Pages 277-279

Question 1

if a phenotype is due to an autosomal recessive allele, then

Answer 1

(1) Individuals with the trait must be homozygous.

(2) If the parents of an affected individual do not have the trait, then the parents are heterozygous for the trait.

Question 2

carriers

Answer 2

heterozygous individuals who carry a recessive allele for an inherited disease. When two carriers mate, about ¼ of their offspring are expected to express the recessive phenotype.

Question 3

A recessive phenotype should show up in offspring only when

Answer 3

both parents have that recessive allele and pass it on to their offspring.

Question 4

Autosomal recessive trait:

Answer 4

• Males and females are equally likely to be affected
• Affected offspring often have unaffected parents
• Unaffected parents of affected offspring are heterozygous (carriers)
• Affected offspring are homozygous
• If both parents are heterozygous, about ¼ of the offspring will be affected
• Trait often skips generations

Question 5

autosomal dominant trait

Answer 5

• Males and females are equally likely to be affected
• Affected offspring have at least one affected parent
• Affected offspring are heterozygous if only one parent is affected
• Unaffected offspring are homozygous recessive
• If one parent is heterozygous, about ½ of the offspring will be affected
• Trait does not skip generations

Question 6

If a trait appears about equally often in males and females

Answer 6

then it is likely to be autosomal. But if males express the trait in question more often than females, then the allele responsible is likely to be recessive and found on the X chromosome.

Question 7

A key characteristic of X-linked recessive traits is that

Answer 7

males express the trait more often than females.

Question 8

The appearance of an X-linked recessive trait usually

Answer 8

skips a generation in the pedigree.

Question 9

X-linked recessive trait

Answer 9

• Males are affected more frequently than females
• Trait is never passed from father to son
• Affected sons are usually born to carrier mothers
• About ½ of the sons of a carrier mother will be affected
• All daughters of affected males and unaffected non-carrier females are carriers
• Trait often skips generations

Question 10

X-linked dominant trait

Answer 10

• Males and females are equally likely to be affected
• All daughters of an affected father are affected, but no sons
• Affected sons always have affected mothers
• About ½ of the offspring of an affected mother will be affected
• Affected daughters can have an affected mother or father
• Trait does not skip generations