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Human Inheritance
Chapter 9
Overview
Human inheritance patterns:
– Autosomal
– Sex-linked
Sex determination systems
Human Chromosomes
Humans: male & female, 2n
23 pairs of homologous
chromosomes in cells
Each pair is structurally identical
except sex chromosomes
(Female XX, male XY)
Autosomes are same in both
sexes
Human X & Y chromosomes differ in
appearance & genes
Have small region that allows to act like
homologues during meiosis
Inheritance of sex chromosomes in certain
combos determines gender
Karyotyping
Individual’s metaphase chromosomes
organized by length, shape, centromere
location, etc.
Can detect abnormalities in
chromosome structure or altered
chromosome # by comparing
individual’s karyotype against species
standard
Most traits come from autosomal dominant /
recessive alleles inherited in simple
Mendelian patterns
Some of these alleles cause genetic disorders
Genetic Abnormality
Rare / uncommon version of trait
Not life-threatening
e.g. polydactyly
Genetic Disorder
Heritable condition
Mild to severe medical repercussions
Characterized by set of symptoms
= syndrome
Autosomal Dominant Inheritance
Dominant allele
Trait usually appears each generation because
allele is expressed in homozygous dominants
& heterozygotes
Remember: phenotypic ratio 3:1
e.g. Huntington’s disease, lactose intolerance
Huntington’s Disease
Degeneration of neurons in brain
Affects 1/20,000 – 1/1,000,000 people
Results in uncontrolled movements,
emotional problems, loss of brain function
Symptoms include mood swings, difficulty
making decisions & retaining info
No cure
If 1 parent is heterozygous & other is homozygous
recessive, offspring has 50% chance of being
heterozygous
A
a
a
a
Aa
Aa
aa
aa
Some dominant alleles that cause severe problems
persist in populations because:
• Expression of allele doesn’t affect reproduction
• Affected individuals reproduce before symptoms
are evident
• Spontaneous mutations
Autosomal Recessive Inheritance
Recessive allele
Must be homozygous recessive to express trait
If heterozygous for the trait = carrier
e.g. cystic fibrosis, sickle cell anemia
Cystic Fibrosis
Production of very thick, sticky mucus
Affects lungs & digestive system
(clogs lungs & hampers pancreas from breaking
down & absorbing food)
~30,000 people in US are affected
Average lifespan = 35-40 years
Sickle Cell Anemia
Body produces abnormally-shaped RBCs
= break down prematurely & cause anemia
Affects 1/500 African-Americans
If only 1 allele = sickle cell trait
– 1/12 African-Americans have trait
– Resistance to malaria
If both parents are carriers (heterozygous),
offspring has 50% chance of being carrier
(heterozygous) & 25% chance of being affected
(homozygous recessive)
A
a
A
AA
Aa
a
Aa
aa
Sex Determination in Humans
Every normal female egg has 1 X chromosome
½ of sperm cells have X, ½ have Y
Sperm that fertilizes egg determines gender
The SRY Gene
1 of 255 Y chromosome genes
Master gene for male sex determination
When expressed in XY embryos, initiates
testes formation
Testes produce testosterone
(controls expression of male 2 sexual traits)
XX embryo
= no Y, no SRY,  testosterone
= ovaries form
(make estrogens & other sex hormones that
control expression of female 2 sexual traits)
The X Chromosome
1141 genes:
Some associated with sexual traits
e.g. distribution of body hair & fat
Most of genes associated with non-sexual traits
expressed in both males & females
(because males get 1 X chromosome)
X-Linked Inheritance
Thomas Hunt Morgan & Drosophila
Determined that genes for non-sexual traits
are located on X chromosome
X-Linked Inheritance
Males show their only allele
Males inherit only from mother
Fathers pass their only allele to all daughters
X chromosome alleles result in phenotypes
that follow simple Mendelian inheritance
Many recessive alleles cause genetic disorders
e.g. hemophilia A, red-green colour blindness
Hemophilia A
Bleeding disorder
(caused by lack of clotting factor)
Occurs primarily in males (1/10,000)
Severity varies
Red-Green Colour Blindness
Impairment or loss of function in light-sensitive
cone cells in eyes
Little or no perception of reds, greens,
yellows
Affects ~10% of males
Punnett Squares for X-Linked
Crosses
XA
Xa
XAY
XaY
XA XA XA Xa
XA
Y
Set up in much the same way as regular Punnett Squares, but
use X & Y to represent sex chromosomes with superscript
letters to represent the alleles carried on those chromosomes
Unaffected female & affected male
Female offspring:
All carriers
Male offspring:
All normal
Carrier female & normal male
Female offspring:
0.5 carrier
0.5 normal
Male offspring:
0.5 normal
0.5 affected
Female carrier & affected male
Female offspring:
0.5 carrier
0.5 unaffected
Male offspring:
0.5 normal
0.5 affected
More males than females affected
Heterozygous females have dominant allele
on other X that masks recessive allele’s
effects
Males only have 1 X chromosome
(no 2nd X chromosome to counteract effects of
recessive allele)
Females are the bridge between generations of
affected males
Unaffected male
Affected male
Unaffected female
Carrier female
Pedigrees
Genetic connections among individuals
Info from several generations collected
Can predict probability of trait being expressed as well
as trace trait origins backwards
Basic procedure is to
create a family tree &
apply Mendelian
genetics
Can’t assume that an
individual has a trait
or is a carrier without
evidence
A pedigree for a dominant trait
I
Common Symbols
Used in Pedigrees
II
III
A pedigree for a recessive trait
I
II
?
?
?
?
III
?
IV
?
?
How to read pedigrees
I, II, III = generations
= male
= female
= parents
= offspring
or
= shows trait
or
= does not show trait
or
= known carrier (heterozygote) for
recessive trait
? or ? = cannot determine genotype from pedigree
Notice you can use parents to
determine children’s genotypes
or children to determine
parents’.
Looking at this pedigree, is the trait caused by a dominant or recessive allele?
How do you know?
Can you tell anything about the genotypes of these individuals?
Y-Linked Inheritance
Genes can only be passed from father to son
No effect from mother
No effect on daughters
e.g. hairy ear (pinna) syndrome
An Example
In cats, coat colour is determined by an X-linked gene.
The black allele causes black coat colour while the
other allele, orange, causes orange colour, but in
heterozygotes the cats are tortoiseshell (patches of
black & orange).
This is an example of what type of inheritance?
What kind of offspring would you expect from a
black female & an orange male?
Heritable Changes in Chromosome #
Chance events occur before or after cell
division that result in wrong chromosome #
Consequences can be minor or lethal
Most changes in chromosome number occur
because of non-disjunction
= 1 pair of chromosomes do not separate
during mitosis or meiosis
(a) Aneuploidy
Normal # ± 1 chromosome
Usually fatal
Basis of most miscarriages
Chances of non-disjunction  with 
maternal age
e.g. Down Syndrome (Trisomy 21)
Child inherits extra copy of
chromosome 21
(2n for all other chromosomes)
1/900 births
Distinct physical characteristics:
Weak muscle tone, small mouth that can’t accommodate
tongue, uniquely-shaped eyelids
Varying degrees of mental retardation
Often ↓ immune response, heart malformations
(b) Polyploidy
Cells have ≥ 3 of each type of chromosome (e.g. 3n,
4n, etc.)
Many angiosperms, insects, fish, animals are actually
polyploid
Responsible for evolution via speciation
e.g. polyploidy in plants
Fertilized diploid egg duplicates chromosomes
but fails to divide = tetraploid (4n)
Produce diploid gametes that can fuse with
other diploid gametes = 4n offspring
Can self-fertilize or interbreed with other 4n
individuals of same species
If breed with 2n individual from original
species, offspring is triploid
(sterile because meiosis fails)
4n & 2n of original species can’t interbreed
successfully
= new species can form in 1 generation
Polyploidy is common in plants because they
can reproduce asexually
If a 4n animal was produced, it would have to
mate with a 2n individual
↓
All 3n offspring would be sterile
↓
= no speciation occurs
Changes in # of Sex Chromosomes
Non-disjunction causes most of changes in # of
X & Y chromosomes
Relatively frequent:
Often results in learning disabilities & speech
problems
Female Sex Chromosome Abnormalities
Turner Syndrome
Trisomy X
(a) Turner Syndrome
1 X chromosome; no corresponding X or Y
= XO
Affects 1/2500-1/10,000 newborn females
(75% because of non-disjunction from father)
98% of embryos spontaneously abort
Generally, XO females are 4’8” but wellproportioned
↓ sex hormone production & non-functional
ovaries
(2˚ sexual traits do not develop properly)
↑ risk of cardiovascular disease, kidney defects,
hearing loss
Display X-linked recessive disorders more
frequently than XX women
(b) Trisomy X
Women with 1 extra X chromosome
=XXX
1/1000 live births
Some learning disabilities & taller than average,
but otherwise no detectable defects
Fertile adults
(usually bear normal XX & XY children)
Male Sex Chromosome Abnormalities
Klinefelter Syndrome
XYY Syndrome
(a) Klinefelter Syndrome
Inherit extra X chromosome from mother
= XXY
1/500 -1/1000 males
• 2/3 from non-disjunction at meiosis
• Other 1/3 because Y chromosome fails to separate
at mitosis
(XY in some cells, XXY in others)
Syndrome develops during puberty:
– Overweight, tall, small sex organs
– Normal intelligence
(some learning disabilities & short-term memory loss)
Feminizing effects
because  testosterone &  estrogen
( sperm count, sparse hair, high voice,
enlarged breasts)
Testosterone injections can reverse female traits
(b) XYY Males
1/500-1/1000 males
Taller than average, ↑ ↑
testosterone levels,
severe acne
Mild mental impairment
Many XXX, XXY, XYY children not even
diagnosed
= unfairly categorized as underachievers
Why are changes in sex chromosome
number tolerated?
Remember X-chromosome inactivation?
In females, one X chromosome is shut off
Coils up chromosome so can’t be transcribed
So … any extra X chromosomes get turned off
Sex Determination Systems
XX / XY
XX / XO
ZZ /ZW
# of chromosomes
Hermaphrodites
XX / XY
e.g. mammals, fruit flies
Female = XX
Male = XY
Male produces 2 types of
sperm
(one has X, other has Y)
Sex is determined by
sperm cell at fertilization
XX / XO
e.g. some insects
Female = XX
Male = XO
Male produces 2 types of sperm
(one type bears X, other has no sex chromosome)
Sex is determined by sperm cell at fertilization
ZZ / ZW
e.g. some fish, butterflies, birds
Female = ZW
Male = ZZ
Female produces 2 types of egg
(one type has Z, other has W)
Sex is determined by egg cell at fertilization
Chromosome Number
e.g. most ants & bees
Have no sex chromosomes
Sex determined by # of chromosomes:
Female is 2n & comes from fertilized egg
Male is n & comes from unfertilized egg
Hermaphrodites
e.g. many plants & invertebrates
Have both male & female sex organs
All individuals in a species have same complement
of chromosomes
Earthworm
Lily
Banana slug
Possess a mechanism against
self-fertilization so only function
as a single sex at a time
Prefer sexual
reproduction but will
self-fertilize
Both stamen (male) &
pistil (female) found on
same flower