Problem-Solving Tips

1)If you have to decide the type of inheritance:

  1. Memorize pedigree characteristics of each type:
  2. autosomal recessive => afflicted = aa
  3. the disease can appear in the progeny of unaffected parents
  4. the affected progeny include both males and females
  5. both affected mothers and fathers transmit to offspring
  6. autosomal dominant => afflicted = AA, Aa
  7. phenotype usually appears in every generation
  8. afflicted offspring have parents afflicted parents
  9. both affected mothers and fathers transmit to offspring
  10. offspring with disease appear in equal sex ratios
  11. X-linked recessive => afflicted females = XaXa, afflicted males = XaY
  12. Many more males than females show the phenotype
  13. None of the offspring of an affected male are affected, but his daughters are all heterozygote carriers
  14. No sons of an affected male will show disease or pass it to their offspring
  15. X-linked dominant => afflicted females = XAXA or XAXa, afflicted males = XAY
  16. Affected males pass the condition to all of their daughters but to none of their sons
  17. Affected heterozygous females married to unaffected males pass the condition to half their sons and daughters
  18. Now, look at the information seen in the problem
  19. Sex ratio of afflicted
  20. Go through the above checklist and see what you see in the pedigree
  21. Now, use hypothetical: “If the inheritance type is autosomal recessive”
  22. Then list known genotypes under hypothetical inheritance types
  23. Do punnett squares to see possible offspring, and see if known information matches that punnett square
  24. Iterate hypothetical until you’re satisfied with the choice you made for inheritance type

2)If you are given the inheritance type and have to discern a genotype or other information

  1. List all the information you are given
  2. Fill in all genotypes you know for sure
  3. Use process of elimination and Punnett square possibilities to determine remaining unknown genotype

Examples:

21. The first child born to a couple with normal pigmentation is an albino. What are the genotypes of the parents?

FemaleMale

A.AAAA

B. Aaaa

C.AaAA

D.AaAa

E.aaaa

Problem-solving approach:

1)list all known information

  1. albino = autosomal recessive
  2. child’s genotype = aa
  3. parents = normal, genotypes = A_

2)deduce unknown information

  1. To be albino, the child needed an a from both parents, and both parents have an A, so the parents must both be Aa

Note: Use this additional information (plus the information above on skin pigmentation) in answering questions 25-27. T = tongue roller, t = non-tongue roller.

25. A couple who are normal pigmented and can roll their tongues have an albino child who cannot roll his/her tongue. What are the genotypes of the parents?

FemaleMale

A.AATtAATt

B. AaTtAATt

C. AaTtAaTt

D.AattAatt

E.aaTtaaTt

26. The couple in question #25 are expecting a second child and they would like to know the chances that the expected child will be a non-tongue rolling albino.

A. 1/4

B. 3/4

C. 1/16

D. 3/16

E. 9/16

Problem solving approach:

  • 25
  • List all known info
  • Albino is autosomal recessive, toungue-rolling is autosomal dominant
  • Parents = not albino, toungue rollers
  • Genotypes = A_, T_
  • Kid = albino, can’t roll tongue
  • Genotype = aatt
  • Deduce unknown info
  • Kid got a recessive allele from each parent, so each parent must be a carrier, they both must be AaTt
  • 26
  • Unknown info needed: probability that next kid is a non-tonguerolling albino
  • Known info: non-rolling albino = aatt, parents = AaTt
  • To deduce unknown info: I’ll have to write out a punnett square and look at the probability of getting aatt. After writing out the punnett square, I see that the probablility is 1/16