Chapter 12 Section 3-Complex Inheritance of Human Traits

I) Codominance in Humans **Note: phenotypes of both homozygotes are produced in heterozygotes  ex/sickle cell anemia B) Sickle Cell Anemia (SCA) i) Basics of SCA Major health problems in US & Africa Most common among in black Americans w/ origins in Africa & white Americans with origins in the countries that surround the Med. Sea 1/12 African American is heterozygote for SCA

I) Codominance in Humans i)Basics of SCA Hemoglobin is a protein that carries O 2 in red blood cells (RBC) Individuals who are homozygotes for SCA have defective hemoglobins which form crystal-like structures that cause shape changes in RBC Normal RBC Disc Shaped Sickle Cell RBC Half-moon or sickle shaped

I) Codominance in Humans ii)How sickle cell works? 1.Gene for hemoglobin produces protein that differs from normal hemoglobin by 1 AA 2.SCA hemoglobin crystallizes when O­ 2 levels are low after hemoglobin delivers O 2 to cells in narrow capillaries 3.Causes change in RBC from disc shaped  sickle shape (see Fig.12.16) 4.Sickle shaped RBC slow blood flow and block small vessels –Can result in: ◊Pain ◊Tissue damage ◊Short life span of RBC

I) Codominance in Humans iv) Codominance of SCA –Individuals who are heterozygotes for SCA means that they produce both: normal hemoglobin SCA hemoglobin (ex/ of codominance) and they have the SCA trait –These individuals produce enough normal hemoglobins to prevent serious health problems and can live relatively normal lives –These individuals can show signs of SCA related to disorders if O 2 availability reduced

II) Multiple Alleles govern blood type –ABO blood type grp classic example of single gene with multiple alleles in humans i) Human Blood Types –Human blood types determined by presence or absence of certain molecules on surface of RBC –Blood Type Gene: I –(3) alleles that can be present on I gene: I A = surface molecule A I B = surface molecule B i = no surface molecule

GENOTYPES Type A:_____ Type B:____ Type AB:_______ Type O:___ Possibilities Type A:_____ Type B:_____

II) Multiple Alleles govern blood type B) Importance of Blood typing (B.T.) –Blood typing important b/c: Incompatible blood type given through blood transfusion can cause other blood types to clump causing death Can be used to solve parentage: Ex/ Child has blood type AB, Mother has blood type A, what must the father’s blood type be?

II) Sex-linked traits in Humans –Many human traits are determined by genes carried on sex chromosomes –Most genes carried on the X chromosome A) How sex chromosomes are passed on? –MalesFemales XYXX Passed on to Daughters only Passed on to Sons only Can be passed to either sons or daughters

II) Sex-linked traits in Humans If son receives allele on X chromosomes it will be expressed b/c males have only 1 chromosome 2 traits that are X-linked disorders are: 1.Red-green blindness 2.Hemophilia B) Red-green colorblindness –Individual can’t distinguish between red & green –Caused by recessive allele found at 1 of (2) gene sites on X chromosome –Both genes affect red & green receptors in eye cells –Can cause problems w/ driving  traffic lights

Color Blindness Tests

II) Sex-linked traits in Humans C) Hemophilia: An X-linked Disorder –Clotting ability necessary human adaptation –If clotting ability not present, could result in: Excess blood loss due to minor injury Serious blood loss due to internal injury Major blood loss due to bruising –Hemophilia A X-linked disorder causing a blood clotting problem 1/10,000 males have hemophilia 1/100 million female have hemophilia

II) Sex-linked traits in Humans C) Hemophilia: An X-linked Trait –Hemophilia A Males receive hemophilia allele from carrier mothers on X chromosome 1.1 recessive allele = hemophilia in males 2.2 recessive alleles = hemophilia in females –Treatment of hemophilia: 1.Blood transfusions 2.Injections of Factor VIII (this is a blood clotting enzyme that is absent in hemophilic patients

III) Polygenic Inheritance in Humans Many human traits are determined by polygenic inheritance –Ex/ variations in eye color (humans) –Ex/variations in skin color (humans) i) Skin Color: A polygenic trait 1900s-skin color data used to test polygenic inheritance theory Light skinned individual x Dark Skinned individual Intermediate skin color

III) Polygenic Inheritance in Humans –F2 generation: results in range of skin colors from light skinned  dark skinned like P 1 generation w/ most having intermediate skin color –Variation led to skin color being polygenic & has 3-4 genes involved

III) Changes in Chromosome Numbers Remember abnormal chromosomes # usually happens b/c of meiosis accidents (Process Called _________________) Human chromosome: –Pairs of chromosomes = 23 –Total chromosomes = 46 A) Abnormal numbers of Autosomes –Trisomic Humans-those individuals who have either partial/whole extra chromosome –Trisomy usually caused by nondisjunction

III) Changes in Chromosome Numbers i) How abnormal chromosomes are ID’ed? 1.Sample of cells obtained from fetus or individual 2.Metaphase chromosomes photographed & images are enlarged & arranged by length & location of Centromere 3.Chart of chromosomes is called a karyotype Karyotype-def-pg329-chart of metaphase chromosome pairs arranged according to length & location of the Centromere; used to pinpoint unusual chromosome #s in cells Karyotypes valuable in ID’ing unusual chromosome #s in cells

III) Changes in Chromosome Numbers B) Down Syndrome: Trisomy 21 –Most embryo w/abnormal chromosome # result in death of the embryo –Down syndrome only autosomal trisomy that results in embryo surviving to birth and adulthood. –Down syndrome happens in 1/700 live births ii) Symptoms of Down Syndrome: 1.Some degree of mental retardation 2.Short stature 3.Weak muscles through out body 4.Short, wide neck with excess skin 5.Short/stocky arms & legs 6.Facial features: sm. Low set ears/nasal bridge pushed in/ irregular mouth & tongue –Incident of Down Syndrome births is  er in older mothers (especially over 40)

III) Changes in Chromosome Numbers C) Abnormal numbers of sex chromosomes –Many abnormalities in # of sex chromosomes –Any individuals w/ at least one Y chromosome is male –Most individuals w/abnormal # of sex chromosomes lead normal lives, but some may result in: 1.Klinefelter syndrome (XXY)-sm testes, usually sterile 2.XYY-may be taller than normal 3.Turner’s syndrome (XO)- monosomy X-have sex organs which don’t mature/women sterile/short/heart defects common