Transcript Chapter 20

Chapter 20
Patterns of Genetic
Inheritance
Mader, Sylvia S. Human Biology. 13th Edition. McGraw-Hill, 2014.
Points to Ponder
• What is the genotype and the phenotype of an individual?
• What are the genotypes for a homozygous recessive and dominant
individuals and a heterozygote individual?
• Be able to draw a punnett square for any cross (1-trait cross, 2-trait
cross and a sex-linked cross).
• What are Tay-Sachs disease, Huntington disease, sickle-cell
disease, and PKU?
• How are each of the above inherited?
• What is polygenic inheritance?
• What is a multifactorial trait?
• What is sex-linked inheritance?
• Name 3 X-linked recessive disorders.
• What is codominance?
• What is incomplete dominance?
• What do you think about genetic profiling?
20.1 Genotype and phenotype
These traits are genetically inherited
Answer these questions about your inheritance:
• Do you have a widow’s peak?
• Are your earlobes attached or unattached?
• Do you have short or long fingers?
• Do you have freckles?
• Can you roll your tongue?
• Do you have Hitchhiker’s thumb?
Genotype
Genotype – specific genes for a particular trait
written with symbols
– Alleles:
• alternate forms of a specific gene at the same
position (locus) on a gene (e.g. allele for unattached
earlobes and attached lobes)
• alleles occur in pairs
– Dominant gene: will be expressed and will mask a
recessive gene (Tt or TT)
– Recessive allele: allele that is only expressed when
a gene has two of this type of allele
Genotype
• Genotype
– Homozygous dominant genotype:
• 2 dominant alleles (TT or AA)
– Homozygous recessive genotype:
• 2 recessive alleles (tt or aa)
– Heterozygous genotype:
• one dominant allele and one recessive allele
(Tt or Aa)
20.1 Genotype and phenotype
Phenotype
Phenotype – the physical or outward expression of
the genotype
Genotype
EE
Ee
ee
Phenotype
unattached earlobe
unattached earlobe
attached earlobe
What are your genotype and phenotype?
Understanding genotype & phenotype
What about your inheritance?
Crosses
• One-trait cross – considers the inheritance of one
characteristic
e.g. WW x Ww
• Two-trait cross – considers the inheritance of two
characteristics
e.g. WWTT x
WwTT
• Gametes only carry one allele, so if an individual
has the genotype Ww what are the possible
gametes that this individual can pass on?
Answer: either a W or a w but not both
Another example:
20.2 One-and Two-trait inheritance
Punnett squares
• Punnett squares
– use of a grid that
diagram crosses
between individuals
by using the possible
parental gametes
• These allow one to
figure the probability
that an offspring will
have a particular
genotype and
phenotype
20.2 One-and Two-trait inheritance
Practicing punnett squares
F – freckles
f – no freckles
eggs
M/F
sperm
• What would a
punnett square
involving a man (M)
with a genotype Ff
and a women (F)
with a genotype Ff
look like?
F
f
F
FF
Ff
f
Ff
ff
Practicing ratios
• Genotypic ratio:
– number of offspring with
the same genotype
eggs
• Phenotypic ratio:
• What is the genotypic
ratio?
1: 2: 1 (1 FF: 2 Ff: 1 ff)
• What is the phenotypic
ratio?
3: 1 (3 with freckles and 1
with no freckles)
sperm
– number of offspring with
the same outward
appearance
M/F
F
f
F
FF
Ff
f
Ff
ff
20.2 One-and Two-trait inheritance
Monohybrid crosses
Monohybrid cross – an experimental cross in which parents
are identically heterozygous at one gene pair (e.g. Aa x Aa)
One Trait Crosses
Possible gametes for two traits
All Possible Combinations of Chromosomes and
alleles in the gametes (via meiosis)
Dihybrid cross (a type of two-trait cross)
• Dihybrid cross
– experimental cross usually involving parents that are
homozygous for different alleles of two genes and results in
a 9:3:3:1 genotypic ratio for the offspring
Practicing a punnett
square for 2-trait cross
• What would the
punnett square look
like for a dihybrid
cross between a male
that is WWSS and a
female that is wwss?
Phenotypic Ratios of Common Crosses
Genotype
Phenotype
Monohybrid x Monohybrid
(Ww x Ww)
Monohybrid x recessive
(Ww x ww)
Dihybrid x Dihybrid
(WwSs x WwSs)
3:1 (dominant to
recessive)
Dihybrid x recessive
1:1 (dominant to
recessive)
9:3:3:1 (9 both dominant,
3 one dominant, 3 other
dominant, 1 both
recessive
1:1:1:1 (all possible
combinations)
Family Pedigrees for Genetic Disorders
• Autosomal Dominant
– Individual with alleles AA or Aa will have disorder
• Autosomal Recessive
– Only individuals with alleles aa will have disorder
• Key:
–
–
–
–
–
Square = male
Circle = female
Shaded circle/square = affected individual
Line between square and circle = union
Vertical line going downward = child/children
Autosomal recessive disorder
• Individuals must be homozygous recessive to
have the disorder
Autosomal dominant disorder
• Individuals that are homozygous dominant and
heterozygous will have the disorder
Autosomal Recessive Disorders
1.
2.
3.
4.
5.
Tay-Sachs
Cystic Fibrosis
Phenylketonuria
Sickle-cell Disease
Huntington Disease
Genetic disorders of interest
• Tay-Sachs disease:
– lack of the enzyme that breaks down lipids in lysosomes
– results in membranous cytoplasmic bodies (MCB) in the
cells present in the cortical neuron
– eventually death of a baby
• Cystic fibrosis:
– Cl- do not pass normally through a cell membrane, so Na+
and water can not enter the cell
– results in thick mucus in lungs and causes infections, clogs
pancreatic ducts preventing function of digestive enzymes
• Phenylketonuria (PKU):
– lack of an enzyme needed to make a certain amino acid
– affects nervous system development
Genetic disorders of interest
• Sickle-Cell disease:
– red-blood cells are sickle shaped rather than
biconcave that clog blood vessels
– Sickle-cell heterozygotes have sickle-cell traits in
which the blood cells are normal unless they
experience dehydration or mild oxygen deprivation
• Huntington disease:
– Caused by a mutated copy of the gene (on
chromosome 4) for a huntingtin protein resulting in
too many glutamine amino acids
– leads to progressive degeneration of brain cells
20.2 One-and Two-trait inheritance
Genetic disorders
Huntington Disease
Tay sachs
Cystic fibrosis
20.3 Beyond simple inheritance
Polygenic inheritance
• Polygenic traits - two or more sets of alleles govern
one trait
– Each dominant allele codes for a product so these effects
are additive
– Results in a continuous variation of phenotypes
– Environmental effects cause intervening phenotypes
– e.g. skin color ranges from very dark to very light
– e.g. height vary among
• Multifactorial trait – a polygenic trait that is
particularly influenced by the environment
– e.g. skin color is influenced by sun exposure
– e.g. height can be affected by nutrition
Polygenic inheritance
Distribution of phenotypes expected to follow a
bell-shaped curve
20.3 Beyond simple inheritance
Demonstrating environmental
influences on phenotype
• Himalayan rabbit’s coat
color influenced by
temperature
• There is an allele
responsible for melanin
production that appears
to be active only at lower
temperatures
• The extremities have a
lower temperature and
thus the ears, nose paws
and tail are dark in color
20.3 Beyond simple inheritance
Incomplete dominance
• Occurs when the heterozygote is
intermediate between the 2 homozygotes
• Example:
(curly hair) CC
x
SS (straight hair)
CS (wavy hair)
20.3 Beyond simple inheritance
Codominance
• Occurs when the alleles are equally
expressed in a heterozygote
• Example:
(Type A blood) AA
x
BB (Type B blood)
AB (Type AB blood that has characteristics
of both blood types)
20.3 Beyond simple inheritance
Multiple allele inheritance
•
•
•
•
•
The gene exists in several allelic forms
A person only has 2 of the possible alleles
A good example is the ABO blood system
A and B are codominant alleles
The O alleles is recessive to both A and B
therefore to have this blood type you must
have 2 recessive alleles
20.3 Beyond simple inheritance
Multiple allele inheritance
Based on what you know what type of blood
would each of the following individuals have in a
cross between Ao and Bo?
possible genotypes:
AB
Bo
Ao
oo
phenotypes:
Type AB blood
Type B blood
Type A blood
Type O blood
20.3 Beyond simple inheritance
Blood type inheritance
20.4 Sex-linked inheritance
Sex-linked inheritance
• Traits are controlled by genes on the sex
chromosomes
 X-linked inheritance:
 allele is carried on the X chromosome
 Y-linked inheritance:
 allele is carried on the Y chromosome
 Most sex-linked traits are X-linked
20.4 Sex-linked inheritance
X-linked inheritance: Color blindness
Carried on the X chromosome
Cross:
XBXb x
XBY
Possible offspring:
XBXB normal vision female
XBXb normal vision female
XBY normal vision male
XbY normal vision male
X-linked disorders
• More often found in males than females
because recessive alleles are always
expressed
• Most X-linked disorders are recessive:
– Color blindness:
• most often characterized by red-green color blindness
– Muscular dystrophy:
• characterized by wasting of muscles and death by age 20
– Hemophilia:
• characterized by the absence of particular clotting factors
that causes blood to clot very slowly or not at all
X-linked disorders
X-linked disorders: Hemophilia
20.4 Sex-linked inheritance
Bioethical focus: Genetic profiling
• Genetic profiling is a way to look for genetic
disorders that you may have now or in the future
– Discrimination concerns:
• Could insurance companies use this to increase rates or not insure
you?
• Could an employer not hire you based on this knowledge?
– Employer concerns:
• Could the government mandate they provide an environment for
each employee’s need in order to prevent illness or should
employees be required to move from an area or job that could
cause future disease?
– Public benefits:
• Most believe this could lead to better preventative care
• Having this type of information can allow complex studies that can
further our understanding of disease