Chromosomal Inheritance
Inheritance patterns can be traced through pedigrees and are explained mechanistically by the behavior of chromosomes during meiosis, including cases of nondisjunction.
Reading and building pedigrees
A pedigree is a family tree that tracks the inheritance of a particular trait across generations, using standardized symbols: squares represent males, circles represent females, filled shapes indicate individuals expressing the trait, and horizontal lines connecting a square and circle indicate mating pairs, with vertical lines dropping down to their offspring.
By examining the pattern of affected and unaffected individuals across generations, geneticists can often infer the mode of inheritance - autosomal versus X-linked, and dominant versus recessive - even without direct knowledge of the underlying gene or its molecular basis.
Diagnostic pedigree patterns
- Autosomal recessive: the trait can skip generations, and two unaffected (carrier) parents can produce an affected child; affected individuals are often the children of unaffected parents.
- Autosomal dominant: the trait typically appears in every generation, and every affected child must have at least one affected parent.
- X-linked recessive: affected individuals are predominantly male; there is never father-to-son transmission (since fathers pass Y, not X, to sons); daughters of affected fathers are carriers.
- X-linked dominant: affected fathers pass the trait to all of their daughters but none of their sons; affected heterozygous mothers pass the trait to about half of all their children, regardless of sex.
The chromosome theory of inheritance
In the early 1900s, Walter Sutton and Theodor Boveri independently proposed the chromosome theory of inheritance: genes are located at specific positions (loci) on chromosomes, and it is the behavior of chromosomes during meiosis - their segregation during meiosis I and independent assortment among tetrads - that physically accounts for Mendel's Law of Segregation and Law of Independent Assortment.
This theory unified cytology (the microscopic study of cell structures like chromosomes) with genetics (the study of inheritance patterns), and was strongly supported by Thomas Hunt Morgan's work with fruit flies, which demonstrated that a specific gene (for eye color) segregated along with the X chromosome, providing direct physical evidence that genes reside on chromosomes.
Nondisjunction and aneuploidy
Occasionally, chromosomes fail to separate properly during meiosis, a mistake called nondisjunction. This can occur during meiosis I, when both homologs of a pair travel to the same pole instead of separating, or during meiosis II, when sister chromatids fail to separate. The result is gametes with either one extra chromosome (n+1) or one missing chromosome (n−1).
When such an abnormal gamete is fertilized by a normal gamete, the resulting zygote is aneuploid - having an abnormal number of a particular chromosome. Trisomy (three copies of a chromosome) and monosomy (a single copy) are the two main outcomes. In humans, trisomy 21 (three copies of chromosome 21) results in Down syndrome; most other autosomal aneuploidies are lethal early in development, while sex chromosome aneuploidies (such as XXY, Klinefelter syndrome, or XO, Turner syndrome) are more often survivable.
Key terms
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- Pedigree
- A diagram depicting the inheritance of a trait across multiple generations of a family, using standardized symbols.
- Chromosome theory of inheritance
- The principle that genes are located on chromosomes, and that the behavior of chromosomes during meiosis explains the patterns of inheritance Mendel described.
- Nondisjunction
- Failure of homologous chromosomes (meiosis I) or sister chromatids (meiosis II or mitosis) to separate properly.
- Aneuploidy
- A condition in which a cell or organism has an abnormal number of a particular chromosome, such as trisomy or monosomy.
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