Genetics - Pedigrees

Question 1

inheritance of a dominant trait

Answer 1

• usually appears in both sexes with equal frequency
• both sexes transmit the trait to their offspring
• does not skip generations once the trait appears
• when one parent is affected (heterozygous) and the other parent is
  unaffected, approximately half of the offspring will be affected
• unaffected parents do not transmit the trait

Question 2

inheritance of a recessive trait

Answer 2

• usually appears in both sexes with equal frequency
• tends to skip generations
• affected offspring are usually born to unaffected parents
• when both parents are heterozygous, approximately one-fourth of the
  offspring will be affected
• appears more frequently among the children of consanguineous
  marriages

Question 3

X-linked recessive alleles

Answer 3

• expressed in males because males have only one X chromosome
• a recessive mutation is visible in the male’s phenotype because Y does not
  carry an allele of this gene to compensate
• an X chromosome present in a male in one generation must be transmitted
  to a female in the next generation and, in the generation after that, can be transmitted back to a male

Question 4

segregation of the
sex chromosomes

Answer 4

• meiosis in a female results in X-bearing eggs only
• meiosis in a male results in a 1:1 ratio
  of X-bearing and Y-bearing sperm

Question 5

nondisjunction

Answer 5

the separation fails to occur causing both sister chromatids or homologous chromosomes to be pulled to one pole of the cell

Question 6

mechanisms of X-link gene dosage compensation

Answer 6

• in mammals: inactivating one of the two X chromosomes in females
• in Drosophila: hyperactivating the single X chromosome in males
• in C. elegans (a nematode): each of the two X chromosomes has reduced activity, that are collectively equal to the single X in males

Question 7

Barr Body

Answer 7

a highly condensed inactive X chromosome

Question 8

X-linked recessive inheritance

Answer 8

• usually more males than females are affected
• affected sons are usually born to unaffected mothers; thus, the trait
  skips generations
• approximately half of a carrier (heterozygous) mother’s sons are
  affected
• never passed from father to son
• all daughters of affected fathers are carriers

Question 9

X-linked dominant inheritance

Answer 9

• both males and females are usually affected; often, more females than
  males are affected
• does not skip generations. Affected sons must have an affected mother; affected daughters must have either an affected mother or an affected father
• affected fathers pass the trait to all their daughters and none of the
  sons
• affected mothers (if heterozygous; most common) pass the trait to
  half of their sons and half of their daughters

Question 10

Y-linked inheritance

Answer 10

• only males are affected
• passed from father to all sons
• does not skip generations
• neither recessive nor dominant