Chapter 12
HUMAN GENETICS
REVIEW
Humans have 46 chromosomes
 44 are autosomes

 22

pairs of homologous chromosomes
2 are sex chromosomes: X and Y
 Females
have two X chromosomes
 Males have one X and one Y
KARYOTYPES: MALE AND FEMALE
SEX-LINKED TRAITS
Many genes not related to sex are on the X
chromosome
 They are called sex-linked traits
 Example: Drosophila (Fruit fly) eye color

DROSOPHILA EYE COLOR
R
Red eyes ( X ) is dominant
r
 White eyes ( X ) is recessive

XRXR
XrY
XR
XRXr
Xr
XRXr
XRY
XR
XR
Y
X RY
XRXR
Xr
XrXR
XrY
Xr
XR
Y
X RY
X rY
R = red-eye allele
r = white-eye allele
XRXr
XRXr
Xr
Xr Xr
Y
XRY
X rY
HUMAN SEX-LINKED DISORDERS

Most sex-linked human
disorders are due to
recessive alleles
 Examples:
hemophilia,
red-green color blindness
 These
are mostly seen in males
 Why???
Figure 9.23A
SEX – LINKED INHERITANCE



A male receives a single X-linked allele from his mother,
c
and will have the disorder (X Y)
A female has to receive the allele from both parents to be
c c
affected (X X )
A female with the gene for the trait on one of her X
c
chromosomes is called a carrier (X X).

A carrier may not know she carries the gene for the trait until
she has a son with the trait.
HEMOPHILIA
Hemophilia is a deadly disease in which the
individual’s blood does not clot properly.
 It is caused by a recessive gene on the X
chromosome. (Xh)
h
h h
 Women can be XX, XX (carrier), or X X
(affected).
h
 Men can be XY or X Y (affected).

PEDIGREES
A pedigree is a diagram that shows how a trait
is inherited over several generations.
 Circles represent women, squares are men
 Solid (dark) is affected (with disease, etc.)
 Empty (white) is not affected
 Half filled is a carrier (heterozygous)


A high incidence of hemophilia has plagued the
royal families of Europe
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Figure 9.23B
PROBLEMS

A female has the genotype XX and she marries a
c
man who is color blind (X Y). What is the
probability that they will have a child who is
color blind?

A female who is not color blind, but whose father
was color blind marries a color blind male. What
is the probability that they will have a child who
is colorblind?
HUMAN GENETIC DISORDERS

Sex-linked disorders


Autosomal disorders


Affected allele is usually on X chromosome
Can affect men and women equally
Chromosome number disorders

Chromosomes don’t separate evenly in meiosis
SEX-LINKED DISORDERS

Colorblindness – Cannot distinguish between red
& green

Hemophilia – Blood clotting disorder

Muscular Dystrophy – Muscles weaken and
eventually break down.
AUTOSOMAL DISORDERS

Albinism


Cystic Fibrosis


no hair or skin pigment
mucus clogs lungs
Tay-Sachs disease

breaks down nervous system
AUTOSOMAL DISORDERS

Sickle-cell anemia
 abnormal
protein causes RBCs to sickle and clump
together

PKU (Phenylketonuria)
 Body
can’t break down a certain amino acid so toxic
substance accumulates
 nerve damage, retardation

Huntington’s Disease
 Autosomal
Dominant
 Weakness and deterioration of brain
 No symptoms appear until middle age
 A person might not know he or she had the gene until
after having children.
CHROMOSOME NUMBER DISORDERS
 Turner’s

Syndrome
X0 (sterile females)
 Klinefelter’s

XXY (mentally retarded, boy, underdeveloped female
characteristics)
 Down’s

Syndrome
Syndrome
Trisomy 21