Lecture 6 - Pedigrees and Sex Chromosomes

• In a pedigree what is an unaffected male represented by?

  A square
• In a pedigree what is an unaffected female represented by?

  A circle
• In a pedigree what is and unaffected person whose sex is unknown or unspecified represented by?


  A diamond
• In a pedigree what is a male that is affected by the trait represented with?

  A filled-in square (red)
• In a pedigree what is a female that is affected by the trait represented with?

  A filled in circle (red)
• In a pedigree what is a sex unknown or unspecified that is affected by the trait represented with?

  A filled in diamond (red)
• In a pedigree, what symbol is an obligate carrier (carries the gene does not have the trait) male represented by?

  Square with a circle in the middle
• In a pedigree, what symbol is an obligate carrier (carries the gene does not have the trait) female represented by?


  A circle with a filled in circle in the middle
• In a pedigree, what symbol is an obligate carrier (carries the gene does not have the trait) who is sex unknown or unspecified represented by?

  A diamond with a filled in circle
• Since we now have a good feel for what gender belongs to each shape (male-square, female-circle, sex unknown-diamond) how can we tell on a pedigree if they are an asymptomatic carrier (unaffected at this time but may later exhibit the trait?

  A line through the shape
• In a pedigree how can we tell if its multiple persons?

  A number in the middle of the shape
• How can we tell if the person is deceased?

  A diagonal line through the shape
• If its proband (first affected family member coming to attention of geneticist)?

  A "P" with an arrow beside the shape
• If the family history is unknown?

  A question mark inside the shape
• View slides for diagrams and descriptions of figures
• What are the 5 pedigree characteristics of autosomal recessive traits?

  1. Usually appears in both sexes with equal frequency
  2. Tends to skip generations
  3. Affected offspring are usually born to unaffected parents
  4. When both parents are heterozygous, approximately one fourth of the offspring will be affected
  5. Appears more frequently among the children of consanguineous marriages
• Pedigree characteristics of autosomal dominant traits

  1. Usually appears in both sexes with equal frequency
  2. Both sexes transmit the trait to their offspring
  3. Does not skip generations
  4. Affected offspring must have an affected parent unless they possess a new mutation
  5. When one parent is affected (heterozygous) and the other parent in unaffected, approximately half of the offspring will be affected.
  6. Unaffected parents do not transmit the trait
• Human sex chromosomes:

  • Almost none of the genes in the X chromosome have counterparts in the Y chromosome, they have very small regions of homology
• Pedigree characteristics of X-linked recessive traits

  1. Usually more males than females are affected
  2. Affected sons are usually born to unaffected mothers; thus, the trait skips generations
  3. Approximately half of a carrier (heterozygous) mother's sons are affected
  4. Never passed from father to son
  5. All daughters of affected fathers are carriers
• 1. X-linked recessive alleles are expressed in males because males have only one X-chromosome
  2. A mutation seen in the males phenotype because Y does not carry an allele of this gene
  3. 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
• Segregation of the Sex chromosomes

  • 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
  • Random fertilization results in an expected ratio of 1/2 XX (female) and 1/2 XY (male)
• Heterozygous Cross: female carrier and normal male

  • Among progeny from a heterozygous carrier female, half of the daughters are expected to be heterozygous carriers and half of the sons are expected to be affected
• Affected male must pass mutant X chromosome to daughter

  • Among progeny from a normal female with an affected male, all of the daughters are expected to be heterozygous carriers and all of the sons are expected to be normal
• Colour blindness

  • For a rare X-linked recessive trait, most affected individuals are male
  • The heterozygous daughters of affected males can have affected sons
  • The offspring of an affected male are not usually affected
  • The sisters of an affected male can have affected sons