Chapter 12 Patterns of Heredity And Human Genetics

12.1 Mendelian Inheritance and Human Traits

Pedigrees Graphic representation of genetic inheritance

Carrier  heterozygous individual –Individual carries the trait but does not exhibit it –Gg exhibits green; however, carries yellow Most Genetic Disorders are caused by recessive alleles

Cystic Fibrosis Fairly common among white Americans Approx. 1 in 20 carries recessive allele Approx. 1 in 2000 white American children inherit disorder Caused by defective protein in plasma membrane Results in formation and accumulation of thick mucus in lungs and digestive tract

Tay-Sachs Disease Affects central nervous system (CNS) Absence of enzyme that breaks down a lipid that is produced and stored in the CNS Results in accumulation of the lipid in the CNS

Phenylketonuria (PKU) Absence of an enzyme that converts phenylalanine to tyrosine Phenylalanine builds up in the body resulting in damage to the CNS Most common among people whose ancestors came from Norway or Sweden

Simple Dominant Traits A single dominant allele inherited from a parent is all that is needed to exhibit the trait –Tongue Rolling –Free hanging earlobes –Hitchhiker ’ s thumb –Almond shaped eyes –Thick lips –Mid-digit hair

Huntington ’ s Disease Lethal genetic disease caused by rare dominant allele Results in breakdown of certain areas of the brain Onset occurs between the ages of 30 and 50, so individuals may have already passed gene to offspring before discovering they carry the allele

12.2 Heredity following different rules

Incomplete dominance Offspring of two homozygous parents results in an intermediate phenotype Occurs because neither allele is completely dominant

Homozygous red snapdragon (RR) is crossed with a homozygous white (RR) resulting in offspring that are heterozygous pink (RR) R and R are used instead of R and r –R allele codes for a red pigment –R allele is defective; no pigment RR has one R allele that produces ½ the amount of pigment resulting in a flower that appears pink

The F 2 generation produced from RR results in a ratio of 1:2:1  red:pink:white

Codominance Phenotypes of both homozygous parents are displayed in heterozygous offspring Both alleles are expressed equally

Multiple Alleles Traits controlled by more that two alleles

Sex Determination 23 rd pair of chromosomes in humans determines sex –XX = female –XY = male

Sex-linked Inheritance Sex-linked traits are controlled by genes located on the sex chromosomes Traits linked to the X chromosome are masked in heterozygous females and are exhibited in males Traits linked to the Y chromosome are passed from male to male only

Polygenic Inheritance Inheritance pattern of a trait controlled by two or more genes All heterozygotes are intermediate in phenotype Each allele contributes a small, but equal, portion to the trait Results in a continuous range of variability from the minimum to the maximum

12.3 Complex Inheritance

Sickle-Cell Anemia Major health problem in U.S. and Africa Most common in black Americans originating from Africa and white Americans originating from Mediterranean 1 in 12 African Americans is heterozygous for disorder

Homozygous individuals form crystal-like structures in their hemoglobin that change the shape of red blood cells (RBC) Abnormal RBC ’ s are shaped like a sickle (half-moon) resulting in slower blood flow, blockage of small vessels, tissue damage, and pain Sickled cells have short life-span so individual suffers from anemia

ABO Blood typing The gene for blood type, I, has three alleles that code for different proteins Blood types are determined by the presence or absence of certain molecules on the surface of RBC ’ s Not all blood types are compatible so typing is necessary before surgery/transfusion

Sex-linked Traits Most located on X chromosome so trait is passed from mother to son Red-green colorblindness is X-linked –Can ’ t differentiate between red or green Hemophilia A is X-linked –Inability of blood to clot

Polygenic Inheritance in humans Eye Color –Color is actually several different shades Skin color –Light skinned cross with dark skinned results in intermediate skin color

Changes in Chromosome # ’ s Most caused by non-disjunction during meiosis Trisomy 21 (Down ’ s Syndrome) –Extra chromosome #21 –Occurs in approx. 1 in 700 births –Some degree of mental retardation –Incidence  in older mothers (  40)

That ’ s All Folks!!!