does not usually begin unGl middle age.


Figure 9.16

Parents

Sperm

Eggs

d

Dwarf D Dd

d

Dd

dd d

Normal (no achondroplasia) Molly Jo

Dwarf (achondroplasia)

Dd dd

Normal dd

Normal

Dwarf Matt Amy Jacob Zachary

Jeremy

The Process of Science: What Is the Genetic Basis of Coat Variation

in Dogs? •  Observation: Dogs come in a wide variety of

physical types. •  Question: What is the genetic basis for canine

coats? •  Hypothesis: A comparison of genes of a wide

variety of dogs with different coats would identify the genes responsible.



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•  Prediction: Mutations in just a few genes account for the coat appearance.

•  Experiment: Compared DNA sequences of 622 dogs from dozens of breeds.

•  Results: Three genes in different combinations produced seven different coat appearances, from very short hair to full, thick, wired hair.

The Process of Science: What Is the Genetic Basis of Coat Variation in Dogs?


VARIATIONS ON MENDEL’S LAWS

•  Some paeerns of geneGc inheritance are not explained by Mendel’s laws. 1.   Incomplete Dominance: F1 hybrids have an appearance

between the phenotypes of the two parents. 2.   Codominance: F1 hybrids have an appearancein which both

the phenotypes of the two parents appear.


Figure 9.18-2

Figure 9.18-3

F1 Generation

RR rr

Gametes

P Generation

F2 Generation Sperm

Gametes

Red White

R r

Rr Pink

R r

R r

R

r

RR Rr

rr Rr

Eggs

2 1

2 1

2 1

2 1

2 1

2 1


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•  Hypercholesterolemia –  Is characterized by dangerously high levels of cholesterol in the blood. –  heterozygotes have blood cholesterol levels about twice normal, and –  homozygotes have about five Gmes the normal amount of blood cholesterol and may have heart aeacks as early as age 2.

Incomplete Dominance in Plants and People


Figure 9.19

Homozygous for ability to make

LDL receptors

Severe disease Mild disease Cell

LDL receptor

LDL

Homozygous for inability to make

LDL receptors

Heterozygous HH Hh hh

GEN

OTY

PE

PHEN

OTY

PE

Normal

ABO Blood Groups: An Example of Multiple Alleles and Codominance

•  The ABO blood groups in humans are an example of mulGple alleles.

•  The immune system produces blood proteins called anGbodies that bind specifically to foreign carbohydrates.


Figure 9.20a

Blood Group (Phenotype)

Genotypes

Red Blood Cells

O

A

B

AB

ii

IAIB

IBIB

or IBi

IAIA

or IAi

Carbohydrate A

Carbohydrate B

Antibodies Present in Blood

Anti-B

Anti-A

Anti-A Anti-B

—

–  If a donor’s blood cells have a carbohydrate (A or B) that is foreign to the recipient, the blood cells may clump together, potenGally killing the recipient.

– The clumping reacGon is the basis of a blood-‐typing lab test.

– The human blood type alleles IA and IB are codominant, meaning that both alleles are expressed in heterozygous individuals who have type AB blood.

ABO Blood Groups: An Example of Multiple Alleles and Codominance



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VARIATIONS ON MENDEL’S LAWS

•  Some paeerns of geneGc inheritance are not explained by Mendel’s laws. 3.   Pleiotropy is when one gene

influences several characters. •  EX: Sickle-‐cell disease

4.   Polygenic inheritance is the

addiGve effects of two or more genes on a single phenotype.


Figure 9.18-2

Figure 9.22c

Pleiotropy and Sickle-Cell Disease

– Pleiotropy is when one gene influences several characters.

•  EX: Sickle-‐cell disease

– Results in abnormal hemoglobin proteins, and

– Causes disk-‐shaped red blood cells to deform into a sickle shape with jagged edges.


Figure 9.21

Sickled cells can lead to a cascade of symptoms, such as weakness,

pain, organ damage, and paralysis.

Abnormal hemoglobin crystallizes into long flexible chains, causing

red blood cells to become sickle-shaped.

Individual homozygous for sickle-cell allele

Sickle-cell (abnormal) hemoglobin

Col

oriz

ed S

EM

Pleiotropy and Sickle-Cell Disease

•  Pleiotropy is when one gene influences several characters.

•  Sickle-‐cell disease –  exhibits pleiotropy, –  results in abnormal hemoglobin proteins, –  causes disk-‐shaped red blood cells to

deform into a sickle shape with jagged edges.


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Figure 9.22

F1 Generation

P Generation

F2 Generation Sperm

Eggs

Frac

tion

of p

opul

atio

n

aabbcc (very short)

AABBCC (very tall)

AaBbCc (medium height)

AaBbCc (medium height)

= short allele = tall allele

Very short Very tall Adult height

20 64

15 64

6 64

1 64

1 64

6 64

15 64

20 64

15 64

6 64

1 64

1 8

1 8

1 8

1 8

1 8

1 8

1 8

1 8

1 8

1 8

1 8

1 8

1 8

1 8

1 8

1 8

The Role of Environment

– Many human characters result from a combinaGon of

•  heredity and •  environment.

– Only geneGc influences are inherited.


Figure 9.23: As a result of environmental influences, even idenGcal twins can look different.

Biology and Society: Our Longest-Running Genetic...

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