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Chapter 14. Beyond Mendel’s Laws of Inheritance AP Biology 2004-2005 1 Extending Mendelian genetics Mendel worked with a simple system peas are genetically simple most traits are controlled by a single gene each gene has only 2 alleles, 1 of which is completely dominant to the other The relationship between genotype & phenotype is rarely simple AP Biology 2004-2005 2 Incomplete dominance Heterozygotes show an intermediate phenotype RR = red flowers rr = white flowers Rr = pink flowers make 50% less color AP Biology 2004-2005 3 Incomplete dominance P X true-breeding red flowers F1 true-breeding white flowers 100% pink flowers 100% generation (hybrids) self-pollinate 25% red F2 50% pink 25% white 1:2:1 generation AP Biology 2004-2005 4 Incomplete dominance CRCW x CRCW female / eggs male / sperm CR CW CR CW CRCR CRCW CRCW CWCW % genotype CRCR CRCW 25% 25% 50% 50% CRCW CWCW 25% 25% 1:2:1 AP Biology % phenotype 1:2:1 2004-2005 5 Co-dominance 2 alleles affect the phenotype in separate, distinguishable ways ABO blood groups 3 alleles = IA , IB, and i both the IA & IB alleles are dominant to the i allele IA & IB alleles are codominant to each other AP Biology determines presences of oligosaccharides on the surface of red blood cells 2004-2005 6 Blood type genotype phenotype phenotype status IA IA IA i type A type A oligosaccharides on surface of RBC __ IB IB IB i type B type B oligosaccharides on surface of RBC __ type AB both type A & type B oligosaccharides on surface of RBC universal recipient type O no oligosaccharides on surface of RBC universal donor IA IB ii AP Biology 2004-2005 7 Blood compatibility Matching compatible blood groups is critical for blood transfusions A person produces antibodies against foreign blood factors = oligosaccharides AP Biology if donor’s blood has an A or B oligosaccharide that is foreign to the recipient, antibodies in the recipient’s blood will bind to the foreign molecules cause the donated blood cells to clump together & can kill the recipient 2004-2005 8 Blood donation AP Biology 2004-2005 9 Pleiotropy Most genes are pleiotropic one gene affects more than one phenotypic character wide-ranging effects due to a single gene: dwarfism (achondroplasia) gigantism (acromegaly) AP Biology 2004-2005 The genes that we have covered so far affect only one phenotypic character, but most genes are pleiotropic 10 Acromegaly: André the Giant AP Biology 2004-2005 11 Pleiotropy It is not surprising that a gene can affect a number of organism’s characteristics consider the intricate molecular & cellular interactions responsible for an organism’s development cystic fibrosis sickle cell anemia Timothy disease AP Biology 2004-2005 12 Epistasis One gene masks another coat color in mice = 2 genes pigment (C) or no pigment (c) more pigment (black=B) or less (brown=b) cc = albino, no matter B allele AP Biology 2004-2005 13 Polygenic inheritance Some phenotypes determined by additive effects of 2 or more genes on a single character phenotypes on a continuum human traits skin color height weight eye color intelligence behaviors AP Biology 2004-2005 14 Albinism Johnny & Edgar Winter albino Africans AP Biology 2004-2005 15 Nature vs. nurture Phenotype is controlled by both environment & genes Human skin color is influenced by both genetics & environmental conditions Color of Hydrangea flowers is influenced by soil pH AP Biology 2004-2005 • The relative importance of genes & the environment in influencing human characteristics is a very old & hotly contested debate • a single tree has leaves that vary in size, shape & color, depending on exposure to wind & sun • for humans, nutrition influences height, exercise alters build, suntanning darkens the skin, and experience improves performance on intelligence tests • even identical twins — genetic equals — accumulate phenotypic differences as a result of their unique experiences 16 It all started with a fly… Chromosome theory of inheritance experimental evidence from improved microscopy & animal breeding led us to a better understanding of chromosomes & genes beyond Mendel Drosophila studies AP Biology A. H. Sturtevant in the Drosophila stockroom 2004-2005 17 Chromosome theory of inheritance Experimental evidence from improved microscopy & animal breeding led us to a better understanding of chromosomes & genes beyond Mendel AP Biology Drosophila studies A. H. Sturtevant in the Drosophila stockroom 2004-2005 18 Thomas Hunt Morgan Morgan was an embryologist at Columbia University 1st to associate a specific gene with a specific chromosome Drosophila breeding AP Biology prolific 2 week generations 4 pairs of chromosomes XX=female, XY=male 2004-2005 19 Morgan’s first mutant… Wild type fly = red eyes Morgan discovered a mutant white-eyed male AP Biology trace a gene for eye color to a specific chromosome 2004-2005 20 Discovery of sex linkage red eye female x white eye male all red eye offspring 75% red eye female x 25% white eye male How is this possible? Sex-linked trait! AP Biology 2004-2005 21 Sex-linked traits Although differences between women & men are many, the chromosomal basis of sex is rather simple In humans & other mammals, there are 2 sex chromosomes: X & Y 2 X chromosomes develops as a female: XX redundancy an X & Y chromosome develops as a male: XY no redundancy AP Biology 2004-2005 22 Sex chromosomes autosomal chromosomes sex chromosomes AP Biology 2004-2005 23 Genes on sex chromosomes Y chromosome SRY: sex-determining region master regulator for maleness turns on genes for production of male hormones pleiotropy! X chromosome other traits beyond sex determination hemophilia Duchenne muscular dystrophy color-blind AP Biology 2004-2005 Duchenne muscular dystrophy affects one in 3,500 males born in the United States. • Affected individuals rarely live past their early 20s. • This disorder is due to the absence of an X-linked gene for a key muscle protein, called dystrophin. • The disease is characterized by a progressive weakening of the muscles and loss of coordination. 24 Map of the Y chromosome? AP Biology 2004-2005 25 Sex-linked traits HXh x X HY HH XHh sex-linked recessive XH male / sperm female / eggs XH Y XH XHXH XHY Xh XhY AP Biology XHXh XHXh Xh XH XHY Y 2004-2005 26 Sex-linked traits summary X-linked follow the X chromosomes males get their X from their mother trait is never passed from father to son Y-linked very few traits only 26 genes trait is only passed from father to son females cannot inherit trait AP Biology 2004-2005 27 AP Biology 2004-2005 28 AP Biology 2004-2005 Hemophilia is a sex-linked recessive trait defined by the absence of one or more clotting factors. • These proteins normally slow and then stop bleeding. Individuals with hemophilia have prolonged bleeding because a firm clot forms slowly. • Bleeding in muscles and joints can be painful and lead to serious damage. Individuals can be treated with intravenous injections of the missing protein. 29 X-inactivation Female mammals inherit two X chromosomes one X becomes inactivated during embryonic development condenses into compact object = Barr body AP Biology 2004-2005 30 X-inactivation & tortoise shell cat 2 different cell lines in cat AP Biology 2004-2005 31 Male pattern baldness Sex influenced trait autosomal trait influenced by sex hormones age effect as well: onset after 30 years old dominant in males & recessive in females B– = bald in males; bb = bald in females AP Biology 2004-2005 32 Mechanisms of inheritance What causes the differences in alleles of a trait? yellow vs. green color smooth vs. wrinkled seeds dark vs. light skin Tay sachs disease vs. no disease Sickle cell anemia vs. no disease AP Biology 2004-2005 33 Mechanisms of inheritance What causes dominance vs. recessive? genes code for polypeptides polypeptides are processed into proteins proteins function as… enzymes structural proteins hormones AP Biology 2004-2005 34 How does dominance work: enzyme = allele coding for functional enzyme = allele coding for non-functional enzyme = 50% functional enzyme • sufficient enzyme present • normal trait is exhibited AP Biology Aa carrier = 100% non-functional enzyme • normal trait is not exhibited aa = 100% functional enzyme • normal trait is exhibited AA 2004-2005 35 How does dominance work: structure = allele coding for functional structural protein AP Biology = allele coding for non-functional structural protein = 50% functional structure • 50% proteins malformed • normal trait is not exhibited Aa = 100% non-functional structure • normal trait is not exhibited AA = 100% functional structure • normal trait is exhibited aa 2004-2005 36 Prevalence of dominance Because an allele is dominant does not mean… it is better it is more common Polydactyly: dominant allele AP Biology 2004-2005 37 Polydactyly individuals are born with extra fingers or toes dominant to recessive allele for 5 digits recessive allele far more common than dominant → 399 individuals out of 400 have only 5 digits → most people are homozygous recessive (aa) AP Biology 2004-2005 38 Hound Dog Taylor AP Biology 2004-2005 39