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Degrees of Dominance • Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Extending Mendelian Genetics for a Single Gene • Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: – Sex Linked Traits -When alleles are on the sex chromosomes . (color blindness) – Incomplete Dominance -When alleles are not completely dominant or recessive (pink flowers from red and white phenotype) – Multiple Alleles - When a gene has more than two alleles. (Blood Type) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sex linked traits *Hemophilia *Muscular dystrophy *Red/green colorblindness *Rickets Discovery of Sex Linkage Thomas Hunt Morgan • First to associate a specific gene on a specific chromosome (early 1900’s) • Mated flies Sex linked genes • Genes located on sex chromosomes • Most are usually genes on X chromosomes • Have unique patterns of inheritance – Mothers pass sex-linked alleles to daughters and sons – Fathers pass sex-linked alleles to daughters, NOT sons Muscular Dystrophy • • • • Absence of a key protein called dystrophin Symptoms: weakening of muscles 1 out of 3500 males in US Rarely live past 20’s Hemophilia • Sex linked recessive • Absence of proteins required for blood clotting • Sons born to women with a family history of hemophilia • Queen Victoria of England – Introduced to royal family thru a mutation in one of sex cells: carrier of allele – The royal families of Prusia, Russia, Spain had Hemophilia genes that spread thru their families • In Codominance, phenotypes of both alleles are exhibited in the heterozygote Example: Pink/white flower, Roan horse both colors are being expressed. How can I make lots of money? • If I was a horse breeder and I wanted to sell Roan horses what horse color do I want for my breeding stock? RR (red), rr (white), Rr (Roan)? Draw Punnent Square. Incomplete Dominance • Offspring has a phenotype that is intermediate between the traits of its two parents. • Ex. Red flowering Snapdragon is crossed with a white flowering Snapdragon the offspring is pink flowers. Neither the red or the white Allele is dominant. There is just less pigment. Fig. 14-10-1 P Generation Red CRCR Gametes White CWCW CR CW Fig. 14-10-2 P Generation Red CRCR Gametes White CWCW CR CW Pink CRCW F1 Generation In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties Gametes 1/2 CR 1/ 2 CW Fig. 14-10-3 P Generation Red CRCR White CWCW CR Gametes CW Pink CRCW F1 Generation Gametes 1/2 CR 1/ CW 2 Sperm 1/ 2 CR 1/ 2 CW F2 Generation 1/ 2 CR Eggs 1/ 2 CRCR CRCW CRCW CWCW CW Incomplete Dominance What would happen? • Could a plant breeder produce only pink flowering snapdragons by crossing pinkflowering snapdragons and white – flowering snapdragons? • Why? Multiple Alleles • Most genes exist in populations in more than two allelic forms • For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. • Multiple alleles control the ABO blood groups. Different combinations of three alleles result in four blood phenotypes (A, AB, B, and O) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-11 Allele IA IB Carbohydrate A B i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Genotype Red blood cell appearance Phenotype (blood group) IAIA or IA i A IBIB or IB i B IAIB AB ii O (b) Blood group genotypes and phenotypes • Antigens – Located in Plasma. “Bad Guy” • Antibodies – located on surface of red blood cells. “Anti-A antibodies” Blood Group Antigens Antibodies Can give blood to… Can receive blood from… AB A, B NONE AB AB, A, B, O Universal Receiver A A B A, AB A, O B B A B, AB B, O O NONE A&B A,B,AB,O Universal Donor O What Blood Types will be… • An Universal Donor? O. Does not contain any A or B antigens (bad guys) at all. • An Universal Recipient? AB. Can receive blood from all other blood groups. Did you know? • Another inheritance pattern demonstrated by the ABO blood group is CODOMINANCE. • Explain why? Oh No! We need to go the ER! • You need blood now what? ABO Blood System Rh Factor • Rh is a protein that is attached to a red blood cell. • Rh- is the absence of the Rh factor on the red blood cell. (Just like the O blood cell.) • You can give Rh- blood to a patient with Rh+ blood but not vice versa. • Mother has 1st child…the child is Rh+ (from the father)…baby’s and mother’s blood mix at time of delivery. Once exposed to Rh+ mother makes Rh antibodies. • Mother is pregnant with 2nd child (Rh+, same dad) The mother has Rh antibodies so her body attacks the babies blood cells and she miscarries. Rh factor • Treatment: Rh Immune Globulin- It destroys baby cells in mom’s blood and suppresses mom’s Rh antibody production. ` Fig. 14-15b 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) Ww ww ww Ww ww ww Ww Ww Ww ww 3rd generation (two sisters) WW or Ww Widow’s peak ww No widow’s peak (a) Is a widow’s peak a dominant or recessive trait? Fig. 14-15c 1st generation (grandparents) Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff Ff ff ff Ff Ff Ff ff ff FF or Ff 3rd generation (two sisters) Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait?