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
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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