Modes of Genetic InheritanceThere are six modes in which an offspring can inherit a trait from his/her parents.
|Mode of Inheritance||Examples|
|Autosomal Dominant||Polycystic kidney disease, Huntington's disease, hereditary spherocytosis|
|Autosomal Recessive||Sickle Cell anemia, cystic fibrosis, Tay-Sachs disease|
|X-Linked Dominant||Rett Syndrome, Vitamin-D resistant rickets, Alport syndrome|
|X-Linked Recessive||hemophilia, color blindness|
|Y-Linked||Hypertrichosis pinnae, Azoospermia, Retinitis Pigmentosa|
|Maternal (Cytoplasmic)||All the children are affected if mother is affected.|
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Symbols Used in Pedigree Chart
Horizontal lines between square and circles represents husband and wife. Children are one rung lower.
Generation are depicted as Roman numerals. A digit stands for an individual in a generation.
A darkened shape is an affected individual - male or female. A hollow shape is a normal individual.
Identical twins are represented with slanting lines and a horizontal line between them.
Non-identical twins are slanting lines but without horizontal line between them
The figure (circle or square) of a deceased individual is represented by striking it.
UsagePedigree analysis is used for various purposes - criminal, legal, scientific, research, academic, etc.
Pedigree Analysis is used to determine the mode of transmission of a genetic diseases - whether is is dominant or recessive, or whether is X-linked or Y-linked or autosomal, or whether it is maternal (cytoplasmic).
Method usedPedigree Analysis is a method which heavily based on rules of logic. Based on the facts depicted by the family tree, either a definite conclusion can be drawn regarding the nature of disease, or alternatively, if the disease is known, we can estimate the percentage chance of an offspring acquiring the disease.
Rules Governing Genetic Transmission of DiseasesThere are certain rules governing the genetic transmission of diseases.
1. A normal individual cannot have (one or both) alleles of the dominant trait. This is logical, because by definition, even if one allele of a dominant disease trait is present, the individual would be affected.
2. A normal individual, however, can be a carrier of a recessive trait. This means that an individual may have one allele of a recessive trait and still be phenotypically normal. The recessive allele do not express and is masked by the dominant allele. This may show in subsequent generations. So the distinctive feature of recessive trait is that, it skips generations.
3. If trait is X-linked, the male would be affected even by a single recessive allele. This is because male is heterozygous regarding the sex chromosome. So a single allele on X-chromosome (even recessive) will create the expression and male would be affected.
4. When father transmits a trait to his son, it is always an Autosomal trait.
Inheritance Patterns in Pedigree AnalysisBased on the above rules, there are a few patterns of genetic inheritance which gives clue to the nature of the trait.
1. If there is skipping of generations, meaning that the trait is observed in generation-I and not in generation-II, but re-appears in subsequent generations, then the trait is "Recessive".
2. If the affected individual has both parents normal, then we can conclude that the trait is "Recessive".
3. If the normal parents have the affected "son" ('B' in figure above), then the trait is "Recessive". We cannot conclude that its Autosomal or X-Linked. If Autosomal, he got it from both parents. But if X-linked he got it from his mother only.
4. Trait is dominant, if each generation has a trait. Remember! For a trait to express only one allele is sufficient.
5. However, when an affected father transmits the disease to a daughter ('E' in figure above), it may be X-linked or autosomal; we can't say for sure. We need have to have more information to ascertain the case.
6. When a mother is affected and "all" the children - sons as well as daughters, are affected ('F' in figure above), then the trait is maternal (cytoplasmic) inheritance.
Multiple Choice Questions
1. Mother is heterozygous.
2. Parents could not have a normal daughter for this character
3. The trait under study could not be color-blindness
4. Father is homozygous dominant
Ans: (1) Mother is heterozygous.
Cystic Fibrosis, where the principle organ affected is pancreas (caused due to mutation of a gene, located on chromosome-7) is inherited as _____________. (1) Recessive Autosomal Gene disorder (2) Dominant Autosomal Gene disorder (3) Recessive Sex-Linked Gene disorder (4) Dominant Sex-Linked Gene disorder Ans: http://zoomgenetics.apphb.com/?id=m296
Congenital Night Blindness, which causes reduced development of visual pigment (rhodopsin) is a case of _____________. (1) Recessive Autosomal Gene disorder (2) Dominant Autosomal Gene disorder (3) Recessive Sex-Linked Gene disorder (4) Dominant Sex-Linked Gene disorder Ans: http://zoomgenetics.apphb.com/?id=m302
Who discovered Incomplete dominance and Cytoplasmic dominance (1) Carl Correns (2) Regnier de Graaf (3) Carl Wilhelm von Nageli (4) Archibald Garrod Ans: http://zoomgenetics.apphb.com/?id=m28
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