HUMAN BIOLOGY

9. The mating between two carriers of a lethal trait
   is Ll 3 Ll.

Progeny genotypes: 1/4 LL 1 1/2 Ll 1 1/4 ll.

Phenotypes:

1/4 homozygous survivors (LL)

1/2 heterozygous survivors (Ll)

1/4 lethal (ll) nonsurvivors

10. Bill’s genotype: Aa Ss Ee
    Marie’s genotype: AA SS EE

No matter how many children Bill and Marie have,
the probability is 100 percent that each child will
have the parents’ phenotype. Because Marie can
produce only dominant alleles, there is no way that
a child could inherit a pair of recessive alleles for
any of these three traits—and that is what would be
required in order for the child to show the recessive
pheno type. Thus the probability is zero that a child
will have short lashes, high arches, and no achoo
syndrome.

11. The white-furred parent’s genotype is bb; the black-
    furred parent must be Bb; because if it were BB all
    offspring would be heterozygotes (Bb) and would
    have black fur. A monohybrid cross between a het-
    erozygote and a homozygote typically yields a 1:1
    phenotype ratio. Four black and three white guinea
    pigs is close to a 1:1 ratio.

chapter 20

1. The probability is the same for each child: 1/2, or
   50 percent.


2. a: A male can produce two types of gametes with
   respect to an X-linked gene. One type will possess

only a Y chromosome and so lack this gene; the

other type will have an X chromosome and will

have the X-linked gene.

b: A female homozygous for an X-linked gene will

produce just one type of gamete containing an

X chromosome with the gene.

c: A female heterozygous for an X-linked gene will

produce two types of gam etes. One will contain an

X chromosome with the dominant allele, and the

other type will contain an X chromosome with the

recessive allele.

3. Most of chromosome 21 has been trans located to
   chromosome 14. While this individual has 46 chromo-
   somes, there are in fact three copies of chromosome
   21. The third copy of chromosome 21 is attached to
   chromosome 14.


4. No. Many traits are sex-influenced and controlled by
   genes on autosomes.


5. Fifty percent. His mother is heterozygous for the
   allele, so there is a 50 percent chance that any male
   offspring will inherit the allele. Since males do not
   inherit an X chromosome from their fathers, the geno-
   type of the father is irrelevant to this question.


6. The allele for cystic fibrosis is recessive. Most of the
   carriers are heterozygous for the allele and do not
   have the cystic fibrosis phenotype.


7. Males inherit their X chromosome from their mother, so
   there is no chance that a son of this female will be color
   blind because she doesn’t carry this X-linked trait.

Appendix V A-13

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