C12- Mendelian InheritanceOf Human Traits

Chp 12- pp. 308- 335

Contents

• 12-1 Mendelian Inheritance• 12-2 Complex Inheritance• 12-3

Complex Inheritance of Human Traits

C12-Mendelian Inheritance of Human Traits

• Making a Pedigree• Symbols identify sex,

genotype and relationship between parents, siblings & spouses.

• Half-shaded Carrier• Horizontal row

generation• Practice on p. 310.

Analyzing a Pedigree

• Determine likelihood of being a carrier.

• Probability for offspring being affected.

Simple Recessive Heredity

• Most genetic diseases are caused by recessive alleles:

• Cystic fibrosis• Tay Sachs• Phenylketonuria

Simple Dominant Heredity

• Almond Eyes• Thick Lips• Digital Hair

• Cleft Chin• Widow’s Peak• Hanging Earlobes• Hitchhiker’s Thumb

Simple Dominant Heredity

• Huntington’s Disease• Lethal genetic disorder• Onset between 30 to 50

12-2 When Heredity Follows Different Rules

• Complex Inheritance– Incomplete

Dominance– Codominance– Multiple Alleles– Sex Determination– Sex-linked– Polygenic Inheritance

Complex Inheritance

• Incomplete dominance • Appearance of a third phenotype blending the

first two

Complex Inheritance

• Codominance• Both alleles

expressed equally in heterozygotes

• Use uppercase for both

• Ex. Checkered chickens

Complex Inheritance

• Multiple Alleles• Traits controlled by

more than 2 alleles• Ex. Feather color in

pigeons

Complex Inheritance

• Sex Determination• 23rd pair of

chromosomes• Female 2 X s• Male X Y

Complex Inheritance

• Sex-Linked Traits controlled by genes on sex chromosomes

• Thomas Hunt Morgan (1910) discovered rules governing SLT by studying fruit flies eye color.

• Females red eye males ½ red ½ white

Complex Inheritance

• Polygenic Inheritance

• Skin color & height controlled by many genes

• Heterozygotes intermediate phenotype

Environmental Influences

• Can determine how or whether gene is expressed:

• Internal Environment– Hormones, structural

differences, age• External

– Temperature, nutrition, light, chemicals, infectious agents.

12-3 Complex Inheritance of Human Traits

• Codominance– Sickle-cell anemia– Differs from normal

hemoglobin by one amino acid

– Moon-shaped cells block blood flow causing tissue damage and pain.

Multiple Alleles

• Blood Type- A, B, AB or O types determined by presence or absence of molecules

• Gene has 3 alleles IA, IB, I• Incompatible blood causes clumping

Sex-linked Traits in Humans

• Red-green colorblindness

• Hemophilia-blood clotting disorder

Sex-linked Traits in Humans

• ♀ Carrier X ♂ C• 50% offspring CB in

this example• Responsible gene on

X

Polygenic Inheritance

• Eye Color• Skin Color• 3 or more genes

involved

Changes in Chromosome Number

• Abnormal number autosomes

• Trisomy usually results from nondisjunction.

• Metaphase chromosomes are photographed & charted into a Karyotype.

Changes in Chromosome Number

• Down’s Syndrome- Trisomy 21

• Only autosomal trisomy that survives to adulthood.

• Occurs 1 in 700 live births

• Some degree of mental retardation

Changes in Chromosome Number

• Abnormal number of Sex chromosomes• XO female Turner Syndrome• XXX female• XXY male Klinefelter Syndrome• XYY male• Infertility• Some mental retardation possible