HUMAN BIOLOGY

384 Chapter 19

7. Soon after a couple marries, tests show that both the man
   and the woman are heterozygotes for the recessive allele
   that causes sickling of red blood cells; they are both HbA/
   HbS. What is the probability that any of their children will
   have sickle-cell trait? Sickle-cell anemia?


8. A man is homozygous dominant for ten different genes
   that assort independently. How many genotypically
   different types of sperm could he produce? A woman
   is homozygous recessive for eight of these genes and is
   heterozygous for the other two. How many genotypically
   different types of eggs could she produce? What can you
   conclude about the relationship between the number of
   different gametes possible and the number of heterozygous
   and homozygous gene pairs that are present?


9. As is the case with the mutated hemoglobin gene that causes
   sickle-cell anemia, certain dominant alleles are crucial to
   normal functioning (or development). Some are so vital
   that when the mutant recessive alleles are homozygous,
   the combination is lethal and death results before birth or
   early in life. However, such recessive alleles can be passed
   on by heterozygotes (Ll). In many cases, these are not
   phenotypically different from homo zygous normals (LL).
   If two heterozygote parents mate (Ll 3 Ll), what is the
   probability that any of the children will be heterozygous?


10. Bill and Marie each have flat feet, long eyelashes, and
    “achoo syndrome” (chronic sneezing). All are dominant
    traits. The genes for these traits each have two alleles,
    which we can designate as follows:

Dominant Recessive
Foot arch A a
Sneezing S s
Eyelash length E e

In diabetes, severe asthma, many

cancers, and some other common

diseases, affected people typically

show multiple gene-based (multifactorial) traits associated with

the disease. For instance, medical researchers have identified

several dozen separate traits commonly seen in patients with

hard-to-manage asthma. Such complex scenarios have made

it extremely difficult to sort out the genetic basis of many

diseases. Researchers in the field of computational biology now

are developing computer programs that can analyze the genetic

underpinnings of several correlated disease traits at one time.

This approach may provide a much clearer picture of the “gene

networks” that collectively cause disease.

your future

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Bill is heterozygous for each trait. Marie is homozygous

for all of them. What is Bill’s genotype? What is Marie’s

genotype? If they have four children, what is the probability

that each child will have the same phenotype as the parents?

What is the probability that a child will have short lashes,

high arches, and no achoo syndrome?

11. You decide to breed a pair of guinea pigs, one black and
    one white. In guinea pigs, black fur is caused by a dominant
    allele (B) and white is due to homozygosity for a recessive
    allele (b) at the same locus. Your guinea pigs have seven
    offspring, four black and three white. What are the genotypes
    of the parents? Why is there a 1:1 ratio in this cross?

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