19 Survey of Human Genetics key: p. 157

    Dimples Dimples: DD/Dd No dimples: dd
    Free Earlobes Free: LL/Ll Attached: ll
    Tongue Roller Roller: RR/Rr Non-roller: rr
    Widow's Peak: WW/Ww No-peak: ww
    Rh factor Positive: RR/Rr Negative: rr
    Freckles Yes: FF/Ff No: ff
    Hitchhiker's Thumb: HH/Hh No-Thumb: hh
    Hair Form (incomplete) Curly: CC Wavy: Cc
    Straight: cc
    Blood (codominance, mult alleles) Type A: IAIA, IAiO
    Type O: iOiO
    Type B: IBIB, IBiO
    Type AB: IAIB
    Hair Color (polygene) Dark Brown/Black: JJKK/JJKk
    Sandy/Titan: jjkk
    Brown: JjKk/JjKK
    Red: JJkk/Jjkk
    Blond: jjKK/jjKk
    Thiourea Taste Taster: TT/Tt Non-Taster: tt
    PTC Taste Taster: PP/Pp Non-Taster: pp
    Sodium Benzoate Taster: SS/Ss Non-Taster: ss
    Gender Male: XY Female: XX
    Color Vision (X-linked) Normal: XCXc, XCXC, XCY Colorblind: XcXc, XcY
    Interlocking Fingers (X-linked) Left over Right: XIXI, XIXi, XIY Right over Left: XiXi, XiY

    Questions p. 158

    1. Can parents who are non-tongue rollers have a child who is a tongue roller?
      No: parents are homozygous recessive.

    2. Can a parent who is a taster and a parent who is a non-taster have children who are tasters?
      Yes: taster parent has at least one dominant allele.
      How about children who are non-tasters?
      Yes: taster parent is carrier.

    3. Can parents who have wavy hair have a child who has straight hair?
      Yes. Wavy parents are heterozygous.
      How about a child who has curly hair?
      Finally, how about a child who has wavy hair?

    4. Can a parent who has Type A positive blood and a parent who has Type B positive blood have a child who is Type O negative?
      Yes: parents can be IAiO (+-) and IBiO (+-)
      How about Type O positive?
      Yes: parents can be IAiO (+?) and IBiO (+?)
      How about Type A negative?
      Yes: parents can be IAiO (+-) and IBiO (+-)

    5. X-linked recessive traits appear more often in males than females. Why?
      Males have only 1 X chromosome.

    6. In respect to the parents, who is responsible for donating an X-linked recessive trait on to the son?
      The daughter?
      Both parents.

    Questions 1-4 are in reference to Figure 1, (autosomal recessive characteristic like sickle cell anemia).

    1. Individuals showing the autosomal recessive characteristic do not appear in all generations. How is this possible?
      Some individuals are carriers (heterozygous).

    2. From which couple in generation 1 did individual III-5 likely receive the autosomal recessive characteristic?
      III-5 from I-3 and 4 (because II-5 shows characteristic).
      How about individual III-6?
      III-6 from I-5 and 6 (because II-6 and II-7 exhibit the characteristic)

    3. An offspring showing the autosomal recessive characteristic was produced from the marriage between III-7 and III-8. What must be the genotype of III-7?
      III-7 is a carrier of sickle cell - Ss (III-8 must also be a carrier).

    4. The predicted outcomes of a genetic cross may not match the actual result. For example, consider parents I5 and I6 who both must be heterozygous. The phenotypic ratio from this cross predicts a 75% probability of a child expressing the dominant characteristic and a 25% probability of a child expressing the recessive characteristic. However, 2 of the 3 progeny express the recessive characteristic. Does the result of the cross between I5 and I6 violate the rules of statistics?
      No. The predicted ratios assume random fertilization and large numbers of progeny. Small samples can deviate from the predicted result.
    Questions 5-8 are in reference to Figure 2, (autosomal dominant characteristic like Huntington's disease).

    1. Is it possible that an autosomal dominant characteristic can skip a generation?
      No, dominant allele always shows its trait, including heterozygotes.

    2. Why do so few of the individuals in the pedigree show the dominant characteristic? p. 160
      Most diseases are low in frequency due to natural selection.

    3. 37 of the individuals in the pedigree do not show the dominant characteristic. How many of these 37 individuals carry at least one "hidden" allele?
      0: none of the unaffected individuals can carry a "hidden" allele (heterozygote).

    4. Family member I-8 is the original source of the allele in the pedigree. If individual III-1 is unrelated to I-8, how can she (III-1) produce an offspring who shows the dominant characteristic?
      Her husband has the dominant trait. She is homozygous recessive; he is heterozygous.

Lab notes made Jul 14, 2010 by Peter Chen