Pedigree analysis

Class 12 Biology (Principle of Inheritance and Variation)

 

Pedigree analysis is a powerful tool used to study the inheritance patterns of genetic traits and disorders within families. This method involves constructing a family tree, or pedigree, that illustrates the relationships among family members and the presence or absence of the trait in question. By analyzing the pedigree, we can determine the mode of inheritance, whether it is dominant or recessive, and whether it is autosomal or sex chromosomal.

Dominant vs. Recessive Inheritance When analyzing a pedigree, it is important to determine whether the mode of inheritance is dominant or recessive. Autosomal dominant traits typically show vertical transmission, meaning that the trait is passed from generation to generation without skipping a generation. An affected individual has a 50% chance of passing the trait on to each of their children, regardless of gender. In contrast, autosomal recessive traits may skip generations and affect individuals who are not related. Carrier status may be present in unaffected individuals who have one copy of the mutated gene. An affected individual typically inherits two copies of the mutated gene, one from each parent. The trait may only be expressed if both copies are present.

4.2 Symbols used in Pedigree Charts – Open Genetics

Fig :Pedigree Symbols

To determine whether the trait is dominant or recessive, we need to look at the pattern of inheritance in the pedigree. If the trait is present in every generation and affects both males and females equally, it is likely to be autosomal dominant. If the trait appears to skip generations and affects males and females equally, it is likely to be autosomal recessive.

Autosomal vs. Sex Chromosomal Inheritance Another important aspect of pedigree analysis is determining whether the mode of inheritance is autosomal or sex chromosomal. Autosomal inheritance refers to traits that are located on the autosomes, which are the non-sex chromosomes. Sex chromosomal inheritance, on the other hand, refers to traits that are located on the X or Y chromosomes.

Autosomal traits can be dominant or recessive, while sex chromosomal traits can be dominant, recessive, or X-linked. X-linked traits are located on the X chromosome and can be either dominant or recessive. X-linked dominant traits affect both males and females, but affected males transmit the trait to all of their daughters but none of their sons. Affected females have a 50% chance of transmitting the trait to both their sons and daughters. X-linked recessive traits typically affect males more frequently than females, as males only have one X chromosome. The trait may be passed from an unaffected carrier mother to her sons.

To determine whether the trait is autosomal or sex chromosomal, we need to look at the pattern of inheritance in the pedigree. If the trait affects both males and females equally and does not show a gender bias, it is likely to be autosomal. If the trait shows a gender bias, affecting primarily one gender, it may be sex chromosomal.

Pedigree analysis is a valuable tool used to study the inheritance patterns of genetic traits and disorders within families. By analyzing the pedigree, we can determine the mode of inheritance, whether it is dominant or recessive, and whether it is autosomal or sex chromosomal. The rules to follow when conducting pedigree analysis include collecting information, using standard symbols, analyzing the pattern of inheritance, constructing the pedigree, interpreting the results, verifying the findings, and communicating the results. A thorough analysis of the pedigree can provide valuable information for understanding the genetic trait or disorder in question and may help guide medical management and counseling for affected individuals and their families.

Rules to know whether it is autosomal

Here are some rules to follow when analyzing a pedigree to determine whether the inheritance pattern is autosomal:

  1. Autosomal traits affect males and females equally: If the trait appears to affect males and females equally, it is likely to be autosomal.
  2. Autosomal traits show vertical transmission: Autosomal dominant traits typically show vertical transmission, meaning that the trait is passed from generation to generation without skipping a generation. In contrast, autosomal recessive traits may skip generations and affect individuals who are not related.
  3. Autosomal traits may have carriers: For autosomal recessive traits, carrier status may be present in unaffected individuals who have one copy of the mutated gene. This can help identify individuals who may pass on the trait to their children.
  4. Autosomal traits can affect siblings differently: If siblings have different phenotypes (i.e., one is affected and one is not), it is possible that the trait is autosomal recessive, and each sibling has inherited a different combination of alleles from their parents.

Rules to know whether it is sex chromosomal

Here are some rules to follow when analyzing a pedigree to determine whether the inheritance pattern is sex chromosomal:

  1. Sex chromosomal traits show gender bias: Unlike autosomal traits, sex chromosomal traits often show a gender bias in their inheritance pattern. For example, X-linked traits may affect males more frequently than females because males only have one X chromosome.
  2. X-linked traits can be dominant or recessive: X-linked dominant traits affect both males and females, but affected males transmit the trait to all of their daughters but none of their sons. Affected females have a 50% chance of transmitting the trait to both their sons and daughters. X-linked recessive traits typically affect males more frequently than females, as males only have one X chromosome. The trait may be passed from an unaffected carrier mother to her sons.
  3. Y-linked traits affect only males: Y-linked traits are rare and only affect males, as females do not have a Y chromosome.
  4. Analysis of multiple generations can help identify sex chromosomal traits: Analysis of multiple generations can help identify sex chromosomal traits, as these traits may appear to skip generations or affect primarily one gender.

 

Rules to know whether it is recessive

Here are some rules to follow when analyzing a pedigree to determine whether the inheritance pattern is recessive:

  1. Recessive traits can skip generations: Unlike dominant traits, which usually appear in every generation of a pedigree, recessive traits may skip generations because carriers can be unaffected and pass the gene mutation to their offspring.
  2. Affected individuals can have unaffected parents: For a recessive trait to be expressed, an individual must inherit two copies of the mutated gene (one from each parent). Therefore, affected individuals can have unaffected parents who are carriers of the gene mutation.
  3. Consanguinity increases the likelihood of recessive traits: Consanguinity (i.e., when two individuals who are closely related have children) increases the likelihood of recessive traits appearing in offspring because the chances of inheriting two copies of a gene mutation are higher.
  4. Males and females are affected equally: Recessive traits typically affect males and females equally.

Rules to know whether it is dominant

Here are some rules to follow when analyzing a pedigree to determine whether the inheritance pattern is dominant:

  1. Dominant traits usually appear in every generation: Unlike recessive traits, which may skip generations, dominant traits typically appear in every generation of a pedigree.
  2. Affected individuals often have affected parents: For a dominant trait to be expressed, an individual only needs to inherit one copy of the mutated gene from an affected parent.
  3. Males and females are affected equally: Dominant traits typically affect males and females equally.
  4. A vertical pattern of inheritance: Dominant traits usually show a vertical pattern of inheritance, meaning that the trait is passed down from generation to generation without skipping a generation.
  5. Variable expressivity: Some dominant traits may show variable expressivity, which means that the trait may be expressed differently among affected individuals.

 

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