Patterns of Heredity Can Be Complex

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Transcript Patterns of Heredity Can Be Complex

Patterns of Heredity Can
Be Complex
Section 8-4
Most Traits Are Not Controlled by
Simple Dominant-Recessive Alleles
► Some
traits display more complex patterns
of heredity than the simple dominantrecessive patterns discussed
Traits Influenced by Several Genes
► Polygenic
trait – when several genes
influence a trait
► The genes for a polygenic trait may be
scattered along the same chromosome or
located on different chromosomes.
► Determining the effect of any one of these
genes is difficult. Due to independent
assortment and crossing-over, many
combinations appear in offspring.
► Human polygenic traits – eye color, height ,
weight, hair color, and skin color
Intermediate Traits
► Incomplete
Dominance – when an individual
displays a trait that is intermediate between
two parents
Ex: Red snapdragon crossed with white
snapdragon – not red nor white, but pink
Traits with Two Forms Displayed at
the Same Time
► Codominance
– when two dominant
alleles are expressed at one time
► Different from incomplete dominance in that
both traits are displayed, not the two traits
being mixed
► Ex: Roan color in horses: Red hair parent
and white hair parent, offspring has red and
white hair or roan color hair
Traits controlled by Genes with
Three or More Alleles
► Multiple
Alleles – genes with three or more
► Ex: ABO blood groups/blood types
Determined by 3 alleles – IA, IB, i
Letters A and B refer to a carbohydrate on
the surface of the red blood cell. The letter
i means there is no carbohydrate present.
A and B are dominant over i, but neither A
nor B is dominant over the other. When
they are both present, they are codominant.
Only 4 blood types – A, B, AB, O
Traits Influenced by the Environment
► Phenotype
can be influenced by the
► Hydrangeas – blue if in acidic soil, pink if in
basic soil
► Arctic Fox – changes coat color in seasons
Some traits are Caused by Mutations
► In
order for a person to develop and
function normally, the proteins encoded by
his or her genes must function precisely.
► Sometimes genes are damaged or copied
incorrectly, resulting in faulty proteins.
► Changes in genetic material are called
► Mutations are rare because most of the time
the body can correct them, but sometimes
they may have harmful effects.
► The
harmful effects produced by inherited
mutations – genetic disorders
► Many mutations are carried by recessive
► Two phenotypically normal people who are
heterozygous carriers of a recessive
mutation can produce children who are
homozygous for the recessive allele.
Sickle Cell Anemia
► Condition
caused by a mutated allele that
produces a defective form of the protein
► Causes red blood cells to bend into a sickle
► Red blood cells that are sickle rupture easily
► Results in less oxygen being carried by the
► Tend to get stuck in blood vessels, so can
cut off supply of blood to organs
Sickle Cell Anemia
► The
recessive allele that causes sickleshaped RBC also helps protect cells of
heterozygous individuals from effects of
► Cause death of the parasite that causes
condition that impairs the blood’s ability
to clot
► Sex-linked trait
Huntington’s Disease
► Caused
by a dominant allele located on an
► Begins in thirties and forties with mild
forgetfulness and irritability
► Causes loss of muscle control,
uncontrollable physical spasms, severe
mental illness, and eventually death
Detecting and Treating Genetic
► Most
genetic disorders cannot be cured,
although progress is being made
► Can undergo genetic counseling – form of
medical guidance that informs people about
genetic problems that could affect them or
their offspring
Phenylketonuria (PKU)
► Lack
an enzyme that converts the amino
acid tyrosine
► Phenylalanine builds up in the body and
causes severe mental retardation
► If newborn is diagnosed soon after birth,
the newborn is placed on a lowphenylalanine diet, ensuring that the baby
will get enough phenylalanine to make
proteins, but not enough to cause enough