1. Determining Inheritance using PunnettSquares

A Punnett is a means to determine the genetic inheritance ofoffspring if the genotypes of both parents are known. Using Punnettsquares answer the questions about the following scenarios. Inorder to properly answer some of the questions more than 1 Punnettsquare might be needed. With every Punnett square provide a key foryour alleles.

1. Neurofibromatosis (NF), sometimes called von Recklinghausendisease, is one of the most common genetic disorders. It affectsroughly 1 in 3,000 people. It is seen equally in every racial andethnic group throughout the world. At birth or later, the affectedindividual may have six or more large tan spots on the skin. Suchspots may increase in size and number and become darker. Smallbenign tumors called neurofibromas may occur under the skin or inthe muscles. They are made up of nerve cells and other cell types.NF is a dominant disorder. Jane has NF and wantsto figure out what the chances are that her children will inheritthe disorder if her husband, John, does not have NF.

1. The condition of sickle cell anemia isrecessive and only the homozygous recessiveindividuals are adversely affected. Interestingly, heterozygousindividuals have increased resistance to Malaria which is caused bya Plasmodium parasite that infects red blood cells. Aheterozygous individual has red blood cells that are minimallyaffected but are altered enough so that Plasmodium cannotinfect the cells. This is one reason why this allele persists inthe population – in the heterozygous state it gives the individualan advantage for survival in some environments.

Veronica, who has the sickle cellcondition, and Mason who does not have the condition have a child.They are worried about the having a child with sickle cell anemiaand malaria because they are moving to a part of Africa where thePlasmodium is common. Who in this family should be worriedabout sickle cell anemia and contracting malaria?

1. All newborn babies are tested to see if they have a defectiveenzyme which cannot break down the amino acid phenylalanine. Ifphenylalanine accumulates in the blood then high levels can causemental retardation. The condition is called phenylketonuria or PKU.PKU is a recessive allele and only homozygous recessive individualsare affected. A family living in a country where PKU testing is notdone wanted to know if they should have their baby in the US. Thepregnant mother has a parent who has the PKU condition and thefather also has one parent with the condition. Neither the pregnantmother nor the father has the condition. Draw a punnett squaredemonstrating the likelihood that this new baby will inherit PKUand discuss why or why not the parents should have the baby in theUS. (K = dominant, non-disease allele; k = recessive, diseaseallele)
2. Huntington’s disorder is caused by a dominant allele.Heterozygous individuals with one copy of this allele suffer fromprogressive degeneration of the nervous system and premature death.The phenotype or symptoms do not appear until the affectedindividual reaches ~40 years of age. This is after child-bearingage, so people have children before they know they are carrying theaffected gene allele.

Jermaine is very worried about passingthe dominant Huntington’s gene allele to his children. IfJermaine’s father has the Huntington’s Disorder, his mother doesnot, and his wife has no history of Huntington’s, does Jermaineneed to be tested for Huntington’s before he has children? Note:homozygous dominant is lethal and a fetus will not survive. Draw apunnett square representing the Jermaine’s father and mother, todetermine his chances of carrying the Huntington’s disease allele.(H = dominant, disease allele; h = recessive, non-diseaseallele)

What is the probability that Jermainehas the Huntington’s disease allele? Should he be tested?

If Jermaine is positive for theHuntington’s allele, what are the chances he will pass it on to hischildren? Do a second punnett square to illustrate this cross.