24.1 Sex-Linked Inheritance Chapter 24

1

Chapter 24Chromosomal 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

24.1 Sex-Linked Inheritance• 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

" #

"#


• X-linked Recessive Disorders of Interest

– 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

• Missing clotting factor now available as a biotechnology

product

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

BeatriceVictoria Frederick III

(Germany)

Louis IV

(Hesse)

Princess

Helena of

Waldeck

Leopold

(died at 31)

Leopold

(died at 32)

VictoriaHenry Irene Frederick

(died at 3)

Alexandra Nicholas II

(Russia)

Alfonso XII

(Spain)

Alexander

(Earl ofAthlone)

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 alleles

in a definite fixed order

• Linkage group – all the alleles on one

chromosome that are inherited together

• Alleles that are linked do not show

independent assortment


97% 3%

A Aa

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 linkagea. 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• 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 chromosome 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

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

• 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


© Jose Carrilo/PhotoEdit


• Chances of survival are greater when trisomy or

monosomy involves the sex chromosomes

– Normal XX females have one X inactive

• 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)

• Changes in sex chromosome number

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

determines maleness

• SRY gene (on Y chromosome) produces testis-

determining factor

24.3 Changes in Chromosome Number24.3 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

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

fg

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

24.1 Sex-Linked Inheritance Chapter 24

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