Tuesday, February 3, 2009

Know the Pedigree Analysis of Humans

Humans are unique among organisms in many ways. But one way, which is near and dear to a geneticist's heart, is that humans are not susceptible to genetic experimentation. In practice, we humans actually share this characteristic with many long-lived organisms who delay first births. In short, it is not terribly convenient to perform experimental crosses if one has to wait 15 years between generations. However, for humans, one also has to add that our system of morality uniquely does not allow such experimentation on humans. This is an unfortunate state of affairs since there is no other organism for which practical knowledge of their genetics would be more useful, especially in the case of the genetics of heritable diseases. It has been found that human genetics may readily be inferred so long as good records have been kept within large families. This formal mechanism of inference is called pedigree analysis.

Here we are presenting many aspects of human genetics, with particular consideration to strategies of pedigree analysis whereby we are attempting to infer the genetics of human conditions based on knowledge of marriage (mating) and affliction in large extended families.

Procedures for Pedigree Analysis:
Pedigree analysis is one of the central tasks of the human geneticist. It involves the construction of family trees. Family history information is often collected at major family gatherings. A pedigree is used to trace inheritance of a trait over several generations.

Three primary patterns of inheritance in man are the following:

1) Autosomal recessive
2) Autosomal dominant
3) Sex-linked (X-chromosomal)

Autosomal Dominant Inheritance: A dominant condition is transmitted in unbroken descent from each generation to the next. Most matings will be of the form M/m x m/m (i.e., heterozygote to homozygous recessive). We would therefore expect every child of such a mating to have a 50". chance of receiving the mutant gene and thus of being affected.In this pedigree, an affected father passes the trait to half of his six children, including two daughters and a son. One of the daughters passes the same trait to one of her three children.

Autosomal Recessive Pedigree:A recessive trait will only manifest itself when homozygous. If neither parent has the characteristic phenotype (disease) displayed by the child, the trait is recessive. If it is a severe condition it is unlikely that homozygotes will live to reproduce and thus most occurrences of the condition will be in matings between two heterozvgotes (or carriers). An autosomal recessive condition may be transmitted through a long line of carriers before, by the ill chance, two carriers mate. Then there will be a one fourth chance that any child will be affected.

If the parents are related to each other, perhaps by being cousins, there is an increased risk that any gene present in a child may have two alleles identical by descent. The degree of risk that both alleles of a pair in a person are descended from the same recent common ancestor is the degree of inbreeding of the person.

Pedigree of Sex-linked Traits: The transmission of X-linked traits is in a zigzag manner. Females transmit X chromosomes to both sons and daughters. Males transmit the X chromosomes only to daughters and Y chromosomes to sons. The X-linked traits, which are recessive are preferentially seen in males, who are always homozygous for the X chromosomes. Females are heterozygous and form the "carriers" of that trait. Most X-linked traits are recessive.

A color-blind man is the father of "carrier" daughters and normal sons. Carrier daughters have a 50'N, chance to have color-blind sons and a cross between a color-blind male and carrier female can produce color-blind daughters. Hemophilia also has the same type of inheritance. Duchenne muscular dystrophy is another example of X-linked inheritance.Transmission of Y-linked Genes
Men are homozygous for Y-linked genes present on the non-homologous parts. All these genes will be expressed in all conditions. These genes are always transmitted from father to sons and never to daughters. There are no essential genes in the Y chromosomes except the locus for the maleness and fertility.

Pseudoautosomal Inheritance:
There are some homologous regions in the X chromosomes and Y chromosomes. These homologous parts pair during meiosis and may undergo crossing over. Therefore, genes in these homologous regions show inheritance similar to autosomal genes and are called pseudoautosomal inheritance. Such genes or characters are very rare.

Tag: Bio Technology, Bio Genetics, Pedigree Analysis

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