Beyond Mendels Laws of Inheritance AP Biology 2006

Beyond Mendel’s Laws of Inheritance AP Biology 2006 -2007

Extending Mendelian genetics Mendel worked with a simple system peas are genetically simple u most traits are controlled by a single gene u each gene has only 2 alleles, 1 of which is completely dominant to the other u The relationship between genotype & phenotype is rarely that simple AP Biology

Alleles For every gene, there are many different alleles versions of the same gene that differ in their DNA base sequence Some alleles differ in the protein product of the gene Your combination of alleles is unique, and makes you Alleles are generated by mutations Variation preceded adaptation AP Biology

Dominant vs. Recessive Common misconception: Recessive allele is not expressed. This is not always true! Tay sachs: lipids can’t be metabolized Organismal recessive, biochemically incomplete dominant, molecularly codominant Half of wild type is enough AP Biology 4

Lethal Alleles Essential genes are those that are absolutely required for survival u The absence of their protein product leads to a lethal phenotype It is estimated that about 1/3 of all genes are essential for survival A lethal allele is one that has the potential to cause the death of an organism u These alleles are typically the result of mutations in essential genes AP Biology

Lethal Alleles Many lethal alleles prevent cell division Some lethal alleles exert their effect later in life u Huntington disease Characterized by progressive degeneration of the nervous system, dementia and early death The age of onset of the disease is usually between 30 to 50 A lethal allele may produce ratios that seemingly deviate from Mendelian ratios Another example is the “creeper” allele in chicken AP Biology

Phenotypic Ratios Associated with Lethal Alleles Creeper X Normal Creeper X Creeper 1 creeper : 1 normal Creeper is a dominant allele AP Biology 1 normal : 2 creeper Creeper is lethal in the homozygous state

Incomplete dominance Heterozygote shows an intermediate, blended phenotype u example: RR = red flowers rr = white flowers Rr = pink flowers w make 50% less color AP Biology RR Rr rr

1: 2: 1 phenotypic ratio NOT the 3: 1 ratio observed in simple Mendelian inheritance In this case, 50% of the CR protein is not sufficient to produce the red phenotype Figure 4. 2 AP Biology

Multiple Alleles The term multiple alleles is used to describe situations when three or more different alleles of a gene exist Examples: ABO blood u Coat color in many species u Eye color in Drosophila u Every gene? http: //ghr. nlm. nih. gov/Browse. Genes, Click on OMIM and the allelic variants table. u AP Biology

Multiple Alleles coat color in rabbits C (full coat color) u cch (chinchilla pattern of coat color) u Partial defect in pigmentation u ch (himalayan pattern of coat color) Pigmentation in only certain parts of the body u c (albino) Lack of pigmentation AP Biology

AP Biology

Co-dominance 2 alleles affect the phenotype equally & separately not blended phenotype u example: ABO blood groups u 3 alleles u I A, I B, i IA & IB alleles are co-dominant to each other w both antigens are produced both IA & IB are dominant to i allele u AP Biology produces glycoprotein antigen markers on the surface of red blood cells

Figure 14. 11 (a) The three alleles for the ABO blood groups and their carbohydrates Allele Carbohydrate IA IB i none B A (b) Blood group genotypes and phenotypes Genotype IAIA or IAi IBIB or IBi IA IB ii A B AB O Red blood cell appearance Phenotype (blood group) AP Biology

Pleiotropy Most genes are pleiotropic u one gene affects more than one phenotypic character wide-ranging effects due to a single gene dwarfism (achondroplasia) gigantism (acromegaly) AP Biology

Epistasis One gene completely masks another gene u coat color in mice = 2 separate genes C, c: pigment (C) or no pigment (c) B, b: more pigment (black=B) or less (brown=b) cc = albino, no matter B allele 9: 3: 3: 1 becomes 9: 3: 4 AP Biology How would you know that difference wasn’t random chance? Chi-square test!

Polygenic inheritance Some phenotypes determined by additive effects of 2 or more genes on a single character u human traits AP Biology skin color height weight eye color intelligence behaviors

Eye Color in Humans Controlled by at least 8 genes Each gene has multiple alleles AP Biology

Sex linked traits Genes are on sex chromosomes u u u as opposed to autosomal chromosomes first discovered by T. H. Morgan at Columbia U. Drosophila breeding good genetic subject w prolific w 2 week generations w 4 pairs of chromosomes w XX=female, XY=male AP Biology

Genetics of Sex AP Biology

Genes on sex chromosomes Y chromosome u few genes other than SRY sex-determining region master regulator for maleness turns on genes for production of male hormones w many effects = pleiotropy! X chromosome u AP Biology other traits beyond sex determination mutations: w hemophilia w Duchenne muscular dystrophy w color-blindness

Human X chromosome Sex-linked Duchenne muscular dystrophy Becker muscular dystrophy usually means “X-linked” u more than 60 diseases traced to genes on X chromosome u Chronic granulomatous disease Retinitis pigmentosa-3 Norrie disease Retinitis pigmentosa-2 Hypophosphatemia Aicardi syndrome Hypomagnesemia, X-linked Ocular albinism Retinoschisis Adrenal hypoplasia Glycerol kinase deficiency Ornithine transcarbamylase deficiency Incontinentia pigmenti Wiskott-Aldrich syndrome Menkes syndrome Sideroblastic anemia Aarskog-Scott syndrome PGK deficiency hemolytic anemia Anhidrotic ectodermal dysplasia Agammaglobulinemia Kennedy disease Pelizaeus-Merzbacher disease Alport syndrome Fabry disease Immunodeficiency, X-linked, with hyper Ig. M Lymphoproliferative syndrome Albinism-deafness syndrome Fragile-X syndrome AP Biology Ichthyosis, X-linked Placental steroid sulfatase deficiency Kallmann syndrome Chondrodysplasia punctata, X-linked recessive Androgen insensitivity Charcot-Marie-Tooth neuropathy Choroideremia Cleft palate, X-linked Spastic paraplegia, X-linked, uncomplicated Deafness with stapes fixation PRPS-related gout Lowe syndrome Lesch-Nyhan syndrome HPRT-related gout Hunter syndrome Hemophilia B Hemophilia A G 6 PD deficiency: favism Drug-sensitive anemia Chronic hemolytic anemia Manic-depressive illness, X-linked Colorblindness, (several forms) Dyskeratosis congenita TKCR syndrome Adrenoleukodystrophy Adrenomyeloneuropathy Emery-Dreifuss muscular dystrophy Diabetes insipidus, renal Myotubular myopathy, X-linked

Map of Human Y chromosome? < 30 genes on Y chromosome Sex-determining Region Y (SRY) Channeling Flipping (FLP) Catching & Throwing (BLZ-1) Self confidence (BLZ-2) Devotion to sports (BUD-E) Addiction to death & destruction movies (SAW-2) note: not linked to ability gene Air guitar (RIF) Scratching (ITCH-E) Spitting (P 2 E) Inability to express affection over phone (ME-2) AP Biology linked Selective hearing loss (HUH) Total lack of recall for dates (OOPS)

X-inactivation Female mammals inherit 2 X chromosomes u one X becomes inactivated during embryonic development condenses into compact object = Barr body which X becomes Barr body is random w patchwork trait = “mosaic” X HX h Xh AP Biology

Functions of Mitochondrion Site of cellular respiration Make DNA components Detoxify ammonia in the liver Required for heme synthesis Cholesterol metabolism Steroid synthesis AP Biology

Mitochondria are Passed from Mother to Offspring In most animals father and mother each contribute the same number of chromosomes to the zygote. Thousands of mitochondria per egg cell, and only a few in the sperm Once the sperm enters the egg, those sperm mitochondria are usually destroyed. The zygote ends up filled with mitochondria from the egg, therefore the zygote inherits the maternal mitochondrial DNA. AP Biology

Examples myoclonic epilepsy and ragged red fiber disease (MERRF) u Leber’s hereditary optic neuropathy (LHON) u Kearns–Sayre syndrome (KSS) u AP Biology

MERRF Cells Normal Skeletal Muscle. Crosssection. http: //img. medscape. com/pi/emed/ckb/neurology/11348151189245 -1189246 -1189490. jpg AP Biology

Nature vs. nurture Phenotype is controlled by both environment & genes Human skin color is influenced by both genetics & environmental conditions Coat color in arctic fox influenced by heat sensitive alleles Color of Hydrangea flowers APinfluenced Biology is by soil p. H