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Mendelian Inheritance of
Human Traits
• Pedigree
– Inheritance map of genetic traits
• Genetic “Family tree”
Mendelian Inheritance of
Human Traits
– Simple Recessive Heredity
• Trait is expressed when organism is homozygous
recessive (rr) for the trait
• Carrier
– Organism is heterozygous (Rr), “carries” the recessive
allele
• Examples
– Cystic Fibrosis
– Tay-Sachs disease
– Phenylketonuria (PKU)
Mendelian Inheritance of
Human Traits
– Simple Dominant Heredity
• Trait is expressed when organism is homozygous
dominant (RR) or heterozygous (Rr) for the trait
• No carriers
• Examples
– Attached earlobes
– Hitchhiker’s thumb
– Widow’s peak
– Rolling of tongue
– Huntington’s disease
When Heredity Follows
Different Rules
– Incomplete Dominance
• Heterozygous genotype results in a blending of the
two phenotypes
– RR = red
– rr = white
– Rr = pink
When Heredity Follows
Different Rules
– Codominance
• No recessive alleles
• Heterozygous genotype results in a blending of the
two phenotypes
– RR = red
– WW = white
– RW = red & white
When Heredity Follows
Different Rules
– Multiple Alleles
• More than 2 alleles for the trait exist
• Phenotypes are variant
– MR is dominant to both M and md
– M is dominant to md but recessive to MR
– md is recessive to both MR and M
When Heredity Follows
Different Rules
– Sex determination
• Sex chromosomes determine the sex of the
organism
– XX = female
– XY = male
When Heredity Follows
Different Rules
– Sex-linked inheritance
• Traits are controlled by genes on the sex
chromosomes
• X-linked
– Red-green color blindness
– Hemophilia
» Inability to form blood clots
• Y-linked
– Hairy ears
When Heredity Follows
Different Rules
– Polygenic Inheritance
• Trait is controlled by
more than one gene
– Genes may be on same
or different
chromosomes
– Genes may have 2 or
more alleles
» Skin color
» Hair color
» Height
» Eye color
When Heredity Follows
Different Rules
– Environmental Influences
• Phenotype is affected by the organism’s
environment (internal and external)
– Temperature
– Chemicals
– Light
– Nutrition
– Sex of organism
» Sex-influenced – pattern baldness
Complex Inheritance of
Human Traits
• Sickle-cell anemia
– Hemoglobin in the red blood cell
is abnormally shaped
» Slow blood flow
» Capillary blockage
» Tissue damage & pain
Complex Inheritance of
Human Traits
• Down Syndrome
– Trisomy 21
» 3 #21 chromosomes
» 47 XX or 47 XY
– 1 in 800 births
– Decreased muscle tone
– Distinct facial characteristics
– Lowered mental capacity
– Numerous abnormalities
» Heart defects
» Intestinal defects
Complex Inheritance of
Human Traits
• Turner Syndrome
– Monosomy X
» 45 X
» 1 X chromosome
– 1 in every 2000 females
– Short stature
– Lack of sexual
development
– Heart defects
– Other abnormalities