NOTES: 14.1-14.2 HUMAN HEREDITY
Key Terms Autosomal Recessive Autosomal Dominant Pedigree Tay-Sachs Cystic Fibrosis Phenylketonuria Key Concepts How to read and interpret a pedigree How diseases are caused
Recessive and Dominant Alleles: Some common genetic disorders are autosomal recessive This means that you need two recessive alleles (on any of the 44 chromosomes NOT the sex chromosomes) to express the disease EX: Cystic Fibrosis Other genetic disorders are autosomal dominant Only one allele is needed for the trait to be expressed EX: Huntington s Disease
Autosomal Recessive Disorders: In order to develop an autosomal recessive trait, an individual must have the genotype: aa To be born with a homozygous recessive genotype, both parents must be heterozygotes ( carriers ) or homozygous recessive themselves (although with most disorders, that is not the case) WHY NOT?
From gene to molecule: In both cystic fibrosis and sickle cell anemia, a small change in the DNA of a single gene affects the structure of a protein, causing a serious genetic disorder
Cystic Fibrosis: Caused by a recessive allele on chromosome #7 It is an autosomal genetic disorder Causes digestive and respiratory problems Death around 20-30 years of age How does it happen? Three bases are deleted from the protein, which removes one amino acid The protein cannot fold properly anymore, and is destroyed Result: airway is clogged with mucus
CF Example: Cystic fibrosis heterozygotes (Ff) just one copy of the normal (dominant) allele is enough to supply the cell with the proper proteins to function. Because of this, the normal allele is considered dominant over the recessive allele Therefore, a person who is heterozygous does not suffer from Cystic Fibrosis
Sickle Cell Anemia: One DNA base has been changed Amino acid is valine, instead of glutamic acid Result = abnormal hemoglobin The abnormal hemoglobin forms crystallike structures that change the shape of the red blood cells
Sickle Cell Anemia: The abnormal red blood cells are shaped like a sickle or a half-moon; These RBCs slow blood flow, block small blood vessels, and result in tissue damage and pain.
Sickle Cell Anemia: Three genotypes possible: 1) HH = all healthy hemoglobin 2) Hh = ½ healthy hemoglobin; ½ abnormal hemoglobin; **CODOMINANCE!! 3) hh = all abnormal hemoglobin
Sickle Cell Anemia: Individuals who are Hh do not have serious health problems and can lead relatively normal lives, but- They do show some signs of sickle cell anemia if the availability of oxygen is reduced (i.e. high altitude; strenuous exercise)
Pedigrees Graphical representation recording the line of ancestors Family tree Used for breeding animals (e.g. dogs, cats, horses) Trace genetic disorders
Pedigree Chart: Shows how a trait is transmitted from generation to generation Each row is a generation Circles represent females Squares represent males Shaded in: person expresses that trait Half shaded in: person is only a carrier Clear: person does not carry or express that trait
Symbols to Pedigrees
Tay-Sachs Disease: Autosomal recessive disorder Recessive allele results in the absence of an enzyme that normally breaks down lipids in the central nervous system Without this enzyme, the lipids accumulate in the nervous system and the affected individual dies
Phenylketonuria (PKU): Autosomal recessive disorder; Absence of an enzyme to break down the amino acid phenylalanine The accumulation of phenylalanine causes damage to the nervous system By avoiding phenylalanine in the diet, affected infants can avoid the symptoms of the disorder
Autosomal Recessive Traits & Pedigrees: May skip a generation Affected individuals are born to 2 carriers Males and females affected equally KNOWN carriers will be half-shaded in it is not always possible to know if an individual is a carrier
What about autosomal dominant human traits? A single dominant allele inherited from 1 parent is all that is needed for a person to show the dominant trait.
Simple Dominant Traits: Tongue rolling Unattached earlobes Hitchhiker s thumb Hair in the middle section of fingers Ability to taste PTC
Autosomal Dominant Traits & Pedigrees: Typically seen in every generation, affecting multiple people Affected individuals are born to affected parent(s) Males and females affected equally There are NO carriers! (you either have it AA or Aa or you don t aa)
Autosomal Dominant Disorder: HUNTINGTON S DISEASE Rare, but lethal, dominant allele; Results in a breakdown of parts of the brain; Onset between the ages of 30 and 50; No known treatment or cure; There is a test available to see if you have it may help with the decision to start a family; Every child of an affected individual has a 50/50 chance of being affected (and passing it down)
Sex-Linked Traits and Pedigrees: only (or mostly) males are affected; affected males are born to carrier females; typically not seen in all generations
Queen Victoria s Legacy in Royal Families of Europe