Inheritance Patterns &
Human Genetics
Chapter 12
Chromosomes &
Inheritance
Section 12.1
What makes human males different
than females?

Sex chromosomes (X and Y)



Male: XY
Female: XX
Gametes:


Egg: carry only X
Sperm: carry either X or Y
Who Discovered Sex
Chromosomes?

Thomas Morgan



Early 1900s
Columbia University (USA)
Worked with fruit flies
Sex Linkage:



When genes are found on the sex
chromosomes
X-linked Genes: genes on the X
chromosome
Y-linked Genes: genes on the Y
chromosome
Sex Linked Traits


Most sex linked genes
are found on the X
chromosome
Only genes on the Y
chromosome are for
male reproductive
organ development
Sex Linked Genetic Problems

In flies: R = red eyes, r = white eyes

Gene located on the X chromosome
X
X
Y
X
Example 1:

White eye male mates with a red
homozygous dominant female
 XrY
x
Xr
Y
XR
XRXr
XRY
XR
XRXr
XRY
XRXR
100 % red female
0 % white female
100 % red male
0 % white male
Example 2:

Red eye male mates with a red
heterozygous female
 XRY
x
XR
Y
XR
XRXR
XRY
Xr
XRXr
XrY
XRXr
100 % red female
0 % white female
50 % red male
50 % white male
Example 3:

White eye male mates with a red
heterozygous female
 XrY
x
Xr
Y
XR
XRXr
XRY
Xr
XrXr
XrY
XRXr
50 % red female
50 % white female
50 % red male
50 % white male
Linkage Groups


Genes located on the same chromosome
and therefore inherited together
Goes against Mendel’s Law of
Independent Assortment
How do linked genes get
“unlinked”?


Crossing Over
The frequency of
crossing over
between certain
genes is used to
make a chromosome
map

Which two genes have the highest probability
of crossing over? The lowest?
a
A
b
B
Highest: A & C
Lowest: A & B
c
C
Chromosome Map:
Diagram of the linear order of genes on a chromosome
Sex Linkage Problems!!!!
Use these genotypic symbols for the sex
linked trait of red-green color blindness in
humans to solve the problems that follow.





"Normal" female = XBXB
Carrier female = XBXb
Color-blind female = XbXb
Normal male = XBY
Color-blind Male = XbY
1) A normal female marries a color blind
male. What are the chances that the
offspring will be color blind if they are
females? What are the chances that the
offspring will be color blind if they are
males?
2) A color blind female marries a normal
male. How many of the female offspring
will be carriers of the color blind allele?
3) A man whose mother is color blind
marries a woman with normal vision. What
is the genotype of the husband? What
percent of their offspring can be expected
to be color blind? What percentage of
their offspring can be expected to be
carriers?
How do biologist keep track of inherited
traits over generations in a family?

Pedigree (page 241)
Pedigree Key
Normal male
Marriage
Affected male
Unmarried
Normal female
Affected female
Dead
Let’s try some pedigree
problems!
R = Tongue Roller
r = No Tongue Roller
John Jones, a tongue roller, marries Jill Smith, a
woman that cannot roll her tongue. John and
Jill have four children that can each roll their
tongue: John Jr., Alice, Lisa, and Sean. John Jr.
later marries non-tongue roller Pamela, and
they have four children: Jessica, Sherri, Mary,
and John III. Sherri and Mary both can roll
their tongues, and Jessica and John III are
non-tongue rollers. Sean marries Robin, a nontongue roller. Both Robin’s parents are nontongue rollers also. Sean and Robin have four
children: Nicholas, Harry, Donna, and Sean Jr.
Nicholas, Harry and Donna each have the
ability to roll their tongues. Sean Jr. cannot.
Human Genetics
Section 12.2
Human genetics is not as
easy as Mendel’s peas!
Many
patterns of inheritance
Human Patterns of Inheritance
1.
2.
3.
4.
5.
Single allele trait
Multiple allele trait
Polygenic trait
X-linked trait
Nondisjunction
1. Single Allele Trait

A trait that is controlled by a single allele
of a gene

Normal dominant-recessive (Mendel)

Example Genetic Disorders:


Huntington’s Disease (autosomal dominant)
Cystic Fibrosis (autosomal recessive)
2. Multiple Allele Trait


3 or more alleles of the same gene code
for a single trait
Example: ABO Blood Type
IA = type A (dominant)
IB = type B (dominant)
i = type O (recessive)
Blood Type Problems




If a person is type A blood….what is his/her
genotype?
IAIA or IAi
If a person is type B blood….what is his/her
genotype?
IBIB or IBi
If a person is type O blood….what is his/her
genotype?
ii
If a person is type AB blood….what is his/her
genotype?
IA IB
Blood Type Problems

A mother gives birth to a type O child.
The mother is type A blood. The two
potential fathers are type A (father 1) and
type AB (father 2).
Who’s the daddy?
3. Polygenic Trait

Trait that is controlled by 2 or more genes

Range of phenotypes


Influenced by environmental factors too
Examples:
skin color
eye color
human height
4. X-Linked Trait


Trait controlled by a gene on the X
chromosome
Examples:
colorblindness (recessive)
hemophilia (recessive)
Hemophilia Pedigree
5. Nondisjunction

The failure of chromosomes to separate
during meiosis resulting in one gamete
with too many chromosomes and one
gamete with too few chromosomes
Trisomy
Monosomy



Trisomy: cell with 3 copies of a
chromosome (too many chromosomes)
Monosomy: cell with 1 copy of a
chromosome (too few chromosome)
Example Genetic Disorders:
Down Syndrome (Tri-21)
Klinefelter’s Syndrome (XXY)
Turner’s Syndrome (X__)
Mutations that Lead to Genetic
Disorders:



Mutation: a change in the DNA of an
organism
Can involve an entire chromosome or a
single nucleotide
Can lead to genetic disorders
Mutation Types
1.
Germ-cell mutation: occurs in the germ
cells (gametes)


2.
Somatic-cell mutation: occurs in the
organism’s body cells


3.
Does not affect the organism
Does affect the organism’s offspring
Does affect the organism
Does not affect the organism’s offspring
Lethal mutation: causes death, often
before birth
4.
Chromosome mutation: change in the
structure of a chromosome
a. Deletion
b. Inversion
c. Translocation
d. Nondisjunction
5. Gene mutation: involves large segments
of DNA or a single nucleotide of DNA
a. Point mutation: single
nucleotide mutation
within a codon
b. Frame shift mutation:
cause the misreading of
codons during
translation thus making
the wrong protein
Detecting Human Genetic
Disorders

Before Pregnancy:
1.
2.

During Pregnancy:
1.
2.

Genetic Screening
Genetic Counseling
Amniocentesis
Chorionic Villi Sampling
After Birth:
1.
Genetic Screening