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Patterns of Heredity and

Human Genetics

Chapter 12

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 Recessive Heredity

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

Mendelian Inheritance of

Human Traits

– Simple Dominant Heredity

Mendelian Inheritance of

Human Traits

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

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

When Heredity Follows

Different Rules

When Heredity Follows

Different Rules

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

• ABO Blood Group

– Codominance & multiple allele

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

Complex Inheritance of

Human Traits

• Klinefelter’s Syndrome

– 47 XXY

– 1 in every 500 males

– Low testosterone levels

– Infertility