HUMAN BIOLOGY

394 Chapter 20

inheritance of Genes on the X Chromosome

F i g u r e 20.10 Here is one pattern for X-linked inheritance.

It shows the outcomes possible when the mother carries

a recessive allele on one of her X chromosomes (red).

(© Cengage Learning)

Duke of

Saxe-Coburg-Gotha

Edward

Duke of Kent

(1767–1820)

George III

Victoria

Albert (1819–1901)

VictoriaEmpress Edward VI

Frederick

Kaiser

Wilhelm

II

George V

George

Duke ofWindsor VI

Elizabeth

II

A

Prince

Philip

Margaret

Earl

Mountbatten

of Burma

Prince

Sigismund

of Prussia

Henry

Frederick

Waldemar William

Irene

Princess

Henry

Alice of Hesse

Anastasia

Alexandra

(Czarina

Nicholas II)

Helena

Princess

Christian

Leopold

Duke

of Albany

Beatrice

Leopold

Maurice

Viscount

Trematon

Alexei

Lady

May

Abel

Smith

Rupert Alfonso

Alice of

Athlone

Victoria

Eugénie,

wife of

Alfonso XII

Louis II

Grand Duke of Hesse Carrier female

Hemophilic male

Status uncertain

Three females

??

3

?

???

?

????

3????

32

I

II

III

IV

V

VI

VII

VIII

F i g u r e 20.11 Hemophilia is an X-linked disorder. A Partial pedigree of Queen Victoria’s descendants, showing carriers and affected
males who carried the X-linked allele for hemophilia A. This pedigree does not include many members of later generations. B (facing page)
Photograph of the Russian royal family in the early 1900s. (© Cengage Learning)

20.6

n Genes on the X chromosome also are inherited

according to predictable patterns.

n Links to Muscle contraction 6.3, blood clotting

8.7, eye disorders 14.10

some disorders are recessive

X-linked traits

When a recessive allele on an X chromosome

causes a trait, two clues often point to this source.

First, many more males than females are affected.

This is because a recessive allele can be masked

in females, who may inherit a dominant allele on

their other X chromosome. It cannot be masked

in males, who have only one X chromosome (Fig-

ure 20.10). Second, only a daughter can inherit the

recessive allele from an affected father, because

his sons will receive a copy of his Y chromosome,

not the X.

Two forms of the bleeding disorder

hemophilia are X-linked recessive traits. The

most common one, hemophilia A, is caused by

a mutation in the gene for factor VIII, a blood-clotting

protein ( Section 8.7). A male with a recessive allele on his

X chromosome is always affected, and he risks death from

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).