NOTES: : 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 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 crystal- like 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-. MotherFatherChild 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 and I A, I B Cross: IAIA IAIA IAIBIAIB IBIB IAIA IAIAIAIA IAIAIAIA IAIBIAIB

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 XCXCXCXC XCXcXCXc XCYXCY XcYXcY

The Test

More Human Sex-Linked Gene Disorders… 2. HEMOPHILIA 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:MEN: BB: not baldBB: not bald Bb: not baldBb: bald bb: baldbb: 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