Blood Type and Sex Linked Inheritance Multiple Alleles Multiple alleles for a particular gene means that more than two alleles exist in the population. This does not mean that the gene in a particular individual possesses more than two alleles. An individual can only have a maximum of two of alleles, one maternal and one paternal, no matter how many alleles exist in the population. Example: a population of dogs can have 5 alleles for coat color at one gene. The Classic Example of Codominance and Multiple Alleles in Humans is BLOOD TYPE Gene that controls ABO blood type codes for an enzyme that makes an antigen on blood cells. Two alleles (IA and IB) are called “A” and “B” which are codominant. A and B can both be expressed on the same cell. Third allele (i) is called “O”) and it is recessive to both A and B Possible Genotypes for Blood Types If the Phenotype: A Genotype(s): AA, Ao If the Phenotype: AB Genotype(s): AB If the Phenotype: B Genotype(s): BB,Bo If the Phenotype: O Genotype(s): oo Possible combinations of alleles A Real Problem: Mom is Type B, Dad is Type O … Baby is Type AB. Is this possible? Bo X ____ oo •Cross: ____ MOM B o o Bo oo o Bo oo Remember, when in doubt, heterozygous is more likely Check your understanding Could an AB father have a type O child? Why? Are the father’s alleles codominant or recessive? O is homozygous recessive. A or B would hide the o allele. Sex Chromosomes: the 23rd pair in humans All others (122) are called autosomes Boy Girl  The Sex Chromosomes The “X” Chromosome Has essential genes Can’t live without it! Females have a 23rd pair = XX (one usually deactivated) The “Y” Chromosome Has few genes Male 23rd pair = XY What’s the probability of having a boy or girl? X X X XX XX 50% Y XY XY 50% Some X-Linked Traits: Hemophilia (missing clotting enzymes) Bleeding disorders in which it takes a long time for the blood to clot. Colorblindness Defect in color vision “Male-pattern” baldness Most are recessive.  A man with normal vision marries a colorblind woman. What is the probability that they will have a colorblind SON? Xc Xc XY •Cross: ____ X ____ MOM Xc Xc Remember, Mom only shows if she is homozygous! X XcX Focus only XconXSONS Y Xc Y (not daughters)! Xc Y 100% (2/2) SONS A hemophiliac man marries a healthy woman. What is the probability that they will have any hemophiliac children? 0% All sons healthy All daughters carriers Colorblindness Red-green colorblindnessRed and green appear to be the same. X-linked Red-green affects: 1 in 10 males in the US 1 in 100 females in the US Remember: Males have just one X chromosome. All X-linked alleles are expressed in males. Other forms: Total Colorblindness Blue-Yellow colorblindness Blue-yellow appears to be the same. It is autosomal Chromosomes 1 to 22 What do you see? http://www.ansci.cornell.edu/usdagen/mult_alleles.html Medline Plus, 2014, US National Library of Medicine, National Institutes of Health http://www.nlm.nih.gov/medlineplus/ency/article/002051.htm Virtual Fruit Fly Lab, 2014, http://www.mhhe.com/biosci/genbio/virtual_labs/BL_15/BL_1 5.html