Polygenic and Multifactorial Inheritance

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, dies within a few hours or days from heart and breathing problems

Spina Bifida

Type of neural tube defect, birth defect involving nervous system Problems in development of spinal cord, brain, or meninges Occurs during 1st month of embryonic development Different types of spina bifida Opening and damage can be surgically repaired, damage to nervous system is permanent Varying degrees of paralysis Learning disabilities Bowel & Bladder problems Tends to cluster in families Risk of second child with SB or another neural tube defect increases significantly In mice, gene VANGL1 on chromosome 1 acts early in development of neural tube, confirmed in human -curly tail mouse (neural tube defects) Environmental factors- dietary deficiencies in folic acid (vitamin B) Unknown how folic acid interacts with genes in formation of neural tube

Donation of a Baby's Organs

Ultrasound showed a baby with anencephaly, baby will not live Should pregnancy be aborted? Carried to term for organs donation? Legal and Ethical Issues: -cant force a pregnant woman not to take drugs, even if illegal -cant force a woman to take medication that is beneficial to fetus but not her -physicians can take mothers to court & file for treatment of newborns or children, but not fetuses

Risk Factors for Cardiovascular Disease

Uncontrollably -age -genes (lipid metabolism, lipoproteins) Controllable -fatty diet -cholesterol level -stress -no exercise

Examples of Multifactorial Traits

-height -skin color -fingerprint -spina bifida -obesity -hypertension & cardiovascular disease

Obesity

Complex disorder involving interaction of multiple genes and environment 33% of US adults obese, 68% overweight (determined by BMI- weight in proportion to height) 70% concordance -mice with mutant obese (ob) gene and diabetic (db) gene are obese -in ob mice, lack of leptin (produced by fat cells) Human gene for leptin, equivalent to mouse ob gene, is on chromosome 7 Important genes for obesity located on chromosomes 2,3,5,6,8,10,11,17,18 Further work to ID additional genes and how these genes interact with environmental factors (diet, exercise)

Concordance

Concordance- trait found more in both individuals of monozygotic twins (MZ) than in a pair of dizygotic twins (DZ) or in the general population -higher the difference, greater the heritability -has limitations, assumes similar environment in MZ -MZ twins often share more similar environment -studies of twins raised apart separate nature from nurture

Multifactorial Trait

Controlled by 2 or more genes AND affected by environmental factors ex. height Interaction of genes with environment produces many different phenotypes Characteristics: -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 -many phenotypic differences in trait -distributions of phenotypes form bell curve

Polygenic Traits

Determined by 2 or more genes ex. immune system, skin, hair, eye color Cause slight & variable range of differences throughout population ex. taste to phenylthiocarbamide (PTC) Trait value- measurable aspect of the phenotype (height, size of body parts) Bell curve- most individuals in a population grouped in phenotype classes & frequency of each one can be calculated -few individuals at extremes of phenotype

Intelligence as a Multifactorial Trait

Many genetic, environmental influences Head size used to determine intelligence in past Early 20th century, psychological methods Intelligence Quotient (IQ)- assumes intelligence is biological property, based on ability to perform tasks Concordance in MZ twins (80%) raised together and apart indicates genetic and environmental factors

Describing Traits

Mendelian- primarily controlled by a single gene Polygenic- controlled by 2 or more genes Multifactorial- controlled by many genes (polygenic) but with an environmental component In polygenic inheritance: -mendelian genotypic ratios persist for each gene but their contribution to phenotype may vary (not equal) because of interactions between genes and environment

Separating Genes and Environment

Monozygotic twins -same sex & identical genotype Dizygotic twins -50% of genes Twins raised apart -shared genotype but not environment Adopted individuals -shared environment but not genes

Skin Color

More than 100 genes involved Melanin production by melanocytes Skin pigmentation- protects skin from UV radiation, no melanin in albinos skin Exposure to sun increases melanin formation, adaption Race based on skin color has no genetic meaning

Fingerprints

Ridges, arches and loops on fingers, palm, toes, and feet make fingerprints/dermatoglyphics Form in the first 3 months of embryonic development Different in every human (even identical twins) Controlled by genes Also influenced by prenatal environment -nutrition of mother -rate of finger formation and growth

Genes that Control Intelligence

Searching for single genes that control aspects of learning, memory, and spatial perception Drosophila and mouse models -drosophila has many biochemical pathways identical to those in humans -pathways play important roles in learning and memory Use info from Human Genome Project Genes on chromosomes 4,6, and 15 for cognitive ability Mutation in gene on chromosome 4 associate with dyslexia (reading disorder) Accumulated results indicate intelligence is a multifactorial trate