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Chapter 13
Mendel did not examine plant height and pod shape in his dihybrid crosses. The genes for
these traits are very close together on the same chromosome. How would this have
changed Mendel’s results? (Page 242)
Answer: There would probably be very little if any recombination so the expected
assortment ratios would have been skewed from the expected 9:3:3:1.
What would Mendel have observed in a dihybrid cross if the two loci were 10 cM apart
on the same chromosome? Is this likely to have led him to the idea of independent
assortment? (Page 245)
Answer: About 10% of the progeny would have been recombinants, based on the
relationship of 1 cM (map unit or centimorgan) equals 1% recombination frequency.
When gene loci are separated by greater distances, the frequency of recombination
between them increases to the extent that the number of recombinant gametes roughly
equals the number of parental gametes. In that instance, the genes would exhibit
independent assortment. With a recombination frequency of only 10%, it is doubtful that
it would have led Mendel to the concept of independent assortment.
Over a five-year period between ages 20 and 25, the incidence of Down syndrome
increases 0.1 per thousand; over a five-year period between ages 35 and 40, the incidence
increases to 8.0 per thousand, 80 times as great. The period of time is the same in both
instances. What has changed? (Figure 13.14)
Answer: What has changed is the mother’s age. The older the woman, the higher the risk
she has of nondisjunction during meiosis. Thus, she also has a much greater risk of
producing a child with Down syndrome.
Can you think of two nondisjunction scenarios that would produce an XXY male?
(Figure 13.15)
Answer: XY egg is fertilized by an X sperm. A normal X egg is fertilized by an XY
sperm.
Based upon what you read in this chapter, what reasons could a mother have to undergo
CVS, considering its small but potential risks? (Page 252)
Answer: Advanced maternal age, a previous child with birth defects, or a family history
of birth defects.