Polygenic and Multifactorial Inheritance Chapter 10

Central Points  Polygenic traits controlled by two or more genes  Multifactorial traits are polygenic with an environmental component  Spina bifida is a multifactorial trait  Many other multifactorial traits

Case A: Prenatal Pills  Vera Smith found out she is pregnant  Went to doctor after three months  Nurse asked if she was taking vitamins with folic acid  Reduces chance baby born with spina bifida (SB)

10.1 Polygenic Traits  Determined by two or more gene pairs  Examples: immune system, color of skin, hair, and eyes  Cause slight and often variable range of differences throughout population  Trait value: measurable aspect of the phenotype (height, skin color, and sizes of body parts)

Bell curve  Most individuals are clustered at ~average  Few individuals at extremes of the phenotype Typical Polygenic Trait in a Population

10.2 What Is a Multifactorial Trait?  Controlled by two or more genes and affected by environmental factors  Example: Height  Genes inherited in Mendelian fashion  Interaction of genes with environment produce many different phenotypes

Variation in Height

Height as a Multifactorial Trait

Characteristics of Multifactorial Traits (1)  Several genes control trait  Not inherited as dominant or recessive  Genes controlling trait contribute a small amount to phenotype  Environmental factors interact with genes to produce phenotype

Characteristics of Multifactorial Traits (2)  Many phenotypic differences in trait  Distributions of phenotypes form a bell-shaped curve

How Many Genes Control Trait?  As the number of genes increases, phenotypic differences among people decreases  As differences among people decrease, more likely environmental factor, blending the phenotypes together  Environmental factors for height: hormones and diet

Multifactorial Diseases  Diabetes  Spina bifida  Club foot  Cancer  Hypertension and cardiovascular disease

Some Multifactorial Traits

p. 167 Diabetes

p. 167 Cleft lip and palate

p. 167 Club foot

Animation: Observing Patterns in Genetic Traits (Continuous Variation in Height)

10.3 Spina Bifida  Birth defect involving nervous system  Occurs first month of embryonic development  Type of neural tube defect  Problems in development of spinal cord and related parts of nervous system

Neural Tube  Gives rise to: Brain Spinal cord Meninges: membranes that cover and protect brain and spinal cord

Formation of Neural Tube

Anencephaly  Extreme form of a neural tube defect, head end of neural tube does not close  Major portions of brain and skull do not form, remaining portions may not be enclosed in skull  Can survive only within mother, most stillborn  If survive, die within a few hours or days from heart and breathing problems

Types of Spina Bifida

Spina Bifida  Opening and damage can be surgically repaired, damage to nervous system permanent  Varying degrees of paralysis  Learning disabilities  Bowel and bladder problems  No cure for SB but most live into adulthood

Is Spina Bifida a Multifactorial Trait?  Yes  Tends to cluster in families  Risk of second child with SB or another neural tube defect increases significantly  Environmental factors include dietary deficiencies in folic acid

Environmental Risk Factor for SB  Diets deficient in folic acid, a B vitamin  Need 0.4 mg/day for at least three months before pregnancy, and until week 12 Reduces risk of SB and related conditions by ~70%  How folic acid interacts with genes in formation of neural tube unknown

Case A Questions  Should Vera be skeptical about folic acid because its action is unknown?  Should Vera just take the pills?  What if she takes the pills and her child is born with SB?  See the textbook for further questions on this case

Video: ABC News: All in the family: mixed race twins

10.4 Other Multifactorial Traits  Fingerprints: a polygenic trait  Dermatoglyphics: ridges on fingers, palm, toes, and feet  Influenced by prenatal environment Nutrition of the mother Rate of finger formation and growth  Even identical twins have unique fingerprints

Identical Twins and Fingerprints

Obesity: A Multifactorial Trait?  Twin studies used to estimate how much of obesity is genetic  Identical twins, monozygotic (MZ) twins, genetically identical, form from same zygote  Occurs in both MZ twins ~70% of the time: 70% concordance

Obesity in Mice Pedigree

Mouse ob Gene  Encodes weight-controlling hormone leptin, produced in fat cells  Along with cell receptors in brain, controls how energy used  Human gene for leptin, equivalent to mouse ob gene, is on chromosome 7  Mutations result in obesity

Obesity  Complex disorder involving action and interaction of multiple genes and environment  Important genes for obesity located on chromosomes 2, 3, 5, 6, 7, 10, 11, 17, and 20  Further work to ID additional genes and how these genes interact with environmental factors

Obesity Genes

Is Intelligence a Multifactorial Trait?  Head size was used to determine intelligence  Early 20th century, psychological rather than physical methods  Intelligence quotient (IQ) assumes that intelligence is a biological property  Concordance in MZ twins raised together and apart indicates genetic and environmental factors

Genes that Control Intelligence  Searching for single genes that control aspects of learning, memory, and spatial perception  Drosophila and the mouse models Drosophila has many biochemical pathways identical to those in humans  Pathways play important roles in learning and memory

Quantitative Trait Loci (QTLs)  Use information from Human Genome Project  Associated with reading disability (developmental dyslexia)  Genes on chromosomes 6, 15, and 4 for cognitive ability  Accumulated results indicate intelligence is polygenic and multifactorial trait

Case B: Donation of a Baby’s Organs  Samantha’s baby has anencephaly, baby will not live  Doctor mentions donating baby’s organs  What should she do?  Should she carry the baby to term?  See the textbook for further questions on this case

10.5 Legal and Ethical Issues

IQ and the Bell Curve