Chapter 11: Human Heredity
Section 3: Sex-Linked Inheritance
Sex-Linked Inheritance

Genes that are located on the _______________________________________ of
an organism are inherited in a sex-linked pattern

As in many organisms, the sex in humans is determined by the
__________________________________________________

In females, meiosis produces _________________________ that contain one
_______________________________ and ______________________________

In males, meiosis produces ___________________________
of which
____________________ contain one X chromosome and 22 autosomes

The sex of a person is determined by whether an egg cell is fertilized by an Xcarrying sperm or a Y-carrying sperm
The Human XY System

Although meiosis is a precise mechanism that separates the two sex
chromosomes of a diploid cell into single chromosomes of haploid gamete cells,
errors sometimes do take place

The most common of these errors is ____________________________________

Nondisjunction is the failure of chromosomes to separate properly during one of
the stages of meiosis
Nondisjunction Disorders

Roughly __________________________________ is affected by an abnormality
involving nondisjunction of the sex chromosomes
o ____________________________________________

Female in appearance but their female sex organs do not develop
at puberty and they are sterile

___________ or __________________
o ____________________________________________


Male in appearance, and they, too, are sterile

____________________
What can we learn from these abnormalities of the sex chromosome?
o An X chromosome is absolutely essential for survival
o Sex seems to be determined by the _______________________________
__________________________ of a Y chromosome and not by the
number of X chromosomes
o The
Y
chromosome
contains
a
gene
that
_______________________________ the male pattern of growth during
embryological development

If this gene is absent, the embryo follows a female pattern of
growth
Sex-Linked Genetic Disorders

Genes that are carried on either the X or the Y chromosome are said to be
_____________________________________

In humans, the small Y chromosome carries very few genes

The much larger X chromosome contains a number of genes that are vital to
proper ___________________________________________________

Recall that males have one X chromosome

Thus all X-linked alleles are expressed in males, even if they are recessive

In order for a recessive allele to be expressed in females, there must be two
copies of it
Colorblindness

Colorblindness is a recessive disorder in which a person cannot distinguish
between certain colors

Most types of colorblindness are caused by sex-linked genes located on the X
chromosome

The alleles for colorblindness render people unable to make some of the
pigments in the eye necessary for color vision

Most common is ___________________________________________________

In humans, color vision depends on the varying sensitivity of three groups of
specialized ________________________________ in the retina of the eye

One group is sensitive to _________________ light, one to _____________
light, and one to _____________________ light

Colors of any given shade excite a specific level of activity from each of the
three groups of nerve cells

Because the gene for color vision is carried on the X chromosome, the dominant
allele for normal color vision is represented as ____________ and the recessive
allele for red-green color blindness is represented as ____________

Homozygous (ZCZC) and heterozygous (XCXc) females have _________________
color vision

A female who is heterozygous for colorblindness is said to be a
_______________________ because she carries the recessive allele but does not
express it

Although she is not colorblind, she is capable of passing on the allele for
colorblindness to her offspring

Only homozygous recessive females _____________________ are colorblind

Because males have only one X chromosome, they are either colorblind (XcY) or
have normal color vision (XCY)
Hemophilia

Another recessive allele on the X chromosome produces a disorder called
hemophilia, or ________________________________________

In hemophilia, the protein ______________________________________ (AHF)
necessary for normal blood clotting is __________________________

People with hemophilia can bleed to death from minor cuts and may suffer
internal bleeding from bumps or bruises

Hemophilia can be treated by injecting AHF isolated from donated blood
Muscular Dystrophy

Muscular dystrophy is an inherited disease that results with the progressive
wasting away of ______________________________________

Children with muscular dystrophy rarely live past early adulthood

The most common form of MD is caused by a defective version of the gene that
codes for a muscle protein known as ____________________________

This gene is located on the X chromosome

Researchers are now using molecular techniques to _______________________
________________________________________ of the dystrophin gene into
muscle cells
Sex-Influenced Traits

Many traits that may seem to be sex-linked, such as male pattern baldness, are
actually caused by genes located on ____________________________, not on
sex chromosomes

Why then is baldness so much more common in men than it is in women?

Male pattern baldness is a ____________________________________________

A sex-influenced trait is a trait that is caused by a gene whose
__________________________________ differs in males and females