NOTES: 14.1-14.2: HUMAN HEREDITY
Human Genes: The human genome is the complete set of genetic information -it determines characteristics such as eye color and how proteins function within cells
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
What makes an allele dominant or recessive? Gene = sequence of DNA that codes for a protein Usually: Dominant alleles code for the correct production of that protein Recessive alleles code for no / wrong protein production Heterozygous condition the normal (dominant) allele will still cause correct protein production
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)
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
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)
Can t trace every human trait through a pedigree because some genes are polygenic Shape of your eyes Shape of your ears Height Eye color Also, phenotype is influenced by your environment (ex: nutrition & exercise) Average height is 10 cm more than it was in the 1800 s in the US & Europe due to nutritional improvements Genes that are denied a proper environment in which to reach full expression in 1 generation, can achieve full potential in a later generation (genes are inherited, the environment is not)
REVIEW: Human Blood Groups
Blood Groups Human blood comes in a variety of genetically determined blood groups Using the wrong blood during a blood transfusion can be fatal A number of genes help determine blood type but we will focus on two: ABO blood groups Rh blood groups
Blood Groups Rh factor: The Rh blood group is determined by a single gene with 2 alleles positive and negative The positive allele is dominant You need two Rh- alleles (Rh-/Rh-) to be Rh negative
Rh Factor The Rh factor genetic information is also inherited from our parents, but it is inherited independently of the ABO blood type alleles. There are 2 different alleles for the Rh factor known as Rh+ and Rh-. Mother Father Child Rh- Rh+ Rh+ Rh- Rh- Rh-
The Rh Issue Mom = Rh- Baby #1 = Rh+
ABO Blood Groups This is a case of multiple alleles There are 3 alleles for this gene A, B, and O. AND A and B are CODOMINANT! O is recessive to A and B Alleles A and B produce antigens (which are carbohydrates) on the surface of red blood cells O produces NO antigens
ABO Blood Groups the wrong blood can be FATAL Antigens are recognized by the immune system and induce an immune response If the wrong blood is transfused, the body will respond to these antigens by producing antibodies Antibodies are named for what they attack Antibodies bind to the foreign molecule (the antigen) and blood clumping will occur, which leads to blood clotting, which leads to death
ABO Blood Groups If you have blood type A, then you have: The A antigen on the surface or your RBCs You have anti-b antibodies You can receive type A blood and type O blood Remember: Your antibodies are named for what they attack so if you received type AB or type B blood then clumping would occur.
ABO Blood Groups Try this cross Cross a person who s genotype is I A I A with a person who is I A I B. Give the possible genotypes and phenotypes
Cross a person who s genotype is I A I A with a person who is I A I B. Gametes = I A, I A Cross: and I A, I B I A I A I A I A I A I A I A I B I A I B I A I B
Genotypes= ½ I A I A, ½ I A I B Phenotypes = ½Type A, ½ Type AB
Human Genes & Chromosomes: Vocabulary: Sex-linked gene Sex-influenced gene Key Concepts: Why are sex-linked disorders more common in males than in females? Lou Gehrig: died at 37 of ALS
Human Genes & Chromosomes: Chromosomes 21 & 22 are the smallest human autosomes Chromosome 22 contains approximately 43 million DNA bases (approx. 545 genes) Chromosome 21 contains approximately 32 million DNA bases (approx. 225 genes) 1 of these genes is associated with ALS (Amyotrophic Lateral Sclerosis) Causes a progressive loss of muscle control due to the destruction of nerves in the brain and spinal cord
Remember Humans have 46 chromosomes (23 pair) 2 of them are sex chromosomes they determine what sex you are XX = female XY = male 44 of them are autosomes they do not determine what sex you are
Who determines the sex of a child? The mother or the father? THE FATHER!!!!
Why does the father determine the sex of the offspring??? Mom is XX, she can donate either one X chromosome or the other X chromosome Dad is XY, he can donate either an X chromosome or a Y chromosomes. If the offspring receives the father s X, it is female If the offspring receives the father s Y, it is male
The Y chromosome: If a Y chromosome is present, the person is male X chromosomes contain genes necessary for growth / development No cases of a person born with being 45,0Y Probably spontaneously aborted (miscarriage)
Sex Linked Genes: Genes carried on the X or Y chromosome are sex-linked because they are on the sex chromosomes Many sex-linked genes are found on the X-chromosome
Sex Linked Genes: All X-linked traits are expressed in males WHY??????? Males only have 1 copy of the X chromosome, while females need 2 copies of the defective gene
Human Sex-Linked Gene Disorders: 1. COLORBLINDNESS: 3 human genes associated with color vision are located on the X-chromosome In males, a defective version of any one of these produces colorblindness Females must receive 2 copies of the allele to be colorblind X C X C X C X c X C Y X c Y
More Human Sex-Linked Gene 2. HEMOPHILIA Disorders 2 important genes on the X-chromosome that code for proteins that control blood clotting A recessive allele in either of these 2 genes may lead to hemophilia - bleeders disease -injections of normal clotting proteins prevent death
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
Sex-Influenced Genes: A person s phenotype is affected by their sex (internal environment is different hormones!) Ex: Pattern Baldness WOMEN: BB: not bald MEN: BB: not bald Bb: not bald Bb: bald bb: bald bb: bald
X-Chromosome Inactivation: Females have 2 X-chromosomes if 1 is enough for males, how does the cell adjust to the extra x-chromosome in females? One X-chromosome is randomly switched off Condenses and is called a Barr body
X-Chromosome Inactivation an example: Tortoise-shell cats: X B Y = brown male X b Y = orange male
X-Chromosome Inactivation an example: Tortoise-shell cats: X B X B = brown female X b X b = orange female X B X b = tortoise-shell female -Can a male cat have tortoise-shell fur?
Tortoise-shell cats! (a.k.a. Torties ) X B X b