Genetics

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Genetics is the science of heredity

• These black Labrador puppies are purebred—their parents and grandparents were black Labs with very similar genetic makeups

– Purebreds often suffer from serious genetic defects

Purebreds and Mutts — A Difference of Heredity


• The parents of these puppies were a mixture of different breeds

– Their behavior and appearance is more varied as a result of their diverse genetic inheritance


• Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants

9.2 Experimental genetics began in an abbey garden

Figure 9.2A, B

Stamen

Carpel


• Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation

Figure 9.2C

• This illustration shows his technique for cross-fertilization

1 Removed stamensfrom purple flower

White

Stamens

Carpel

PurplePARENTS(P)

OFF-SPRING

(F1)

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

3 Pollinated carpel matured into pod

4 Planted seeds from pod


• Mendel studied seven pea characteristics

Figure 9.2D

• He hypothesized that there are alternative forms of genes (although he did not use that term), the units that determine heredity

FLOWER COLOR

FLOWER POSITION

SEED COLOR

SEED SHAPE

POD SHAPE

POD COLOR

STEM LENGTH

Purple White

Axial Terminal

Yellow Green

Round Wrinkled

Inflated Constricted

Green Yellow

Tall Dwarf


• From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic

– One characteristic comes from each parent

9.3 Mendel’s principle of segregation describes the inheritance of a single characteristic

P GENERATION(true-breedingparents)

F1 generation

F2

generation

Purple flowers White flowers

All plants have purple flowers

Fertilization among F1 plants(F1 x F1)

3/4 of plantshave purple flowers

1/4 of plantshave white flowers

Figure 9.3A


• A sperm or egg carries only one allele of each pair– The pairs of

alleles separate when gametes form

– This process describes Mendel’s law of segregation

– Alleles can be dominant or recessive

GENETIC MAKEUP (ALLELES)

P PLANTS

F1 PLANTS(hybrids)

F2 PLANTS

PP pp

All P All p

All Pp

1/2 P 1/2 p

EggsP

p

P

PPp

Sperm

Pp Pp

pp

Gametes

Gametes

Phenotypic ratio3 purple : 1 white

Genotypic ratio1 PP : 2 Pp : 1 pp

Figure 9.3B


• Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes

9.4 Homologous chromosomes bear the two alleles for each characteristic

GENE LOCI

Figure 9.4

P a B

DOMINANTallele

RECESSIVEallele

P a b

GENOTYPE: PP aa Bb

HOMOZYGOUSfor thedominant allele

HOMOZYGOUSfor therecessive allele

HETEROZYGOUS


• The offspring of a testcross often reveal the genotype of an individual when it is unknown

9.6 Geneticists use the testcross to determine unknown genotypes

TESTCROSS:

B_GENOTYPES bb

BB Bbor

Two possibilities for the black dog:

GAMETES

OFFSPRING All black 1 black : 1 chocolate

B

b

B

b

b

Bb Bb bb

Figure 9.6


• The inheritance of many human traits follows Mendel’s principles and the rules of probability

9.8 Connection: Genetic traits in humans can be tracked through family pedigrees

Figure 9.8A


• Most such disorders are caused by autosomal recessive alleles

– Examples: cystic fibrosis, sickle-cell disease

9.9 Connection: Many inherited disorders in humans are controlled by a single gene

Figure 9.9A

D D

d d

NormalDd

NormalDd

DDNormal

DdNormal(carrier)

DdNormal(carrier)

ddDeaf

Eggs Sperm

PARENTS

OFFSPRING


• A few are caused by dominant alleles

Figure 9.9B

– Examples: achondroplasia, Huntington’s disease


• Karyotyping and biochemical tests of fetal cells and molecules can help people make reproductive decisions

– Fetal cells can be obtained through amniocentesis

9.10 Connection: Fetal testing can spot many inherited disorders early in pregnancy

Figure 9.10A

Amnioticfluid

Fetus(14-20weeks)

Placenta

Amnioticfluidwithdrawn

Centrifugation

Fetalcells

Fluid

Uterus Cervix Cell culture

Severalweeks later Karyotyping

Biochemicaltests


• In a population, multiple alleles often exist for a characteristic

– The three alleles for ABO blood type in humans is an example

9.13 Many genes have more than two alleles in the population


• When an offspring’s phenotype—such as flower color— is

in between the phenotypes of its parents, it exhibits incomplete dominance

9.12 Incomplete dominance results in intermediate phenotypes

P GENERATION

F1 GENERATION

F2 GENERATION

RedRR

Gametes R r

Whiterr

PinkRr

R r

R R

r r

1/21/2

1/2

1/21/2

1/2 SpermEggs

PinkRr

PinkrR

Whiterr

RedRR

Figure 9.12A


Figure 9.13

– The alleles for A and B blood types are codominant, and both are expressed in the phenotype

BloodGroup(Phenotype)

O

Genotypes

AntibodiesPresent inBlood

Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left

O A B AB

A

B

AB

ii

IA IA

orIA i

IB IB

orIB i

IA IB

Anti-AAnti-B

Anti-B

Anti-A


9.14 A single gene may affect many phenotypic characteristics

Individual homozygousfor sickle-cell allele

Sickle-cell (abnormal) hemoglobin

Abnormal hemoglobin crystallizes,causing red blood cells to become sickle-shaped

Sickle cells

Breakdown of red blood cells

Clumping of cells and clogging of

small blood vessels

Accumulation ofsickled cells in spleen

Physical weakness Anemia Heart

failurePain and

feverBrain

damageDamage to

other organsSpleen damage

Kidney failureRheumatism

Pneumonia and other infections

ParalysisImpaired mental

function

• A single gene may affect phenotype in many ways, a phenomenon called pleiotropy

– The allele for sickle-cell disease is an example

Figure 9.14


Figure 9.16

P GENERATION

F1 GENERATION

F2 GENERATION

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

Eggs Sperm

Fra

cti

on

of

po

pu

lati

on

Skin pigmentation


• The chromosomal basis of Mendel’s principles

Figure 9.17


Figure 9.21A

X Y

Male

(male)

Parents’diploidcells

(female)

Sperm

Offspring(diploid)

Egg


• Other systems of sex determination exist in other animals and plants

Figure 9.21B-D

– The X-O system

– The Z-W system

– Chromosome number


– Their inheritance pattern reflects the fact that males have one X chromosome and females have two

Figure 9.22B-D

– These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait

Female Male Female Male Female Male

XrYXRXR

XRXr

XRY

XR Xr

Y

XRXr

XR

Xr XRXR

XR

Y

XRY

XrXR XRY

XrY

XRXr

XR

Xr

Xr

YXRXr

XrXr XRY

XrY

XrY

R = red-eye alleler = white-eye allele


• Most sex-linked human disorders are due to recessive alleles

– Examples: hemophilia, red-green color blindness

– These are mostly seen in males

– A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected

9.23 Connection: Sex-linked disorders affect mostly males

Figure 9.23A

Genetics

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