Transcript Presentation

Dominant and Recessive Physical Traits
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Pedigree Diagrams: I

Basic Symbols
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normal male
normal female
affected male
affected female
male carrier
female carrier
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Pedigree Diagrams: II
Basic Symbols for offspring and the expression of a trait.
 Offspring are depicted below the parents.
 Filled in symbol indicates the expression of the studied
trait.
 Roman numerals – Generations
Parents
 Arabic numerals – Individuals in
a certain generation
Generation
Siblings
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Factors to Consider in Pedigrees

Is the trait located on a sex chromosome or an
autosome?
 Autosomal – not on a sex chromosome #1-22
 Sex Linkage – located on one of the sex
chromosomes
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Y-linked - only males carry the trait.
X-linked (recessive) - sons inherit the disease from normal
parents
How is the trait expressed?
 Dominant - the trait is expressed in every
generation.
 Recessive - expression of the trait may skip
generations.
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Autosomal Dominant Disorders
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Requires only ONE allele for the disorder to be passed
Probability of having offspring with the disorder: 50%
if one parent has the defective allele
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Huntington’s Disease
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Results in damage to the brain
Symptoms generally occur after the affected
reaches 30 years old
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Many have children before they show symptoms
Rate of Occurrence: 1 in 15,000
Life expectancy: 10-20 years after onset of
symptoms
Genetic screening can identify those affected
before the onset of symptoms
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Marfan’s Syndrome: An Example
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Expressed in both sexes.
 Thus, autosomal or sex linked??
Expressed in every generation.
 Thus, dominant or recessive?
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Marfan
Syndrome
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Marfan’s: Genotype the Normal Individuals
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Assign codes for the alleles.
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Code “m” for the recessive normal allele.
Code “M” for the dominant allele for Marfan’s
syndrome.
Normal individuals must be “_____.”
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Marfan’s: Genotype the Affected Individuals
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Affected individuals must have at least one “____.”
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Marfan’s: Parent-Offspring Relationships
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Possibilities for #1 and #2: Heterozygote (Mm)
or homozygous for “M?”
If “MM,” all offspring from a normal mate
should be affected.
Therefore, both must be heterozygotes.
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Marfan’s: Parental Genotypes Known
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“M” must have come from the mother.
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The father can contribute only “m.”
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Thus, the remaining genotypes are “Mm.”
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Autosomal Recessive Disorders
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Autosomes: homologous chromosomes 1-22

many disorders are autosomal recessive
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Requires defective recessive allele to be passed
by BOTH parents
Probability of offspring having the disorder:
25% if both parents are carriers/ 0 if only one
parent is a carrier.
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Cystic Fibrosis (CF)
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1 in 20 Caucasians are carriers
 Rate of occurrence: 1 in 2000
Characteristics: accumulation of mucus in lungs and
digestive tract.
Life expectancy: ~30 years (before treatment became
available), but now depends heavily on the age at when
treatment is started.
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Phenylketonuria (PKU)
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Metabolic disorder in which phenylalanine cannot be
broken down
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Rate of occurrence: 1 in 15,000
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Can result in mental retardation
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All infants in US are screened
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Reducing intake of phenylalanine until puberty can
prevent retardation
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PKU
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Sickle cell anemia
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Disorder of the Red Blood Cells
Causes sickling of cells preventing normal
function
Lack of oxygen to organs can cause
tissue damage resulting in intense pain
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Affects mostly those of African descent
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1 in 12 African-Americans are carriers
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>70,000 have the disease
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Albinism: An Example
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Expressed in both sexes at approximately equal frequency.
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Thus, autosomal.
Not expressed in every generation.
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Thus, recessive.
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Albinism: Genotype the Affected Individuals
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Assign codes for the alleles.
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Code “A” for the dominant normal allele.
Code “a” for the recessive allele for albinism.
Affected individuals must be homozygous for “a.”
First generation parents must be “Aa” because they have normal
phenotypes, but affected offspring.
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Albinism: Genotype the Normal Individuals
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Normal individuals must have at least one “A.”
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Albinism: Parent-Offspring Relationships
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#1 must transmit “a” to each offspring.
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The “A” in the offspring must come from the father.
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Normal father could be either heterozygous or
homozygous for an “A.”
**
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Albinism: Parental Genotypes are Known
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Both parents are heterozygous or homozygous?
Normal offspring could have received an “A” from
either parent, or from both.
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Albinism: One Parental Genotype is Known
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Only the genotype of the offspring expressing albinism
are known.
Normal offspring must have received an “a” from their
affected father.
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Tay-Sachs Disease
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Metabolic disorder that affects the nervous system
More common in those of Central and Eastern
European Jewish descent
 1 in 30 are carriers
Life expectancy: ~5 years
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Sex-Linked Inheritance
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Alleles that are inherited on the sex chromosomes
Written as superscripts on the X and Y chromosomes
 XªX or XªY
Most sex-linked traits are associated with the female (X)
chromosome
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Red-Green Color Blindness
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Disorder in which a person cannot differentiate
between red and green
Allele passed on the X chromosome
Disorder more common in males
 Males: one copy results in color blindness
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Females: one copy (carrier)
two copies result in color blindness
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Red-Green Color Blindness
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Hairy Ears: An Example
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Only males are affected.
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All sons of an affected father have hairy ears.
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Thus, hairy ears is Y-linked.
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Hairy Ears: Female Sex Determination
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All females are XX.
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Hairy Ears: Male Sex Determination
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All males are XY.
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Hairy Ears: Gene on the Y Chromosome
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Code “H” indicates the allele on the Y chromosome for
hairy ears.
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Hemophilia
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Disorder that prevents blood from clotting properly
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Allele passed on the X chromosome
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Males more commonly affected
 males: one copy results in disorder
 Females: one copy (carrier)
two copies results in disorder
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Hemophilia: An Example
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In this pedigree, only males are affected, and sons do
not share the phenotypes of their fathers.
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Thus, hemophilia is linked to a sex chromosome–the X.
Expression of hemophilia skips generations.
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Thus, it is recessive.
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Extensive bruising of
the left forearm and
hand in a patient with
hemophilia.
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Hemophilia:
Expression of the Female Sex Chromosomes
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All females are XX.
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Hemophilia:
Expression of Male Sex Chromosomes
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All males are XY.
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Hemophilia: Genotype the Affected Individuals
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Assign codes for the alleles.
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Code “h” for the recessive hemophilia allele.
Affected individuals must have an “h” on an X chromosome.
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Hemophilia: Father-Daughter Relationship
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All daughters of an affected father receive an X
chromosome with the “h” allele.
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Hemophilia: Homozygous or Heterozygous?
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Only males affected
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Not Y-linked
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Skips a generation: recessive
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X-linked
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Incomplete Dominance
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When traits exhibit incomplete dominance,
heterozygotes exhibit intermediate characteristics
Ex. Snapdragons
Red flowers (RR)X White flowers (R´R’)
F1: Pink flowers (RR’)
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Codominance
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Heterozygotes exhibit both characteristics
ex. Chickens
Black rooster (BB) X White hen (WW)
F1 : black & white checkered pattern (BW)
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Polygenic Inheritance
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when traits are controlled by two or more genes
ex. Hair color, eye color, skin color, height, fingerprint
patterns
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Multiple Alleles
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Many traits are controlled by more than two alleles
ex. Fur color in many animals
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Blood Types
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An example of several different hereditary patterns
 Multiple alleles: A, B, and O
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Codominance: A and B
Recessive and Dominant:O is recessive to both A
and B
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Blood Types
*Notice how
the genotype
is written for
each type
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Blood Transfusion Facts
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People with type O- blood can donate to anyone
(universal donor), but can only receive type O- blood.
People with type AB+ blood can receive blood from
anyone (universal recipient), but can only donate to
someone who is type AB+.
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