Chromosomal Basis of Inheritance - Welcome to Biology!cherylchowbiology.weebly.com/uploads/1/7/1/7/17179396/... · 2013-05-15 · 24.1 Sex-Linked Inheritance •Normally, ... Copyright

1

Chapter 24 Chromosomal Basis

of Inheritance

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24.1 Sex-Linked Inheritance

• Normally, both males and females have 23

pairs of chromosomes

– 22 pairs are called autosomes

– One pair is the sex chromosomes

• Males are XY

• Females are XX


• Sex-linked traits are controlled by genes

on the sex chromosomes

– X-linked are found on the X chromosome

• Most sex-linked traits

• No matching gene on the Y

– Y- linked are found on the Y chromosome


• Most X-linked traits are recessive

– Female must receive two alleles

• One from each parent

– Male inherits X from his mother

• Y from his father does not carry an allele for the

trait


• Sex-linked Alleles

– Example for red-green colorblindness

• Well known X-linked recessive disorder

• Carrier – female capable of passing recessive allele

• Color-blind males are more common than color-blind females

Genotypes Phenotypes

XBXB Female who has normal color vision

XBXb Carrier female who has normal color vision

XbXb Female who is color blind

XBY Male who has normal vision

XbY Male who is color blind


All

1 1

Phenotypic Ratio

Normal vision

Color blind

Key

oocytes

sp

erm

Parents

×

Y

Offspring

Females

Males

XB= Normal vision

Xb= Color blind

XBY XBXb

XB Xb

XB XBXB XBXb

XBY XbY

♀

♀


• Pedigree for X-linked Disorders

– Most sex-linked disorders are carried on the X

chromosome

– X-linked recessive disorder

• More males than females will have the disorder

– Recessive allele on X always expressed in males

– X-linked dominant disorder

• Only a few known

• Affected males pass the trait only to daughters

• Females can pass the trait to both sons and

daughters


XbY

K e y

grandfather

daughter

XBXB

XBXb XBY

XBY XBXB XBXb XbY

XbY

XBY XbXb

grandson

XBXB = Unaffected female

XBXb = Carrier female

XbXb = Color-blind female

XBY = Unaffected male

XbY = Color-blind male X-linked Recessive Disorders

• 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.


• X-linked Recessive Disorders of Interest

– Red-green colorblindness

– Duchenne muscular dystrophy

• Characterized by a wasting away of the muscles

• Absence of protein dystrophin

– Fragile X syndrome

• Most common cause of inherited mental impairment

• Most common known cause of autism

– Hemophilia

• Absence or minimal presence of a clotting factor

fibrous

tissue

abnormal muscle normal tissue

Abnormal muscle in muscular dystrophy


(left, right): Courtesy Dr. Rabi Tawil, Director, Neuromuscular Pathology Laboratory, University of Rochester Medical Center;

(center): Courtesy Muscular Dystrophy;

Hemophiliac

Alice

Alice

12 children of 26

are shown

Key

Unaffected male

Unaffected female

Carrier Queen Victoria Prince Albert

4 children of 9

are shown

Prince Henry of

Battenberg

Beatrice Victoria Frederick III

(Germany)

Louis IV

(Hesse)

Princess

Helena of

Waldeck

Leopold

(died at 31)

Leopold

(died at 32)

Victoria Henry Irene Frederick

(died at 3)

Alexandra Nicholas II

(Russia)

Alfonso XII

(Spain)

Alexander

(Earl of

Athlone)

Waldemar

(died at 56)

Henry

(died at 4)

Alexei

(murdered)

Rupert

(died at 21)

Gonzalo

(died at 20)

Alfonso

(died at 31)

6 children of 34

are shown

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


24.2 Gene Linkage

• Each chromosome contains many aleles

in a definite fixed order

• Linkage group – all the alleles on one

chromosome that are inherited together

– Sex-linkage refers to sex chromosomes

• Two trait crosses assume the alleles are

on nonhomologous chromosomes

• Alleles that are linked do not show

independent assortment


A a

B

A

B

a

b

b

50% 50%

no crossing- over

during meiosis

2 types of gametes in

equal proportions a. Complete linkage


97% 3%

A

A

B

a

b

a

B

A

b

B b

a

crossing-over

during meiosis

no crossing- over

during meiosis

recombinant gametes

4 types of gametes in unequal proportions

b. Incomplete linkage


97% 3%

A A a

B

A

B

A

B

a

b

a

b

a

B

A

b

b B b

a

50% 50%

no crossing- over

during meiosis

crossing-over

during meiosis

no crossing- over

during meiosis

recombinant gametes

4 types of gametes in unequal proportions

b. Incomplete linkage a. Complete linkage

2 types of gametes in

equal proportions


Z S R G

z s r g

pair of homologous chromosomes

24.2 Gene Linkage • During meiosis, crossing-over sometimes occurs

between nonsister chromatids in a tetrad

– Chromatids exchange genetic material

• If crossing-over occurs, dihybrid produces four

types of gametes instead of two

• Occurrence of crossing-over can help tell the

sequence of genes on a chromosome

– Crossing-over occurs more often between distant

genes than between closer genes

24.3 Changes in Chromosome Number

• Nondisjunction

– Occurs during meiosis I, when both members

of a homologous pair go into the same

daughter cell

– Or during meiosis II, when the sister

chromatids fail to separate and both daughter

chromosomes go into the same gamete

– Results in trisomy or monosomy when

fertilized


Meiosis I

Meiosis II nondisjunction

2n 2n

pair of

homologous

chromosomes

normal

normal

2n + 1 2n - 1

Zygote

Fertilization


Meiosis I

Meiosis II

nondisjunction

pair of

homologous

chromosomes

Zygote

2n + 1 2n + 1 2n - 1 2n - 1

Fertilization


Meiosis I

Meiosis II

nondisjunction

nondisjunction

2n

a.

2n

pair of

homologous

chromosomes

pair of

homologous

chromosomes

normal

normal

2n + 1 2n - 1

Zygote

b. 2n + 1 2n + 1 2n - 1 2n - 1

Fertilization


• Normal development depends on exactly

two of each kind of chromosome

– Trisomies are tolerated better than

monosomies

• Only trisomy 21 (Down syndrome) has a

reasonable chance of survival after birth

• Chromosome 21 is one of the smallest

chromosomes

– Chances of survival are greater when trisomy

or monosomy involves the sex chromosomes


• Abnormal numbers of sex chromosomes

– Normal XX females have one X inactive

• Barr body

• Cells of females function with 1 X just like males

• Zygote with one X chromosome (Turner syndrome) can

survive

– All extra X chromosomes become deactivated

• Poly-X females and XXY males are seen fairly frequently

– Extra Y chromosomes are also tolerated

• XYY (Jacobs syndrome) is due to nondisjunction during

meiosis II of spermatogenesis

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• Down Syndrome – trisomy 21

– Most common autosomal trisomy seen among

humans

– Easily recognized physical features

– Mild to severe mental impairment

– Chances of a woman having a Down syndrome child

increase rapidly with age, starting at about age 40

– Karyotyping can be used with amniocentesis or

chorionic villus sampling to diagnose a fetus

– Symptoms may be due to expression of Gart gene


© Jose Carrilo/PhotoEdit

21

extra chromosome 21

Gart

gene


© CNRI/SPL/Photo Researchers, Inc.

21

a. b .


a: © Jose Carrilo/PhotoEdit; b: © CNRI/SPL/Photo Researchers, Inc.

extrachr omosome 21

Gart

gene

• Changes in sex chromosome number

– Presence of Y chromosome, not the number of X,

determines maleness

• SRY gene produces testis-determining factor


24.3 Changes in Chromosome

Structure

• Chromosomal mutations occur when

chromosomes break

– Environmental agents or viruses can cause

break

– Ordinarily, break reunites with same

sequence of genes

– Failure to reunite correctly can result in:

• Deletion, duplication, translocation, or inversion


Structure

• Deletion

– Occurs when a single break causes a

chromosome to lose an end piece or when

two simultaneous breaks lead to the loss of

an internal chromosomal segment

– Inheriting one normal chromosome and one

with a deletion can result in a syndrome due

to not having a pair of alleles

– William’s syndrome – loss of small piece of 7

– Cri du chat – 5 missing an end piece


deletion lost

a.

+

a

b

c

d

e

f

g

h

a

b

c

d

e

f

g

h

b. Courtesy The Williams Syndrome Association

Deletion

a

b

c

d

e

d

e

f

a

b

c

d

e

f

g

g

duplication

inversion


Courtesy Kathy Wise

a b Duplication

• Duplication

– Chromosomal segment

is repeated in the same

chromosome or in a

nonhomologous

chromosome

– Individual has more than

two alleles for certain

traits

– Inv dup 15 syndrome

• Inverted duplication on

chromosome 15


Structure


Structure

• Translocation

– Exchange of chromosomal segments between two

nonhomologous chromosomes

– A person who has both of the involved chromosomes

has the normal amount of genetic material and is

healthy

• Unless the chromosome exchange breaks an allele into two

pieces

– The person who inherits only one of the translocated

chromosomes will have only one copy of certain

alleles and three copies of certain other alleles

• One type of Down syndrome is caused by a

translocation between chromosomes 21 and 14

• Alagille syndrome

– Translocation between chromosomes 2 and 20

– Father did not realize he had the syndrome until he

had a child with the syndrome


a

b

c

d

e

f

a

b

c

d

e

f

g

h

m

l

n

o

p

m

l

n

o

p

q

r

g

h

q

r

translocation

Translocation


A

A

A

B

B

B

C

C C

D

D

D

E

E

E

F

F

F

G

G

G

a

a

a

b

b

b

e

e

e

d

d

d

c

c c

f

f

f

g

g

g

inverted

segment

region of

crossing-over

duplication and

deletion in both

homologous

chromosomes Inversion

• Inversion

– Segment of a chromosome

is turned 180 degrees

– Reverse sequence of

alleles can lead to altered

gene activity

– Crossing-over between an

inverted chromosome and

the noninverted homologue

can lead to recombinant

chromosomes that have

both duplicated and deleted

segments


Structure

A

A

A

B

B

B

C

C C

D

D

D

E

E

E

F

F

F

G

G

G

a

a

a

b

b

b

e

e

e

d

d

d

c

c

c

f

f

f

g

g

g

inverted

segment

region of

crossing-over

duplication and

deletion in both

homologous

chromosomes

deletion lost

+

a

b

c

d

e

f

g

h

a

b

c

d

e

f

g

h

a Deletion

a

b

c

d

e

d

e

f

a

b

c

d

e

f g

g

duplication

inversion


a Duplication

a

b

c

d

e

f

a

b

c

d

e

f

g

h

m

l

n

o

p

m

l

n

o

p

q

r

g

h

q

r

translocation

Translocation Inversion

Documents

Chromosomal Basis of Inheritance - Welcome to Biology!cherylchowbiology.weebly.com/uploads/1/7/1/7/17179396/... · 2013-05-15 · 24.1 Sex-Linked Inheritance •Normally, ... Copyright