Pedigree Chart
Introduction
A pedigree chart is very helpful tool in the study of genealogy and it helps geneticists to effectively carry out an analysis of a family’s genetic history over several generations (Merriman, 2010). Therefore, it is plays an essential role in enabling scientists or a genetic counselor in finding out the history of a family by making these family trees that explicitly explain the genetic history (Morgan, 2009). Thus, a pedigree chart is usually used in finding out the probability of a child to inherit a certain disorder in a particular family (Merriman, 2010). This is mainly because it highlights all the affected individuals in the family history. This is due to the fact that an interpretation of a pedigree chart involves determination of whether a trait is autosomal, X-linked or Y-linked and dominant or recessive (Morgan, 2009).
The data shown below shall be used to generate a pedigree chart:
Subject: affected woman
Subject’s two brothers, unaffected
Subject’s father affected, mother unaffected
Subject’s paternal aunt and uncle, unaffected
Subject’s paternal grandfather affected, grandmother unaffected
Pedigree Chart
Genotypes on the pedigree chart
Key:
XHXh: unaffected female (carrier)
XhXh: affected female
XhY: affected male
XHY: unaffected male
Identification
1 = Subject’s paternal grandmother
2 = Subject’s paternal grandfather
3 = Subject’s father
4 = Subject’s mother
5 = Subject’s paternal aunt
6 = Subject’s paternal uncle
7 = Subject’s first brother
8 = Subject’s second brother
9 = Subject
Analysis
The analysis of the pedigree chart will consider all the possible five modes of inheritance.
Autosomal Dominant: The inherited trait appears in both sexes and both sexes transmit the trait to their offspring without skipping any generation where the affected offspring must have a parent who is affected unless when they are in possession of a new mutation. In autosomal dominant inheritance, when one parent is affected (heterozygous) and the other parent is unaffected (a carrier), approximately a half of the offspring will be affected. However, in case of unaffected parents, the trait is not transmitted to the offspring. This implies that autosomal dominant inheritance is a genetic trait that is transmitted from one generation to another without skipping, especially when the trait is passed from both fathers and mothers to daughters and sons. This indicates that this pedigree chart shows an autosomal dominant inheritance.
Autosomal Recessive: The inherited trait appears in both sexes where both sexes transmit the trait to their offspring, and the inherited trait tends to skip generations. Usually, affected offspring are born to parents who are unaffected. This clearly indicates that the inheritance shown in this pedigree chart is not autosomal recessive because the inherited trait does not skip any generation.
X-linked Dominant: In this mode of inheritance, both males and females are affected; but females are often affected more than males. This mode of inheritance does not skip generations and sons who are affected must have an affected mother; whereas affected daughters must have either a mother or father who is affected. Affected fathers will usually transmit the trait on to all their daughters; whereas for affected mothers who are heterozygous, the trait will be passed to half of their daughters and half of their sons. This clearly indicates that the mode of inheritance shown in this pedigree chart is not X-linked dominant because affected do not transmit the trait to all their daughters.
X-linked Recessive: In this mode of inheritance, both males and females are affected; but males are often affected more than females. In X-linked recessive mode of inheritance, affected sons are usually born to mothers who are unaffected which means the trait skips generations. Approximately a half of sons born from carrier mothers are affected. However, a trait that is passed through X-linked recessive mode of inheritance is never passed from father to son, but all affected fathers’ daughters are carriers. This clearly indicates that the mode of inheritance shown in this pedigree chart is not X-linked recessive because the inherited trait does not skip any generation.
Y-linked: When a trait is Y-linked inherited, then it is only inherited from males and never occurs in females, and an affected male passes the trait to all male descendants. This is not the case in this pedigree chart because the pedigree chart shows that the trait has been passed to a female.
Conclusion
In conclusion, it is undoubtedly evident that a pedigree chart is a very powerful and effective tool for the presentation of a family’s genetic history clearly highlighting how a particular trait is inherited while at the same time showing whether the inheritance of the trait is autosomal, X-linked or Y-linked and dominant or recessive. For instance, from the illustrated pedigree chart the mode of inheritance is autosomal dominant because it is transmitted from one generation to another without skipping, especially from both fathers and mothers to daughters and sons.
References
Merriman, B.D. (2010). Genealogical Standards of Evidence: A Guide for Family Historians. 2nd ed. Toronto: Ontario Genealogical Society.
Morgan, G.G. (2009). How to Do Everything: Genealogy. New York, NY: McGraw-Hill.
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