CHAPTER 12
INHERITANCE PATTERNS AND
HUMAN GENETICS
SEX DETERMINATION
 XX = FEMALE
 XY = MALE
 MALE GAMETE DETERMINES SEX AT
FERTILIZATION
X
Y
X
XX XY
X
XX
XY
SEX LINKAGE
 Thomas Hunt Morgan worked with
fruit flies & confirmed that genes
were on chromosomes
a. Fruit flies are cheaply raised in
common laboratory glassware
b. Females only mate once and lay
hundreds of eggs
c. Fruit fly generation time is short,
allowing rapid experiments
 Experiments involved fruit flies with
XY system similar to human system
 Besides genes that determine sex, sex
chromosomes carry many genes for
traits unrelated to sex
 X-linked gene is any gene located on
the X chromosome that is not on the Y
chromosome
 X-linked alleles are designated as
superscripts to X chromosome
Xr
Y
XR
XRXr XRY
XR
XRXr
XRY
 Heterozygous females are carriers
that do not show the trait but can pass
it on
 Males are never carriers but express
the one allele on the X chromosomeso they have the condition
Examples of x-linked
conditions:
 Red-green color-blindness is X-linked
recessive
 In humans, another well-known X-linked
traits is hemophilia (free bleeders that lack
clotting factors in their blood)
 One of the most famous genetic cases involving
hemophilia goes back to Queen Victoria who
was a carrier for the disorder and married Prince
Albert who was normal
 Their children married other royalty, and spread
the gene throughout the royal families of Europe
PEDIGREES
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Also called a family tree
Squares represent males and circles
represent females
Horizontal lines connecting a male and
female represent mating
Vertical lines extending downward from a
couple represent their children
A shaded symbol means the individual
possess the trait
Half-shaded symbols are carriers
Example Sex-Linked
Problems:
 1. What are the results of crossing a
colorblind male with a female carrier
for colorblindness?
 Trait: Red-Green
ColorblindnessAlleles:
 XC normal vision
 Xc colorblindness
 XCXc
x Xc Y
 Genotypes: XCXC ,XCY, XCXc, XcY,
Genotypic Ratio: 1:1:1:1
Phenotypes:
normal vision female, normal vision
male, female carrier, colorblind male
Xc
Y
XC
XCXc XCY
Xc
XcXc
XcY
Colorblind male
 What are the results of crossing a
colorblind male with a colorblind
female?
 XC normal vision
 Xc colorblindness
 XcXc
x
Xc Y
 Genotypes:
XcXc , XcY
 Genotypic Ratio: 1:1 ratio
 Phenotypes:
colorblind female,
colorblind male
 Phenotypic ratio: 1:1 ratio
Xc
Y
Xc
XcXc XcY
Xc
XcXc
XcY
SINGLE-ALLELE TRAITS
 Controlled by a single allele of a gene
 Example:
 Huntington’s disease (HD)- dominant allele on an
autosome
 Look at table 12-1
POLYGENIC TRAITS
 A trait controlled by 2 or more genes –
lots of variation
 Example: skin, hair, eye color
SEX INFLUENCED
TRAITS
 Presence of male or female sex
hormones influence the expression of
these
 Male and females have different phenotypes
even if same genotype
 Gene expression occurs because of
presence of certain hormones
 Ex: patterned baldness and testosterone
SINGLE-ALLELE TRAITS
 Individual has to have 2 copies of the
gene- homozygous recessive
 Examples:
 Cystic fibrosis
 Sickle cell anemia
MULTIPLE ALLELE TRAITS
 Controlled by 3 or more alleles of the same
gene that code for a single trait-ABO blood
type. A and B are codominant to each other
and dominant to i
 IAIA = A
 IAi
= A
 IBIB = B
 IBi = B
 IAIB = AB
 ii = O
LINKED GENES
 Each chromosomes has 1000's of
genes
 All genes on a chromosome form a
linkage group that stays together
except during crossing-over
 Some genes located on the same
chromosome tend to be inherited
together
 Linked genes were discovered by
Thomas Hunt Morgan while studying
fruit flies
 Linked alleles do not obey Mendel's
laws because they tend to go into the
gametes together
 Crosses involving linked genes do not
give same results as unlinked genes
Chromosome Mapping:
 Recombinants result from chromosome
crossing over during prophase I of meiosis
 Geneticists can use recombination data to
map a chromosome's genetic loci (position
on a chromosome)
 A genetic map lists a sequence of genetic
loci along a particular chromosome
 Alfred Sturtevant, a student of Morgan,
reasoned that different recombination
frequencies reflect different distances
between genes on a chromosome
 The farther apart genes are, the greater
likelihood of crossing-over
 The closer together two genes are, the less
likely of crossing-over occurring
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A map unit equals 1% recombination
frequency
If 1% of crossing-over equals one map
unit, then 6% recombinants reveal 6 map
units between genes
To determine the frequency of
recombinants, the following formula is
used:
Recombination
Frequency=
Number of recombinants
----------------------------------Total Number of Offspring
x 100%
Chromosome Mutations:
 Mutations are changes in genes or
chromosomes that can be passed on to
offspring
 Mutations increase the number of variations
that occur
 Chromosomal mutations include changes
in chromosome number and/or structure
 Monosomy occurs when an individual has only
one of a particular type of chromosome
 Turner syndrome (X0) is an example of monosomy
 Trisomy occurs when and individual has
three of a particular type of chromosome
 Examples of trisomy include Klinefelter's
Syndrome (XXY) and Down Syndrome or Trisomy
21 where the individual has three 21st
chromosomes
 Both monosomy & trisomy result when
chromosomes fail to separate during
meiosis; called nondisjunction
 Monosomy and trisomy (aneuploidy) occur in
plants and animals and may be lethal
(deadly)
 Polyploidy where the offspring have more
than two sets of chromosomes occurs often
in plants (3n, 4n )
 Environmental factors including
radiation, chemicals, and viruses, can
cause chromosomes to break causing
a change in chromosomal structure
 Inversion occurs when a piece of a
chromosome breaks off & reattaches to
the same place but in the reverse order
 Translocation occurs when a
chromosome segment breaks off &
attaches to a different chromosome
 Deletions occur when the end of a
chromosome breaks off & is lost
 Cri du chat syndrome (results in retardation
& a cat-like cry) is due to a deletion of a
portion of chromosome 5
 Duplications occur when a section of a
chromosome is doubled
 Fragile X Syndrome caused by an abnormal
number of repeats (CCG) results in
retardation & long, narrow face which
becomes more pronounced with age
Child with Fragile X
syndrome
Gene Mutations
 Change in genes caused by change in
structure of the DNA
 DNA bases may be substituted,
added, or removed to cause gene
mutation
 When genes are added or removed,
the mutation is called a frame shift
mutation
 Adding or Removing genes is called a
point mutation
 Sickle cell anemia (red blood cells are
C-shaped so can't carry as much
oxygen) is an example of a gene
mutation in African Americans


Tay-Sachs (a disorder where the
nervous system deteriorates) is a
fatal gene mutation in Jewish people
of Central European Descent
Phenylketonuria or PKU occurs from
the inability of a gene to synthesize a
single enzyme necessary for the
normal metabolism of phenylalanine
and results in death