Pedigrees Knowledge-Based Study

Study notes

1. A pedigree is a standardized visual chart that displays phenotypic data and biological relationships across multiple generations of a family. In classical Mendelian genetics, traits follow simple dominant or recessive inheritance patterns encoded by autosomal nuclear genes. However, many inherited conditions exhibit non-Mendelian inheritance, including sex-linked transmission, codominance, and organellar (mitochondrial) inheritance.

   Standard pedigree conventions use specific symbols to represent individuals and unions:

   • Squares designate biological males, and circles designate biological females.
   • Shaded symbols represent individuals who express the trait or condition being analyzed, whereas unshaded symbols represent unaffected individuals.
   • Horizontal lines connecting a male and female indicate a mating pair, while vertical lines descend to connect parents to their offspring.
   • Generations are numbered using Roman numerals (I, II, III), and individuals within a generation are numbered from left to right with Arabic numerals (1, 2, 3).

   Recognizing deviations from Mendelian ratios allows geneticists to deduce whether a trait is governed by non-Mendelian mechanisms.

   Genes located on the sex chromosomes do not segregate in the same manner as autosomal genes because males possess hemizygous sex chromosomes ($XY$) and females possess homologous sex chromosomes ($XX$).

   X-Linked Recessive Inheritance

   In X-linked recessive inheritance (such as hemophilia or red-green color deficiency):

   • Higher incidence in males: Because males have only one X chromosome, possessing a single recessive allele causes the phenotype. Females require two copies of the allele to express the phenotype.
   • Absence of father-to-son transmission: Fathers transmit their Y chromosome to sons and their X chromosome to daughters. Therefore, an affected father cannot pass an X-linked condition to his son.
   • Carrier mothers: Heterozygous females are phenotypically unaffected carriers who transmit the allele to approximately 50% of their sons (who will be affected) and 50% of their daughters (who become carriers).
   • Skipped generations: Traits often appear to skip generations, passing from an affected grandfather through a carrier daughter to an affected grandson.

   X-Linked Dominant Inheritance

   In X-linked dominant inheritance:

   • An affected male transmits the condition to all of his daughters (who receive his mutant X) and none of his sons (who receive his normal Y).
   • Affected heterozygous females pass the trait to 50% of their offspring, regardless of the child's sex.

   Not all genetic material resides in the cell nucleus. Mitochondrial DNA (mtDNA) contains essential genes involved in cellular respiration. During fertilization, the sperm contributes nuclear DNA but virtually no cytoplasm or organelles, whereas the egg provides both nuclear material and nearly all cytoplasmic organelles.

   Consequently, mitochondrial traits follow a pattern of maternal (extranuclear) inheritance, which displays distinct pedigree characteristics:

   • Universal transmission from affected mothers: An affected female will pass the trait or condition to all of her biological children (100% of both sons and daughters), provided penetrance is complete.
   • No transmission from affected fathers: An affected male will never pass the trait to any of his offspring, regardless of the biological sex of the child.
   • Persistence across maternal lineages: The trait persists through unaffected maternal branches only if variable penetrance or heteroplasmy occurs; otherwise, unaffected females do not have affected offspring.

   When determining the mode of inheritance in an unfamiliar pedigree, geneticists apply a sequential elimination process:

   1. Test for Mitochondrial Inheritance: Do all children of affected mothers show the trait, while no children of affected fathers show the trait? If yes, maternal inheritance is strongly indicated.
   2. Check Sex Bias: Are affected individuals exclusively or predominantly male? If so, evaluate for X-linked recessive transmission by checking whether affected males have carrier mothers and unaffected sons.
   3. Verify Male-to-Male Transmission: If an affected father passes the condition directly to a son, rule out X-linked traits immediately, as fathers provide only the Y chromosome to male offspring.
   4. Assess Heterozygote Expression: If intermediate or combined phenotypes appear in generations following two distinct parental phenotypes, the trait likely follows incomplete dominance or codominance (such as the $I^A$ and $I^B$ alleles in human ABO blood typing pedigrees).