Unit – IV A

Pedigree Analysis

Deficiencies of Mendelian Theory and Need of Pedigree Analysis:

  • Mendelian crosses can not be applied to humans, due to following reasons.
  • In human, controlled crosses can not be made. i.e. human can not be crossed at will.
  • They have long generation period of nearly 20 years
  • They produce a small progeny.
  • Hence human geneticists often scrutinise family histories called pedigree, in hope that informative matings might have occurred by chance. This process is called pedigree analysis.
  • Thus pedigree analysis is the process of examining a pedigree to determine the pattern of inheritance for a trait. Pedigrees are often used to determine if a trait is dominant or recessive.

Symbols Used in Pedigree Analysis:

Reading Pedigree Chart:

Chart – 1

Chart – 2

  • I, II, III, IV indicates generations.
  • The “founding parents” in this family are the female I-1 and the male I-2 in the first generation at the top.
  • In this pedigree, the unaffected founding mother, I-1, and affected founding father, I-2, are parents to two affected daughters, II-1 and II-2.
  • The affected founding daughter II-2 and the unaffected male II-3 who “marries into the family” have two offspring, an unaffected daughter III-1 and affected son, III-2.
  • Finally, this affected male III-2 and the unaffected female III-3 who “marries in” have an unaffected son, IV-1.

General Assumptions in Pedigree Analysis:

  • Complete Penetrance: 

    An individual in the pedigree will be affected (express the phenotype associated with a trait) when the individual carries at least one dominant allele of a dominant trait or two recessive alleles of a recessive a trait.

  • Rare-in-Population: 

    In each problem, the trait in question is rare in the general population. Assume for the purposes of these problems that individuals who marry into the pedigree in the second and third generations are not carriers. This does not apply to the founding parents – either or both of the individuals at the top of the pedigree could be carriers.

  • Not-Y-Linked: 

    The causative genes in these problems may be autosomal or X-Linked, but are not Y-linked.

Important Rules:

  1. An unaffected individual cannot have any alleles of a dominant trait because a single allele of a dominant trait causes an individual to be affected.
  2. Individuals marrying into the family are assumed to have no disease alleles – they will never be affected and can never be carriers of a recessive trait because the trait is rare in the population.
  3. An unaffected individual can be a carrier (have one allele) of a recessive trait because two alleles of a recessive trait are required for an individual to be affected
  4. When a trait is X-linked, a single recessive allele is sufficient for a male to be affected because the male is hemizygous – he only has one allele of an X-linked trait
  5. A father transmits his allele of X-linked genes to his daughters, but not his sons. A mother transmits an allele of X-linked genes to both her daughters and her sons.

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