Introduction
Pedigree analysis is crucial for genetic studies. However, it is essential to learn what is pedigree analysis and how the trait is transferred from one generation to the next generation. Students often attempt to solve pedigrees analysis by merely observing them, but this is not the proper way to analyze pedigrees. It is always advisable to use a standard method to solve them, even if it takes a little longer to reach a conclusion. This article discusses how to solve pedigree analysis class 12 using both the standard method and a shortcut method.
This is the Pedigree analysis that we want to analyze. As mentioned above, there are two ways to analyze the same. We will first analyze the pedigree using standard method.
This is a four generation pedigree and we have to analyze whether this pedigree is X-linked dominant, X-linked recessive, autosomal dominant or autosomal recessive in nature. Let’s first check for the X-linked dominant case.
Case I: X-linked Dominant Pedigree Analysis
If we consider the above pedigree analysis as X-linked dominant, the affected man in the Ist generation must have XAY genotype where A is a dominant allele. He marries a woman who does not have any disease, and therefore, she will have recessive ‘a’ with XaXa genotype for the recessive trait. They have a daughter in the IInd generation who inherits XA from her father and Xa from her mother — the genotype of the daughter, therefore, would be XAXa as shown in the picture.
The affected daughter (XAXa) marries a normal man whose genotype would be XaY since he does not have any disease. They have a normal daughter in the IIIrd generation whose genotype is XaXa (one Xa came from father and one Xa came from mother). And, her brother got one Xa from mother and Y from the father. Now, the daughter in the IIIrd generation marries a normal man whose genotype is Xa Y. Until now, everything seems perfect, however, the problem arises in the IVth generation.
In the IVth generation, the son must carry Xa chromosome from mother and Y chromosome from his father i.e., XaY. But, the son shown in the IVth generation is affected (XAY), which should not be as per the pedigree. It suggests that this pedigree analysis cannot be X-linked dominant
Case II: X-linked Recessive Pedigree Analysis
In the case of X-linked recessive, the man in the Ist generation must have a genotype XaY to have the trait. And his wife must have XAXa to be a normal individual. The couple have an affected daughter in the IInd generation. The daughter carries one Xq from the father and one one Xa from the mother — XaXa. She has a brother who seems normal, but he can only inherit Xa from mother and Y chromosome from father. If that’s the case, he must carry the trait, which is not the case here. It, therefore, suggests that this pedigree analysis cannot be X-linked recessive.
Case III: Autosomal Dominant Pedigree Analysis
If it is an autosomal trait, the locus of the gene must be present on one of the 22 autosomes and not on X-chromosomes. So, a person with the trait must have atleast one A and the one that does not have the trait must have homozygous recessive aa.
In the first generation, the man has a genotype of Aa and his wife has a genotype of aa. They have a daughter and two sons in the Iind generation. The genotype of the daughter would be Aa (she inherits one small a from mother and capital A from father), as she carries the trait. Her one brother does not carry any trait and thus his genotype would be aa (one small a from mother and the other small a from his father). However, the other brother is carrying the trait (affected), so, his genotype would be Aa (small a from mother and capital A from father).
The normal brother with a genotype aa in the IInd generation marries a normal woman with a genotype aa. In the IIIrd generation, they have a normal daughter and an affected son. The genotype of the daughter would be aa. However, the affected brother must have a genotype Aa to show the trait, but it is not possible, as both his mother and father have a genotype aa only.
Case IV: Autosomal Recessive Pedigree Analysis
In autosomal recessive case, anyone carrying the trait must have a genotype of homozygous aa. And the normal person could have either AA or Aa genotype. Well, in the Ist generation, an affected man with genotype aa marries a normal woman Aa. They have one affected daughter, one affected son and a normal son. Well, all conditions are possible in the IInd generation.
The affected daughter (aa) marries a normal man (Aa). They have a normal daughter and a normal son in the IIIrd generation. The normal daughter marries a normal man and they have two affected son and a normal daughter in the Ivth generation. All conditions are fulfilled. It is therefore an autosomal recessive trait.
Shortcut Method to Solve Pedigree Analysis Problems
Let’s quickly see criteria for X-linked and Autosomal disorders.
X-linked Dominant
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- No skipping of generation
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- Should follow DDD rule — first D means the pedigree analysis is dominant; second D means the dad is affected in the pedigree and the third D means all daughters are affected.
X-linked Recessive:
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- Affected daughter must have an affected father as shown above
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- All affected mom passes disease to all her sons. However, affected mom should also have affected father if given. Shown above in the diagram.
Autosomal Dominant / Autosomal Recessive
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- No skipping of generation for dominant condition only
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- If two unaffected parent have diseased offspring — it will always be a case of autosomal recessive condition.
Now, let’s analyze the pedigree analysis using the shortcut method.
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- If you look at the diagram above, it is a IV generation pedigree. In the IIIrd generation, there is no disease present on the left and right side of the diagram. It means, there is skipping of disease. We know that skipping of generation is present only in recessive case. So, it is now confirmed that the above pedigree is RECESSIVE in nature.
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- On the left side, we can see in the third generation that two unaffected parent have diseased sons in the IVth generation. So, it is an autosomal recessive condition. The same situation is seen in the middle of the pedigree where two unaffected parent in the IInd generation have diseased son in the third generation It is therefore clear that the above pedigree is AUTOSOMAL RECESSIVE in nature.
FAQs (Frequently Asked Questions)
Q. How can you determine if someone is a carrier in pedigree analysis?
A. In Autosomal dominant and X-linked dominant inheritance, there is no concept of a carrier because the affected allele is dominant. In X-linked recessive inheritance, the carrier is always the mother or a female, as she possesses two X chromosomes, with one compensating for the function of the other non-functional chromosome. The father or a male cannot be a carrier because he has only one X chromosome, and any affected allele will be expressed. Determining carriers in Autosomal recessive cases is challenging without adequate information. While we can identify affected individuals as homozygous recessive, we cannot conclusively determine if an unaffected individual is a dominant homozygote or a carrier. An exception is when one parent is homozygous recessive, in which case we can infer that the offspring will be carriers.
Q. How can we determine that a trait is recessive if it appears in all generations?
A. In autosomal recessive inheritance without skipped generations, the other parent must be heterozygous for the trait, increasing the children’s chances of inheriting the recessive trait. Similarly, in X-linked recessive inheritance, one parent is typically a carrier. Evidence of X-linked recessive inheritance includes mother-son and/or daughter-father links, where the affected trait is passed from the mother to the son or from the father to the daughter, respectively. This indicates that if the mother is affected, her son must be, as he inherits one of her affected X chromosomes, and the same applies to daughter-father links.
Q, Is it possible for a male to be a carrier of an X-linked recessive trait without showing any symptoms?
A. No, this scenario is not feasible. Males, having only one X chromosome, will express any X-linked trait they carry, as they lack a second X chromosome to mask the expression. In contrast, females, possessing two X chromosomes, can be carriers for X-linked traits.
Q. What is the distinction between X-linked and sex-linked traits?
A. Genes located on the sex chromosomes are referred to as sex-linked. Typically, males have one X and one Y chromosome, while females have two X chromosomes. Because males inherit a Y chromosome from their fathers, they are the only ones who can inherit Y-linked traits.
Q. How can we determine if it is autosomal?
A. I believe an equal number of males and females are affected.