Chapter 12 Section 3-Complex Inheritance of Human Traits

Chapter 12Section 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) Basics of SCA• Hemoglobin is a protein that carries O2

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 RBCDisc Shaped

Sickle Cell RBCHalf-moon or sickle shaped


ii)How sickle cell works?1. Gene for hemoglobin produces protein that differs from

normal hemoglobin by 1 AA2. SCA hemoglobin crystallizes when O 2 levels are low after

hemoglobin delivers O2 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


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 O2 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:

• IA= surface molecule A• IB= 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?

– Males FemalesX Y X X

Passed on toDaughters only

Passed on toSons only

Can be passed to either sons or

daughters


• 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 blindness2. 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


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


C) Hemophilia: An X-linked Trait– Hemophilia A

• Males receive hemophilia allele from carrier mothers on X chromosome

1. 1 recessive allele = hemophilia in males2. 2 recessive alleles = hemophilia in females

– Treatment of hemophilia:1. Blood transfusions2. 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 P1 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 individual2. 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


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 retardation2. Short stature3. Weak muscles through out body4. Short, wide neck with excess skin5. Short/stocky arms & legs6. 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)


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

sterile2. XYY-may be taller than normal3. Turner’s syndrome (XO)- monosomy X-have sex

organs which don’t mature/women sterile/short/heart defects common

Documents

Chapter 12 Section 3-Complex Inheritance of Human Traits