Chapter 29 - Development & Inheritance Flashcards
1
Q
Conception
A
The start of pregnancy when a male germ cell fertilizes the female germ cell
2
Q
3 Steps of Fertilization
A
- Penetration of secondary oocyte by sperm cell
- Triggering of completion of Meiosis 2
- Syngamy (Fusion of pronuclei) of ovum & spermatozoa -> zygote
3
Q
Capacitation
A
When sperm are acted upon by female secretions in order to:
- Increase motility
- Partially digest membrane covering sperm cell’s head (Aids in penetration of membrane + chemo-attraction of sperm cell)
4
Q
ZP3 Receptors
A
Receptors find in zona pellucida of ovum that sperm cell head proteins bind to
- Sperm cell loses its middle piece & flagellum in zygote formation; all embryo mitochondria comes from secondary oocyte
- Only around 300 out of 300 mil. sperm cells reach secondary oocyte
5
Q
2 Enzymes for Fertilization
A
- Acrosome hyaluronidaise
- Acrosome protease
* Both help access secondary oocyte, digesting intracellular matrix between corona radiata cells & zona pellucida
6
Q
Polyspermy
A
Fertilization of an egg by multiple sperm
7
Q
2 Steps of Polyspermy Prevention
A
- Depolarization of secondary oocyte’s plasma membrane
2. Release of chemicals that deactivate ZP3 receptors
8
Q
Monozygotic Twins, Triplets…
A
=Identical clones, result of separation of developing embryonic cells
-Usually occurs 8 days post-fertilization
9
Q
Dizygotic Twins, Trizygotic Triplets
A
Representations of 2 or 3 separate syngamy events respectively
10
Q
Cleavage
A
=Rapid mitotic divisions of the zygote, resulting in increased numbers of progressively smaller “blastomeres”
- Begins 24 hours after fertilization
- By 96 hours, cleavage -> morula
- Uterine milk -> differentiation of morula -> blastocyst
11
Q
Human Embryo
A
Organism from fertilization to end of 8th week in utero
12
Q
Implantation
A
=Attachment of the blastocyst to the lining of the uterus followed by its burrowing into stratum functionalis
- Usually occurs on posterior wall of uterine fundus or body 6-7 days post-fertilization
- Embryoblast orients itself closest to endometrial lining
- Trophoblast enzymes digest the endometrial surface
- Implantation -> increased endometrial glands -> increased vascularity
- Following implantation, endometrium called the “decidua”
13
Q
Human Fetus
A
Unborn child from 9th week to delivery
14
Q
Gestation
A
Period of intra-uterine development
15
Q
Parturition
A
Childbirth
16
Q
6 Stages of Embryonic Development
A
- Cleavage
- Implantation
- Gastrulation
- Extraembryonic Membrane Formation
- Placenta & Umbilical Cord Formation
- Organogenesis
*First 3 happen within weeks 1-8
17
Q
Morula
A
- Solid ball of totipotent stem cells, capable of making any cell in body or extrasomatic structures of embryofetus
- Morula surrounded by zona pellucida
18
Q
2 Parts of the Blastocyst
A
- Embryoblast: Inner cell mass; made of pluripotent stem cells, become the 3 embyonic germ layers
- Trophoblast: Outer layer; becomes chorion during 2nd week w/ extra-embryonic mesoderm
19
Q
4 Examples of Multipotent Stem Cells
A
- Basal Keratinocytes - Found in skin
- Myeloid Stem Cells - Found in bone marrow
- Lymphoid Stem Cells - Found in lymphatic tissue
- Oogonia & Spermatogonia - Found in gonads
*All can develop into one or more cell type
20
Q
Somatic Cell Nuclear Transfer
A
A type of therapeutic cloning in which the nuclear material from a human somatic cell is used to replace the nuclear material from a human oocyte
21
Q
2 Types of Somatic Cell Nuclear Transfer
A
- Autologous SCNT: Patient’s own somatic cell is used; no immunosuppressive therapy required
- Heterologous SCNT: Somatic cell from different individual used; immunosuppressive therapy required
22
Q
3 Decidual Regions of the Embryo
A
- Decidua Basalis: Located between stratum basalis & embryo; becomes maternal portion of placenta
- Decidua Capsularis: Located between implanted embryo & uterine cavity
- Decidua Parietalis: Rest of endometrium
23
Q
Ectopic Pregnancy
A
=”Out of normal place” pregnancy (e.g., uterine tube, ovary, visceral peritoneum, or cervical canal)
- May result in scarring of uterine tubes
- Smoking may cause implantation within uterine tube
- Leads to a missed menstrual period, then bleeding & acute abdominopelvic pain
24
Q
Eutopic Pregnancy
A
=Normal implantation
-Occurs within endometrium
25
2 Layers of Trophoblastic Cells
1. Synctiotrophoblastic Layer: A multinucleate cell mass w/o a distinguishable cellular membrane boundary
2. Cytotrophoblastic Layer: Inner layer; secures placenta to endometriumn + forms part of chorion w/ synctiotrophoblasts
26
2 Functions of Human Chorionic Gonadotropin (HCG)
1. Prevents degeneration of corpus luteum, maintains production of progesterone & estrogen, inhibin & relaxin
2. Causes corpus luteum to make even more progesterone, which maintans secretory endometrium
*Produced by the placenta & chorion
27
Pregnancy Testing
Tests that determine whether a female is pregnant or not by detecting HCG in urine; can be as early as the 1st day of missed period
28
4 Causes of False Positive Pregnancies
1. Blood or protein in urine
2. Drugs
3. Choriocarcinoma (cancer in the womb/uterus)
4. Missed abortion
29
2 Causes of False Negative Pregnancies
1. Ectopic pregnancy
| 2. Premature testing
30
Gastrulation
=Transformation of the blastula into the gastrula
- Occurs during the 2nd & 3rd weeks post-fertilization
- Conversion of embryoblast initially into bi-laminar embryo
31
2 Layers of the Bi-laminar Embryo
1. Hypoblast = Inner cell mass; made of cuboidal epith.
| 2. Epiblast = Outer layer; becomes meso- and ecto-derm
32
3 Layers of the Tri-laminar Embryo/3 Primary Germ Layers
1. Ectoderm = Formed from epiblast
2. Mesoderm = Also formed from epiblast
3. Endoderm = Formed from hypoblast
*Occurs at around day 16
33
4 Steps of Gastrulation
1. Primitive streak forms; establishes polarity of embryo
2. Epiblast invagination along primitive streak creates space between epiblast & hypoblast
3. Epiblast cells move into space; creating loosely-organized mesoderm
4. Mesoderm + epiblast + hypoblast -> tri-laminar embryo (3 primary germ layers)
34
Ectoderm
- Becomes epidermis & nerve tissue
- Notochord -> neural plate
- Neural plate undergoes neurulation -> neural groove
- Neural groove -> neural tube
- Nervous system develops from neural tube & neural crest
35
3 Primary Brain Vesicles
1. Prosencephalon; becomes telen- & dien-cephalon
2. Mesencephalon
3. Rhombencephalon
*Neural tube below rhombencephalon becomes spinal cord
36
Neural Crest
Develops into PNS, meninges, adrenal medullae, skin melanocytes & certain CTs
37
2 Types of Neural Tube Defects
1. Spina Bifida = Malformation of vertebral arches -> herniation
2. Anencephaly = Malformation of cranial bones & missing cerebrum
*Both often due to folic acid deficiency
38
Mesoderm (2 Parts)
1. Splanchic Mesoderm
2. Somatic Mesoderm
*Both formed from lateral plate mesoderm, which are split up by the intraembryonic coelom
39
Splanchic Mesoderm
Forms the smooth muscle & CT of GI & respiratory tracts, heart, blood vessels & visceral layer of the serous membranes
40
Somatic Mesoderm
Forms the bones, blood vessels & other CTs of the limbs & parietal layer of the serous membranes
41
Somites
Paired, cube-shaped structures that are formed from the paraxial mesoderm
42
3 Regions of Somites Along Neural Tube
1. Myotome: Develops into skeletal muscles
2. Sclerotome: Develops into vertebral column & ribs
3. Dermatome: Develops into dermis
43
Angiogenesis
- Extraembryonic mesoderm -> hemangioblasts -> angioblasts -> blood islands -> network of blood vessels
- Hemangioblasts of yolk sac, allantois & chorion -> blood cells
44
Cardiogenic (Heart) Area
- Becomes endocardial tubes, which become the primitive heart tubes
* Heart becomes connected w/ blood vessel network
45
Endoderm
Develops into epithelia of respiratory tract, digestive tract, liver, pancreas & some endocrine glands
46
4 Extraembryonic Membranes
1. Yolk Sac
2. Amnion
3. Allantois
4. Chorion
47
Yolk Sac
- Formed by hypoblast cells around blastocyst wall
- The nutrient source for human embryo during weeks 2 & 3
- Becomes a site of blood cell formation during weeks 3-6
- Develops into primordial cells that travel to gonads & develop into gametes
- Becomes incorporated into umbilical cord & primitive gut
48
Amnion
- Formed by epiblast cells
- Amniotic cavity forms between amnion & epiblast
- Eventually surrounds the embryo
49
Oligohydramnios
Deficiency of amniotic fluid due to bilateral agnesis of kidneys
50
4 Functions of Amniotic Fluid
1. Mantain constant temperature
2. Fetal movements
3. Shock absorber
4. Prevents desiccation
51
Amniocentesis
- Removal of 10-30 mL of amniotic fuild (containing amniocytes) for analysis
- Performed at 14-18 weeks
- Allows karyotyping of chromosomes
- Risk of spontaneous abortion
52
Allantois
- The caudal vascularized outpocketing of the yolk sac extending into connecting stalk, which connects embryo into trophoblastic tissue
- Connecting stalk becomes umbilical cord
- Blood vessels of allantois become umbilical vein & 2 umbilical arteries
- Allantois eventually incorporated into urinary bladder
*Embrio, amnion & yolk sac suspended within extra-embryonic coelom
53
Chorion
- Is the combination of extraembryonic mesoderm + synctiotrophoblastic & cytotrophoblastic tissues
- Is the outermost fetal membrane
- Forms from trophoblast
- Yolk sac -> mesenchyme, which surrounds bilaminar embryo/ yolk sac/ amnion
- Extra-embryonic coelom -> chorionic cavity
- Helps form the placenta
54
5 Functions of the Placenta
1. Allows exchange of O2/CO2 & nutrients & wastes
2. Protects embryo/fetus from mother's immune system & microbes
3. Makes HCG
4. Produces estrogen & progesterone between months 3-6 of pregnancy
5. Stores nutrients for fetal use
55
Placenta & Umbilical Cord Formation
- Following blastocyst implantation, syntiotrophoblasts open up lacunae, which become lacunar networks
- Endometrial capillaries -> maternal sinusoids
- Synchtiotrophoblasts erode sinusoids & endometrial glands
- Chorionic villi extend into the lacunae
- Blood capillaries develop in chorionic villi, which connect to embryonic heart via blood vessels of connecting stalk
56
Chorionic VIlli
=Fingerlike projections of cytotrophoblastic + synctiotrophoblastic tissue that extend into the lacunae (AKA "Intervillous Spaces")
57
Placentation
- Decidua Basalis & Embryonic Chorion form the placenta
- Connecting stalk becomes umbilical cord, which connects fetus to placenta
- Umbilical vein & two umbilical arteries embedded in Wharton's jelly (made by allantois)
58
Umbilical Vein
Blood vessel that carries O2 and nutrient-rich blood to the embryo/fetus
59
Umbilical Arteries
Blood vessels that return waste-containing O2-poor blood to the placenta
60
3 Actions of Progesterone & Estrogen
1. Inhibit FSH & LH release
2. Maintain endometrial lining
3. Mammary gland development
61
Oral Contraceptives
=Orally ingested progesterone & various progestins
- Used to inhibit GnRH, which inhibits FSH & LH release
- Leads to inhibition of ovarian follicle development
62
3 Commercial/Medical Uses of the Placenta
1. Grafting
2. Cord Blood
3. Hormone Extraction (Human placental growth factors 1 & 2, placental lactogen/hCS)
63
Miscarriage
=Early lacking of HCG, which leads to corpus luteum degeneration
- Leads to cessation of estrogen & progesterone
- Ultimately causes endometrial sloughing
64
Placental Previa
Implantation of blastocyst near internal cervical os
65
Events During 4th Week Post-fertilization
- Embryonic folding (flat tri-laminar embryo) -> cylindrical embryonic form
- Rapid growth of neural tube -> head & tail folds
- Lateral folding around upper yolk sac -> primitive gut -> foregut, midgut, hindgut
- Stomodeum & foregut in close proximity, separated by oropharyngeal membrane
- Hindgut -> cloaca
- Urorectal septum divides cloaca -> urogenital sinus & anorectal canal
- Allantois incorporated into urinary bladder
- Cloaca in close proximity to proctodeum, separated by cloacal membrane
- Development of six paired pharyngeal arches
66
Pharyngeal Clefts
Grooves that separate the pharyngeal arches
67
Pharyngeal Pouches
Outgrowths of the endodermal pharyngeal lining
68
Otic Placodes
Area of the ectoderm that become the future ears
69
Lens Placodes
Area of the ectoderm that become the future eyes
70
Upper Limb Buds
Outgrowths of the mesoderm and ectoderm which become the future upper limbs
71
Lower Limb Buds
Outgrowths of the mesoderm and ectoderm which become the future lower limbs
72
Events During 5th Week Post-fertilization
Head & brain development
73
Events During 6th Week Post-fertilization
Heart develops
74
Events During 7th Week Post-fertilization
Hand & foot digits develop
75
Events During 8th Week Post-fertilization
Eyes & ears develop
76
Events During the end of Week 8
- External genitalia differentiate
- Tail disappears
- Finger/toe webbing also disappear
77
Fetal Development
- 3rd & 4th Months: Skeletal system develops & gender differentiation occurs
- 5th to 9th Months: Rapid weight gain of fetus occurs
78
Calculation of "Due Date"
Usually occurs 280 days from start of last menstruation
79
Teratogens
=Factors that may cause severe congenital anomalies
- Includes substances such as alcohol, nicotine & various drugs
- Are most harmful in first trimester
- Can pass through placenta due to placenta being fairly porous
80
Fetal Alcohol Syndrome (FAS)
=When ethanol enters the placenta & causes:
- Decreased fetal & infant size
- Mental retardation
- Organ defects
- Limb deformity
- Hyperactivity
81
Mechanism of FAS
Ethanol indirectly causes adverse effects by causing the production of acetaldehyde
82
Cocaine Use During Pregnancy
Causes growth retardation, deformity & apnea
83
Cigarette Smoking During Pregnancy
Causes low birth-weight babies & increased risk of feral/infant mortality
84
Radiation & Pregnancy
Causes skeletal deformity & microcephaly
85
Events During the 3 Trimesters
1. First Trimester: Organogenesis
2. Second Trimester: Organ system development
3. Third Trimester: Rapid fetal growth
86
4 Types of Prenatal Diagnostic Tests
1. Ultra-Sound
2. Amniocentesis
3. Chorionic Villi Sampling
4. Non-Invasive Prenatal Testing
87
Ultra-Sound (US)
- High frequency soundwaves -> image ("sonogram")
- Assesses embryo/fetal viability, growth, possible fetal anomalies, maternal reproductive abnormalities & multiparity
- Guide needle insertion in amniocentesis & in chorionic villous sampling
- Some risks to embryo/fetus due to thermal & shear stress
88
Chorionic Villi Sampling
- Ultras-sound guided insertion of catheter through vagina & cervical canal to placenta
- Performed during weeks 8-10
- Has a small risk of spontaneous abortion
89
Non-Invasive Prenatal Testing
- Exampe: Maternal Alpha Fetoprotein Testing
| - If abnormally increased after week 16, a neural tube defect is present
90
4 Types of Hormones of Pregnancy
1. Estrogen, Progesterone & HCG
2. Relaxin
3. Human Chorionic Somaromammotropin
4. Corticotropin Releasing Hormone
91
Estrogen, Progesterone & HCG (Pregnancy)
- During first 3-6 months of pregnancy, corpus luteum secretes estrogen & progesterone
- From months 6-9, placenta secretes estrogen & progesterone
* Increased progesterone causes myometrium to relax & tightens cervix
92
Relaxin
- Inhibits uterine contractions during pregnancy
| - Relaxes the pubic symphysis at parturition
93
Human Chorionic Somatomammotropin
- Prepares mammary glands for lactation
- Stimulates protein synthesis
- Decreases glucose use by mother (hyperglycemia can cause possible gestational diabetes & fetal beta-cell hyperplasia)
- Increase lipolysis & beta-oxidation of fatty acids
94
Corticotropin Releasing Hormone
- Increases in maternal blood until childbirth; functions as a clock-hormone (Determines time of parturition)
- Increasing CRH -> Increasing ACTH -> Increasing cortisol -> Fetal lung development & surfactant production
* Increased cortisol -> Hyperglycemia -> Possible gestational diabetes & fetal beta-cell hyperplasia
95
4 Physical Changes in Pregnant Mother
1. Compression of abdominopelvic organs
2. Increase in total body weight & water
3. Increase in breast size
4. Lordosis of the spine
96
3 Cardiovascular System Changes in Pregnant Mother
1. Increased cardiac output
2. Increased blood volume
3. Possible renal artery compression (possible mechanism of renal hypertension)
97
4 Respiratory Tract Changes in Pregnant Mother
1. Increased tidal volume
2. Increased O2 consumption
3. Decreased bronchial airway resistance
4. Dyspnea (Shortness of breath due to upward compression)
98
4 Gastrointestinal Changes in Pregnant Mother
1. Increased appetite
2. Constipation
3. Nausea/vomiting
4. Increased Risk of Gastro-intestinal Reflux Disease
99
3 Urinary System Changes in Pregnant Mother
1. Increased urinary frequency, urgency & stress incontinence
2. Increased renal blood flow & glomerular filtration rate
3. Increased risk of bladder & kidney infections
100
4 Skin Changes in Pregnant Mother
1. Chloasma (Tanning/dark skin coloration)
2. Linea nigra (Dark line from pubis symphysis to umbilicus)
3. Pigmentation of areolae
4. Striae (Stretch marks)
101
3 Reproductive System Changes in Pregnant Mother
1. Increased uterine weight due to myometrial hyperplasia & hypertrophy
2. Edema of vulva
3. Vascularity of vulva & vagina
102
Pregnancy-induced Hypertension
=Sudden onset of hypertension, proteinuria & generalized edema
- Cause is unknown
- Eclampsia (sudden onset of seizures) can occur if left untreated
103
Initiation of Labor
Increased estrogen & decreased progesterone levels in maternal blood signal the initiation of labor
104
Mechanism of Labor
- Increased placental CRH -> Increased fetal pituitary ACTH -> Increased fetal adrenal cortisol & DHEA
- Placental aromatase converts DHEA into estrogen
- Fetal cortisol -> decreased placental progesterone synthesis, permitting oxytocin-induced uterine contractions
105
3 Actions of Estrogen During Labor
1. Upregulation of uterine oxytocin receptors
2. Uterine smooth muscle cells to form gap junctions
3. PG synthesis -> Enzymatic digestion of cervix
106
5 Steps of Labour Contractions
1. Maternal oxytocin is released
2. Uterine contractions occur
3. Baby's head enters cervical canal
4. Cervix is stretched
5. Stretch receptors signal hypothalamus to release more oxytocin
107
Delayed Delivery
=When parturition is delayed past due date
| -If delayed for > 2 weeks, risk for fetal hypoxia occurs
108
3 Stages of Childbirth
1. Cervical Dilation: Onset of strong, regular rhythmic uterine contractions until cervix is fully dilated
2. Expulsion of the Fetus: From full cervical dilation unti birth
3. Delivery of the Afterbirth: Delivery of placenta & attached fetal membranes; umbilical cord tied off & cut
109
Benefit of Pitocin Medication
Decreased risk of uterine hemorrhage & decreased risk of Sheehan Syndrome
110
Peurperium
=Period of 6 weeks after delivery
- Involution/weight loss of uterus
- Cervical changes (Becomes smaller & firmer)
- Lochia (Discharge of blood from uterus)
111
Dystocia
=Painful/difficult labor
| -Example: Breech presentation (When the feet & buttocks appear first during birth)
112
Respiratory Changes in the Newborn
- First breath triggered by hypercapnia (increase in CO2)
- Ventilatory rate of newborn: 45 breaths/min
- Prematurity problems: If birthweight < 2.5 kg, baby is at risk of developing respiratory distress syndrome of the newborn
113
6 Circulatory Changes in Newborn
1. Foramen ovale closes; becomes fossa ovalis
2. Ductus arteriosus closes
3. Umbilical vein & 2 arteries degenerate (vein becomes ligamentum teres; arteries become medial umbilical ligaments)
4. Ductus venosus becomes ligamentum venosum
5. Blood cell composition changes: Increased red blood cells & decreased white blood cells
6. Heart rate changes: 120-180 beats/min
114
Mechanism of Ductus Arteriosus Closure
-Lung inflation occurs -> Bradykinin is released -> Ductus arteriosus constricts
115
3 Digestive Changes in Newborn
1. Meconium: Earliest stool of the infant
2. Stomach begins to secrete acid
3. Lactose can be digested
116
Inheritance
Acquisition of traits by transmission of genetic information from parents to offspring
117
Genetics
The study of genes & heredity
118
Genome
Complete gene complement of an organism
119
Karyotype
Complete diploid set of chromosomes
120
Gene
Biological unit of heredity
121
Alleles
Alternate forms of the same gene, coding for alternate versions of the same inherited trait
122
Homozygous
Term used when an allele pair is identical for a given trait
123
Heterozygous
Term used when alleles at same locus on 2 homologues differ for a given trait
124
Genotype
Total hereditary information carried by an individual
125
Phenotype
Observable expression of genotype; determined by the genotype, gene interactions, and environmental factors
126
Mutation
Permanent genetically-transmittable change in structure of a gene
127
Nondisjunction
Error in meiosis in which chromosomes are not evenly divided between two daughter cells; causes aneuploidy in resulting zygote
128
Translocation
Fusion of parts of two non-homologous chromosomes
129
Dominant Allele
Version of a gene that is expressed when present in either single or double gene dose
130
Recessive Allele
Version of a gene that must be present in double gene dose to be expressed
131
Genetic Disorders
=Disorders that usually reflect homozygous recessive conditions
*If a dominant allele codes for a lethal defect, the individual will likely die in utero
132
2 Examples of Genetic Disorders (Dominant Alleles)
1. Huntington's disease
| 2. Achondroplasia
133
3 Examples of Recessive Genetic Disorders
1. Cystic fibrosis
2. Tay-Sachs disease
3. Phenylketonuria
* Carriers = heterozygotes who carry a deleterious recessive gene but do not express trait
134
Consanguinous Marriage
A union between two individuals who are related as second cousins or closer
135
Phenylketonuria (PKU)
- A disorder that disables manufacturing of phenylalanine hydroxylase
- Causes inability to utilize phenylalanine, which leads to brain damage
136
Punnett Square
=Diagram used to calculate risk of transmission of PKU to an offspring of a marriage
-For (Pp)*(Pp), on average, if 4 offspring are produced:
1 = normal homozygous dominant
2 = heterozygous dominant
1 = affected homozygous recessive
* P = dominant allele
* p = recessive allele
137
Incomplete Dominance
- Heterozygote phenotype is intermediate between homozygous dominant & homozygous recessive
- Example: In Sickle Cell Disease, SCD heterozygotes express both HbA and HbS phenotypes
*Intra-erythrocytic phase of malarial parasitic infection cannot occur due to HbS in RBCs
138
Multiple Allele Inheritance
- Involves genes that exist in more than 2 allele forms in the population
- Example: Inheritance of ABO blood types (3 possible alleles): I(A), I(B), I(o))
- I(A) and I(B) = co-dominant alleles
- I(O) = recessive allele
139
Genotypes & Phenotypes for Blood Typing
Blood Type A = I(A)*I(A) or I(A)*I(O)
Blood Type B = I(B)*I(B) or I(B)*I(O)
Blood Type AB = I(A)*I(B)
Blood Type O = I(O)*I(O)
140
Sex Chromosome
=23rd pair of chromosomes in a human genome; can be XX or XY
- Traits encoded on these chromosomes are "sex-linked"
- Most gene alleles present only on X chromosome are present in males (X-linked trait)
141
2 Examples of X-linked genetic traits
1. Hemophilia
| 2. Red-green color blindness
142
SRY
=Sex-determining Region of the Y chromosome; a molecular switch to turn on/off male-pattern development
- When SRY is present, embryo will become a male
- When SRY is absent, embryo will become a female
143
X-Chromosome Inactivation
=A random event that causes the inactivation of an X-chromosome
*Barr body = permanently active X-chromosome
144
Polygenic Inheritance
Interaction of several gene pairs to produce phenotypes
145
Complex Inheritance
=When an inherited trait is controlled by many genes and environmental influences
-Examples: height, build, skin color
