Chapter 10

Copyright © 2011 Pearson Education Inc. Biology: Life on Earth, 9e

Chapter 10

Patterns of Inheritance


What Is the Physical Basis of Inheritance?

Inheritance is the process by which traits are passed to the offspring

Genes are sequences of nucleotides at specific locations on chromosomes – A gene is a unit of heredity that encodes information

needed to produce proteins, cells, and entire organisms – The location of a gene on a chromosome is called its

locus (plural, loci)


Homologous chromosomes are usually not identical– Mutations - Changes in nucleotide sequence in DNA of

one homologous chromosome– Mutations are the raw material for evolution– Mutations in gametes may become a part of the genetic

makeup of the species– Mutation gives rise to new alleles, alternative gene forms

that may produce differences in structure or function

DNA in Eukaryotic Chromosomes

gene 1 gene 2

same alleles different alleles


Both chromosomes carry the same alleleof the gene at this locus; the organism ishomozygous at this locus

This locus contains another gene forwhich the organism is homozygous

Each chromosome carries a different alleleof this gene, so the organism is heterozygous at this locus

a pair of homologous

chromosomes

gene loci

the chromosomefrom the male

parent

the chromosomefrom the femaleparent

The Relationship Among Genes, Alleles, and Chromosomes

Fig. 10-1


How Were the Principles of Inheritance Discovered?

Who was Gregor Mendel?

– Pea plants have qualities that make it a good organism for studying inheritance – Pea flower petals enclose both male and female

flower parts and prevent entry of pollen from another pea plant – self-fertilization

intact pea flower flower dissected to showits reproductive structures

Carpel (female,produces eggs)

Stamen (male, producespollen that contain sperm)


How Are Single Traits Inherited?

The language of a genetic cross– A genetic cross is the mating of pollen and eggs (from

same or different parents)– The parents used in a cross are part of the parental

generation (known as P)– The offspring of the P generation are members of the first

filial generation (F1)

– Offspring of the F1 generation are members of the F2 generation


true-breeding,purple-flowered

plant

true-breeding,white-flowered

plant

cross-fertilize

pollen

pollen

all purple-flowered plants

Parental generation (P)

First-generationoffspring (F1)

Cross of Pea Plants True-Breeding for White or Purple Flowers

Fig. 10-4


First- generationoffspring (F1)

3/4 purple 1/4 white

Second- generationoffspring (F2)

self-fertilize

Fig. 10-5

Self-Fertilization of F1 Pea Plants with Purple Flowers


homozygous parent

(a) Gametes produced by a homozygous parent

(b) Gametes produced by a heterozygous dominant parent

A A AA

gametes

heterozygous parent

A a aA

gametes

The Distribution of Alleles in Gametes

Fig. 10-6


(a) Gametes produced by homozygous parents

PP P P

purple parent

all P sperm and eggs

pp p

white parent

all p sperm and eggs

p

+

+

Fig. 10-7a

Segregation of Alleles and Fusion of Gametes


(b) Fusion of gametes produces F1 offspring

p

p

+

+

Pp

Pp

sperm eggs

P

P

or

F1 offspring

Fig. 10-7b



Fig. 10-7c


ppp +

Ppp +

p+

+

gametes from F1 Pp plants

eggs

F2 offspring

p

PP

P

P

PP

sperm

Pp

(c) Fusion of gametes from the F1 generation producesF2 offspring


How Are Single Traits Inherited?

The hypothesis explains Mendel’s results with peas– The particular combination of the two alleles carried by

an individual is called the genotype– The physical expression of the genotype is known as the

phenotype (for example, purple or white flowers)

Simple “genetic bookkeeping” can predict genotypes and phenotypes of offspring– The Punnett square method predicts offspring

genotypes and phenotypes from combinations of parental gametes


P peggs

Ppself-fertilize

12

12

P

p

12

12

P12 P1

2 PP14 PP

14

P12 p1

2 Pp14

purple34

p12 P1

2 pP14

Pp12

p12 p1

2 pp14 pp

14 white

14

sperm eggsoffspring

genotypesgenotypic

ratio(1:2:1)

phenotypicratio(3:1)

PP

pp

Pp14

14

14

14pP

(a) Punnett square of a single-trait cross (b) Using probabilities to determine the offspring of a single-trait cross

sp

erm

Determining the Outcome of a Single-Trait Cross

Fig. 10-8


Seedshape

Seedcolor

Podcolor

Podshape

Flowercolor

Flowerlocation at leaf

junctionsat tips ofbranches

tall(about6 feet)

dwarf(about 8 to16 inches)

Plantsize

smooth

Dominant formTrait Recessive form

wrinkled

yellow

yellow

inflated

green

green

white

constricted

purple

Fig. 10-10

Traits of Pea Plants Studied by Gregor Mendel


14

14

34

14

14

14

14

14

14

116

116

116

116

116

116

116

116

116

116

116

116

116

116

116

116

SY

SY

SSYY SsYY

ssYY

ssyY

SsyY

SSYy SsYy

SsYyself-fertilize

ssYy

ssyy

SsyySSyy

sSyY sSyy

sSYY sSYy

SSyY

sY

sY

sy

sy

Sy

Sy

eggs

seed shape seed color phenotypic ratio(9:3:3:1)

smooth34 yellow

916smooth yellow

316 smooth green

316 wrinkled yellow

1 16wrinkled green

34 yellow

14 green

14 green

34 smooth

14 wrinkled

14 wrinkled

(a) Punnett square of a two-trait cross (b) Using probabilities to determine the

offspring of a two-trait cross

sp

erm

Predicting Genotypes and Phenotypes for a Cross between Parents That Are Heterozygous for Two Traits Fig. 10-11


How Are Multiple Traits Inherited?

Mendel’s results supported his hypothesis that traits are inherited independently– The independent inheritance of two or more traits is

called the law of independent assortment– Multiple traits are inherited independently because the

alleles of one gene are distributed to gametes independently of the alleles for other genes

– Independent assortment will occur when the traits being studied are controlled by genes on different pairs of homologous chromosomes


Independent Assortment of Alleles

Fig. 10-12independent assortment produces four equallylikely allele combinations during meiosis

pairs of alleles on homologouschromosomes in diploid cells

chromosomes replicate

replicated homologouspair during metaphase of

meiosis I, orientinglike this

or like this

meiosis II

meiosis I

S

S

S

S

S

S

SS

S

SY sY

s

s

s

s s

s

s

s

sy Sy

s

ss

Y

Y

Y

Y

Y

S

Y

Y

Y

Y

Y Y

y

y

y

y

y

y

y

y y y

S

y


Do the Mendelian Rules of Inheritance Apply to All Traits?

Many traits do not follow simple Mendelian rules of inheritance– Not all traits are completely controlled by a single gene– Not all traits have only two possible alleles– A trait may not be completely dominant to another



C1 C2eggs

C1

father

C1C2

C1C2 C2C2

C1C1 C1C2

C1C2

mother

C2

sper

m

Incomplete dominance– When the heterozygous

phenotype is intermediate between the two homozygous phenotypes– Hair texture



A single gene may have multiple alleles– A species may have multiple alleles for a given

characteristic– However, each individual still carries two alleles for

this characteristic– The human blood types are an example (Codominance)

Blood Type(Phenotype)

Genotype(alleles)

Antibodies in Plasma

A AA or AO Anti-B

B BB or BO Anti-A

AB AB None

O OO Anti-A and Anti-B



Polygenic inheritance– Some characteristics show a range of continuous

phenotypes instead of discrete, defined phenotypes– Examples of this include human height, skin color, and

body build, and in wheat, grain color – Phenotypes produced by polygenic inheritance are

governed by the interaction of more than two genes at multiple loci


Polygenic Inheritance of Skin Color in Humans

Human skin color is controlled by at least three genes, each with pairs of incompletely dominant alleles

Fig. 10-14


Do the Mendelian Rules of Inheritance Apply to All Traits? The environment influences the expression of

genes– Newborn Siamese cats demonstrate the effect of

environment on phenotype– A Siamese cat has the genotype for dark fur all over

its body– However, the enzyme that produces the dark pigment

is inactive at temperatures above 93°F (34°C)


How Are Genes Located on the Same Chromosome Inherited?

Genes on the same chromosome tend to be inherited together– Mendel’s law of independent assortment works only for

genes whose loci are on different pairs of homologous chromosomes

– Characteristics whose genes tend to assort together are said to be linked

flower-color gene pollen-shape gene

purpleallele, P

longallele, L

redallele, p

roundallele, l


How Are Genes Located on the Same Chromosome Inherited? Crossing over creates new combinations of linked

alleles– Genes on the same chromosome do not always sort

together– Crossing over, or genetic recombination, in prophase I

of meiosis creates new gene combinationsflower-color gene

purple allele, P long allele, L

red allele, p round allele, l

pollen-shape gene

sisterchromatids

homologouschromosomes

(duplicated)at meiosis I

sisterchromatids

(a) Replicated chromosomes in prophase of meiosis I (b) Crossing over during prophase I

P

P

p

p

L

L

l

l


Fig. 10-18c, d

Crossing Over Recombines Alleles on Homologous Chromosomes

recombinedchromatids

P L

p L

P l

p l

unchangedchromatids

(c) Homologous chromosomes separate at anaphase I

P L

P

L

p l

p

l

recombinedchromosomes unchanged

chromosomes

(d) after meiosis II

Each being a different gamete with different alleles


X chromosome

Y chromosome

How Is Sex Determined?

Sex chromosomes that dictate gender– Females have two X chromosomes– Males have an X chromosome and a Y chromosome– A small section of the X and Y chromosomes is

homologous, allowing them to pair in prophase I and segregate during meiosis

– Autosomes - rest of the (non-sex) chromosomes – occur in identical pairs


eggs

female parent

female offspring

male offspring

male parent

X1

X1

X2

X2

Xm

Xm

Y

Y

Xm XmX1 X2

Y YX1 X2

sper

m

Sex Determination in Mammals

Fig. 10-20


How Are Sex-Linked Genes Inherited? Sex-linked genes are found only on the X or only

on the Y chromosome– Genes carried on one sex chromosome, but not on the

other, are sex-linked– In humans, the X chromosome is much larger than the

Y and carries over 1,000 genes– In contrast, the human Y chromosome is smaller and

carries only 78 genes– During embryonic life, the action of the Y-linked gene

SRY sets in motion the entire male developmental pathway– Under normal conditions, SRY causes the male

gender to be linked 100 percent to the Y chromosome


How Are Sex-Linked Genes Inherited?

Sex-linked genes are found only on the X or only on the Y chromosomes– Few of the genes on the X chromosome have a specific

role in female reproduction– Most of the genes on the X chromosome have no

counterpart on the Y chromosome– Some genes found only on the X chromosome are

important to both sexes, such as genes for color vision, blood clotting, and certain structural proteins in muscles


How Are Sex-Linked Genes Inherited?

Sex-linked genes are found only on the X or only on the Y chromosomes– Because females have two X chromosomes, recessive

sex-linked genes on an X chromosome may or may not be expressed

– Because males, with only one X chromosome, have no second copy to mask recessive genes, they fully express all the X-linked alleles they have, whether those alleles are dominant or recessive


Sex-Linked Inheritance of Color Blindness

eggs

female parent

female offspring

male offspring

male parent

XC

XC

Xc

Xc

XC

XC

Y

Y

XC XCXC Xc

Y YXC Xc

(a) Normal color vision (b) Red-green color blindness

(c) Expected children of a man with normal color vision (CY), anda heterozygous woman (Cc)

Can’t distinguish red from green

sper

m

Fig. 10-21


Family Pedigrees - show the genetic relationships among relatives

Fig. 10-22

(a) A pedigree for a dominant trait

(b) A pedigree for a recessive trait

How to read pedigrees

= generations

= male = female

= parents

= offspring

or = shows trait

or = does not show trait

or = known carrier (heterozygote) for recessive trait

or = cannot determine the genotype from this pedigree

? ? ? ?

? ? ? ? ?

How are Human Genetic Disorders Inherited?


How Are Human Disorders Caused by Single Genes Inherited? Some human genetic disorders are caused by

recessive alleles– New alleles produced by mutation usually code for

nonfunctional proteins– Alleles coding for nonfunctional proteins are recessive to

those coding for functional ones– The presence of one normal allele may generate

enough functional protein to enable heterozygotes to be phenotypically indistinguishable from homozygotes with two normal alleles


How Are Human Disorders Caused by Single Genes Inherited?

Some human genetic disorders are caused by recessive alleles– Heterozygous individuals are carriers

of a recessive genetic trait (but otherwise have a normal phenotype)

– Recessive genes are more likely to occur in a homozygous combination (expressing the defective phenotype) when related individuals have children

– Sickle-cell anemia– Mutation in hemoglobin structure– Malaria resistance in heterozygotes


Albinism

Fig. 10-22



Some human genetic disorders are caused by dominant alleles– A dominant disease can be transmitted to offspring if at

least one parent suffers from the disease and lives long enough to reproduce– Dominant disease alleles also arise as new mutations

in the DNA of eggs or sperm of unaffected parents


How Are Human Disorders Caused by Single Genes Inherited? Some human genetic disorders are caused by

dominant alleles– Huntington disease is a dominant disorder that causes

a slow, progressive deterioration of parts of the brain– The disease results in a loss of coordination, flailing

movements, personality disturbances, and eventual death

– The disease becomes manifest in adulthood, ensuring its maintenance in the population



Some human genetic disorders are sex-linked– The X chromosome contains many genes that have no

counterpart on the Y chromosome– Because males have only one X chromosome, they have

no other allele to exert dominance over a sex-linked (X-linked) allele causing disease– Consequently, sex-linked diseases tend to occur in

males


How Are Human Disorders Caused by Single Genes Inherited? Some human genetic disorders are sex-linked

– Sex-linked disorders caused by a recessive allele have a unique pattern of inheritance– A son receives his X chromosome from his mother

and passes it on only to his daughters, since the gene doesn’t exist on his Y chromosome

– Sex-linked genes typically skip generations because the affected male passes the trait to a phenotypically normal carrier daughter, who in turn bears affected sons

– Several defective alleles for characteristics encoded on the X chromosome are known, including red-green color deficiency, muscular dystrophy and hemophilia


Hemophilia Among the Royal Families of Europe

unaffected male

unaffected female

EdwardDuke of Kent

Victoria Princess of Saxe-Coburg

AlbertPrince

of Saxe-Coburg-Gotha

Victoria Queen

of England

Louis IVGrand Duke of

Hesse-Darmstadt

Alice Princessof Hesse

Victoria Mary

ElizabethAlexandra Tsarina

FrederickErnest Mary Victoria

Irene

Olga Tatiana Maria Anastasia Alexis Tsarevitch

Edward VIIKing of

England

Alexandra of Denmark

Leopold Duke

of Albany

Helen Princess of

Waldeck-Pyrmont

Henry Prince of Battenburg

Beatrice

present British royal family (unaffected)

Alexander Albert

AlfonsoXII

Victoria Queen of Spain

Leopold Maurice

Alfonso Crown Prince

Juan Beatrice diedin

infancy

Marie Jaime Gonzalo

carrier daughter

and hemophiliac grandson

several unaffected

chidren

carrier female

hemophiliac male

Nicholas IIof Russia

? ? ? ? ? ?

Fig. 10-25


How Do Errors in Chromosome Number Affect Humans?

The incorrect separation of chromosomes or chromatids in meiosis is known as nondisjunction– Nondisjunction causes gametes to have too many and

too few chromosomes– Most embryos that arise from fusion of gametes with

abnormal chromosome numbers spontaneously abort, but some survive to birth and beyond



Some genetic disorders are caused by abnormal numbers of sex chromosomes (continued)– Turner syndrome (XO) occurs in females with only one

X chromosome– At puberty, hormone deficiencies prevent XO females

from menstruating or developing secondary sexual characteristics

– Hormone treatment promotes physical development, but because affected women lack mature eggs, they remain infertile

– More susceptible to recessive disorders such as red-green color blindness and hemophilia



Some genetic disorders are caused by abnormal numbers of sex chromosomes– Trisomy X (XXX) results in a fertile “normal” woman with

an extra X chromosome– Most affected women show no abnormal symptoms– There is an increased chance of learning disabilities

and a tendency toward tallness– By some unknown mechanism that prevents an extra

X chromosome from being included in their eggs, women with trisomy X bear normal XX and XY children



Some genetic disorders are caused by abnormal numbers of sex chromosomes– Men with Klinefelter syndrome (XXY) have an extra X

chromosome– Most afflicted males show no symptoms, although

some may show mixed secondary sexual characteristics, including partial breast development, broadening of the hips, and small testes

– XXY men are often infertile because of low sperm count but are not impotent



Some genetic disorders are caused by abnormal numbers of sex chromosomes– Males with Jacob syndrome (XYY) have an extra Y

chromosome– Have high levels of testosterone, tend to develop severe

acne, and may be exceptionally tall, more susceptible to learning disabilities



Some genetic disorders are caused by abnormal numbers of autosomes– Nondisjunction of autosomes can occur during meiosis in

the father or mother, resulting in eggs or sperm that are missing an autosome or that have two copies of an autosome– Embryos with one or three copies of an autosome

(trisomy) usually spontaneously abort; however, a small fraction of embryos with three copies of chromosomes 13, 18, or 21 survive to birth

– The frequency of nondisjunction increases with the age of the parents


How Do Errors in Chromosome Number Affect Humans? Some genetic disorders are caused by abnormal

numbers of autosomes – In trisomy 21 (Down syndrome), afflicted individuals

have three copies of chromosome 21

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