Download A single characteristic may be influenced by many genes

A single characteristic may be
influenced by many genes
•Polygenic inheritance
•Additive effect of 2 or more
•genes on a single trait
•Example:
•Skin color or height
Gene with more than 2 alleles
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Human Inheritance can
be controlled by a
single gene that has
more than 2 alleles
Example blood type
Codominance
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2 alleles can combine because alleles are codominant they are written with superscripts
Blood Type IA IB shows co-dominance in
humans
Blood Types
Genotype
IAIA or IAi
IBIB or IBi
A B
I I
ii
Blood Type (phenotype)
A
B
AB – universal recipient
O – universal donor
Traits Controlled by Many Genes
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Some human traits show a large number of
phenotypes because the trait is controlled by
many genes.
The genes act together to produce a single
trait
Examples
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at least 4 genes control height
at least 3 genes control skin color
Effects of the Environment
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Effects of genes can be altered by the
environment.
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Example – people’s diets can effect height
Living conditions
Medical care
SEX CHROMOSOMES
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Chromosomes determine sex in
many species
A human male has one X
chromosome and one Y
chromosome
A human female has two X
chromosomes
Whether a sperm cell has an X
or Y chromosome determines
the sex of the offspring
X and Y Chromosome
Sex-Linked Inheritance
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Single genes located on sex chromosomes
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Actually X-linked
Males have no counterpart on Y chromosome
Females have counterpart on second X
chromosome
Requires gene on both X’s for trait
Hemophilia, Colorblindness, Duchene MS
Sex-linked traits
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Females need two copies of a recessive allele
to show a phenotype
Males need only one
Sex linked traits more common in males
The individual who has one recessive allele
for a trait is called a carrier
Sex-linked disorders affect mostly males
Most sex-linked human
disorders are due to
recessive alleles
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Examples: hemophilia,
red-green color blindness
These are mostly seen in males
A male receives a single X-linked allele from his
mother, and will have the disorder, while a female has
to receive the allele from both parents to be affected
Hemophilia is a sex-linked disease
Pedigrees
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A tool used to trace the
inheritance of traits in
humans
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Symbols used on
pedigree charts