BIOL 1140 1st Edition Lecture 10 Outline of Last Lecture I Housekeeping a DNA b Genotype phenotype c Genes chromosomes alleles II Homologous Homozygous a Dominant traits recessive Outline of Current Lecture I Polygenic Inheritance a Eye color skin color hair II Genotype AND Environment Affect Phenotype a Melanin III Sex Influenced Traits a Genes IV Genes and Its affects V The Genetic Basis of Huntington Disease a effects Current Lecture Two different alleles of hemoglobin gene HbA encodes normal hemoglobin HbS encodes for sickle cell hemoglobin Sickle cell anemia HbsHbs homozygous Sickle cell trait HbaHbs heterozygous o Affected individual makes both types of hemoglobin If sickle cell makes anemic why is selection not eliminating this allele Homozygotic sickle cell is lethal Heterozygotic sickle cell have a limit in oxygen transport o But provide resistance to malaria These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Polygenic Inheritance Inheritance of phenotypic traits that depend on many genes Examples o Eye color skin color o Height body size and shape Polygenic traits are usually distributed within a population as a continuous range of values Genotype AND Environment Affect Phenotype Phenotype isn t determined by genotype alone Environmental factors can profoundly influence phenotype Example o Nutrition affects height body size o Disease can affect height body size Robert Wadlow o 1918 1940 o was 2 72 meters 8 feet 11 inches o died at age 22 o Reason for growth pituitary tumor that produced growth hormone What we learned thus far Alleles are copies of the same gene Alleles can be recessive full incomplete or semi or co dominant Alleles can be identical homozygous or different heterozygous Inheritance works for people identical to peas or flies Complex traits can follow polygenic inheritance Eye color depends on melanin but also on the aggregation of melanin What is melanin Melanin is a pigment produced by an enzyme called tyrosinase out of the amino acid tyrosine L tyosine L DOPA Dopaquinone melanin Different tyrosinase alleles have different rates of melanin production Melanin production is also affected by light and several other proteins OUTLINE Genotype phenotype Mendelian patterns of inheritance o Dominant recessive codominant incomplete dominance polygenic epistatic Sex and its genetics o The genetically challenged sex Sex linked effects through autosomal genes Too much of a good thing can be bad trisomies and genetic imbalance Chromosomes Sex chromosomes o 23rd pair of chromosomes o X and Y chromosomes carry many different genes Males have one X and one Y chromosome Female have two X chromosomes Male o 50 X carrying gametes 50 Y carrying gametes o Male gametes determine the gender of offspring Sex Linked Inheritance Depends on Genes Located on Sex Chromosomes Sex linked genes are located on sex chromosomes o Sex linked or X linked inheritance o Characteristics More males than females have the disease Passed to sons by mother from grandfather to grandson Father cannot pass the gene to sons o Examples Red green color blindness Hemophilia Sex Influenced Traits Sex influenced traits Genes encoding these traits are located on the autosomes not the sex chromosomes Expression of the trait is affected by presence of testosterone estrogen Example Baldness o Several genes influence hair patterns but are also influenced by the presence of estrogen or testosterone o You may have the mutation for baldness but you need high level of testosterone to express the phenotype Nondisjunction During Meiosis failure of a pair of homologous chromosomes or sister chromatids to separate Results in eggs or sperms that contain one too many or one too few chromosomes A child conceived with one of these eggs or sperm will inherit one too many or one too few chromosomes o Usually fatal o Can lead to disorders Occurs in males and females but becomes more frequent in older over 40 women Chromosomes May Be Altered in Number Nondisjunction during meiosis Failure of homologous chromosomes or sister chromatids to separate Examples Down syndrome trisomy 21 Alterations of the number of sex chromosomes o XYY Jacob syndrome o XXY Klinefelter syndrome o XXX Trisomy X syndrome o XO Turner syndrome no Y Deletions Piece of a chromosome breaks off Example Cri du chat syndrome lost piece of 5 Translocations Piece of chromosome breaks off and attaches to a different chromosome Chromosomes can fuse Robertsonian imbalance Human and Great Ape ancestors had 48 Chromosomes Genes And Behavior Genes encode specific proteins Proteins have specific functions leading to phenotypes Protein functions Hormones Enzymes Neurotransmitters Inherited Genetic Disorders Many genetic disorders involve recessive alleles o To develop these diseases one recessive allele is inherited from each parent who most often are themselves heterozygous carriers Phenylketonuria PKU o Lack enzyme to metabolize phenylalanine o May cause mental retardation o Treatment limit phenylalanine from diet Tay Sachs disease o Lack an enzyme to metabolize a brain lipid o Leads to brain dysfunction and death by age 4 Single Genes Can Affect the Nervous System and Behavior Several single gene effects on human behavior are known Most affect the development structure or function of the nervous system A Model for Neurodegenerative Disorders Huntington disease is a model for single gene neurodegenerative disorders Huntington disease o An autosomal dominant disorder associated with progressive neural degeneration and dementia o Adult onset is followed by death 10 to 15 years after symptoms appear The Genetic Basis of Huntington Disease HD is one of eight disorders caused by expansion of a CAG trinucleotide repeat o Polyglutamine expansion causes the protein product to become toxic and kill nerve cells With 40 to 60 repeats HD develops in adults With more than 60 repeats HD develops before age 20 Effects of Huntington Disease HD damages specific regions of the brain Striatum and cerebral cortex HD is a loss of function mutation Gene product Htt loses the ability to stimulate BDNF protein needed to maintain the striatum HD is a gain of function mutation Altered Htt protein becomes toxic Brain Degeneration in Huntington Disease Normal brain left and Huntington brain right