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PEDIGREES Heredity in families INTRODUCTION A pedigree is a diagram of family relationships that uses symbols to represent people and lines to represent genetic relationships. These diagrams make it easier to visualize relationships within families, particularly large extended families. Pedigrees are often used to determine the inheritance of genetic diseases or disorders. SAMPLE PEDIGREE SQUARES AND CIRCLES Female You can do this to show a heterozygote/carrier Male Has the disorder Grandparents Married Siblings In a pedigree, squares represent males and circles represent females. Horizontal lines connecting a male and female represent mating. Vertical lines extending downward from a couple represent their children. Subsequent generations are therefore written underneath the parental generations and the oldest individuals are found at the top of the pedigree. If the purpose of a pedigree is to analyze the pattern of inheritance of a particular trait, it is customary to shade in the symbol of all individuals that possess this trait. Female You can do this to show a heterozygote/carrier Male Has the disorder Grandparents Married Siblings A KEY TO A PEDIGREE MALES FEMALES Normal Male Normal Female Carrier (Heterozygote) Carrier (Heterozygote) Male with condition Female with condition LEFT-HANDEDNESS Being left-handed is a recessive trait. Genotype = Phenotype LL=Right-handed Ll=Right-handed ll=Left-handed LEFT-HANDEDNESS In this family, the parents include a ? left-handed mother and a father who ll is right-handed. Two of the children are left-handed. We can figure out the father’s genotype by looking at the children’s phenotype. ll ? ll Since some of the children are Ll left-handed, the dad must be ll heterozygous because the two lefthanded children must have gotten a little l from each parent, but he is right-handed…must be heterozygous! ll ? ll WHAT GENOTYPE DOES THE OTHER CHILD HAVE? Ll ll ll ? ll WHAT GENOTYPE DOES THE OTHER CHILD HAVE? She must be Ll heterozygous, too! She ll had to get a little l from the mom, because that is the only option. She must have gotten a big L from her dad since she isn’t left handed. ll Ll ll A PERSONAL EXAMPLE Mr. Horan has red-green colorblindness. This means he can’t tell these colors apart very easily. (They appear to be different shades of the same color.) He would not be able to see the 8 in the figure to the right. COLORBLINDNESS As you read in an earlier lesson, this particular gene is found on the X chromosome, meaning it is a sex-linked gene. The patterns of inheritance are a little tricky when a gene is found on the X chromosome because males only have one X, but females have two. MR. HORAN’S FAM Mr. Horan is colorblind. He XY ? has one brother that is colorblind. He has one brother who has normal vision. His dad is not colorblind. What do you think his mom’s genotype is? ? XY XcY XcY Mr. Horan’s mother must XY be heterozygous for XXc colorblindness. We know this because she gave an X to all of her children. ? XY XcY XcY She gave a colorblind X to two sons and a normal X to the other. What about Mr. Horan’s sister? She is married and has two kids. Her husband is not colorblind. Neither of her kids are. What do we know about her genotype? XY XY XY ? XX XY XcY XXc XcY We don’t know if Mr. Horan’s sister is a carrier or normal for colorblindness. If one of her kids was colorblind, we could know for certain that she is a carrier. As it is, we just can be sure. We can only say for certain that she has one normal gene. XY XY XY ? XX XY XcY XXc XcY THE END!
