Pedigree Analysis

Introduction

Pedigree analysis is a scientific approach to studying the inheritance of human genes. Pedigree analysis can be used to easily understand family history and the inheritance of dominant and recessive traits. With the help of a pedigree chart, one can easily identify dominant and recessive traits and also the carriers of these traits.

Pedigree Chart

A pedigree chart is a diagrammatic representation (chart or family tree) where one can easily determine the occurrence of a phenotype in an organism’s genes passed down from their ancestors. One can study family history through pedigree charts that might have faded away over generations.

Pedigree charts are commonly used for humans, racehorses, and dogs. They utilise standard symbols: a circle represents females, and a square represents males.

Diagrammatic Representation of Pedigree Charts

Relationships are presented diagrammatically in a pedigree chart. 

1. The parents are connected by a horizontal line, with a vertical line leading to their offspring.
2. A horizontal zip line connects all the offspring of the same parents. They are placed from left to right according to their order of birth.
3. Twin offspring are connected by a triangle.
4. If an offspring is no more, there is a line across it.
5. A small triangle represents if the offspring was aborted or has not yet been born.

Different generations are represented by Roman letters, and a numeral represents offspring of the same generation.

If the pedigree chart is constructed for a family with a genetic disorder, this is represented by an arrow.

Types of Pedigree Analysis

Based on linked chromosomes, pedigree analysis is classified into:

1) Sex-linked pedigree

It is subdivided into:

• X-linked pedigree
• Y-linked pedigree

Even the X-linked pedigree is further divided into:

• X-linked dominant pedigree
• X-linked recessive pedigree

2) Autosomal pedigree

• Autosomal dominant pedigree
• Autosomal recessive pedigree

Importance of Pedigree Diagrams

The two important uses of pedigree analysis are with:

• Humans
• Animals

Human usage

• Pedigree analysis for humans is used to check the likelihood of a particular disorder and condition
• Pedigree analysis helps locate genes such as X, Y, and autosomal chromosomes
• It helps predict whether the trait is dominant or recessive
• If the likelihood is 50:50 (mother and father), it is called an autosomal condition
• If the pedigree analysis reveals that a male is affected by a genetic disorder, it is known as an X-linked disorder

Animal usage

• The ancestors of animals can be found
• Horse and dog breeds with suitable traits can be obtained

Characteristic Predictions through Pedigree Analysis

While the pedigree analysis does not help predict characteristics like height, other features, such as hair colour, blood group, and eye colour, can be easily found. To understand pedigree analysis, one needs to be clear about dominant and recessive genes.

Let’s consider an example. The colour blindness trait can be found in the X or Y chromosome. However, as this trait is common in the X chromosome, colour blindness commonly occurs in males. However, females can also be affected if they inherit the X chromosome with colour blindness from the affected mother or father. A shaded dot or half-shaded symbol represents the carrying of any defective traits.

Understanding Genes and Alleles

• If the mother and father are both dominant, the children might show recessive traits
• This is because one copy of the gene is from the mother and the other from the father
• Different forms of a gene, such as whether hair curls or not, are called alleles
• Dominant alleles are represented by Q (capital), while recessive alleles are represented by q (small)
• An individual’s collection of genes is called a genotype Here are some of the genotypes:

1. QQ = dominant
2. Q = dominant
3. Q = recessive

• A person with QQ and Qq genotypes has curly hair
• A person with qq will have curly hair
• QQ and qq are known as homozygous, and the person with the genotype Qq is heterozygous; they are usually considered carriers and are represented by a dot or shaded

Autosomal Dominant Pedigree

• Gen II-2, 6, 7 are affected, children. Gen I-2 is affected by a parent
• Gen III-1 is heterozygous
• Gen II-6, 7 produce unaffected child Gen III-6 and affected child GenIII-7.
• Gen II-3, 4 unaffected parents give rise to unaffected children Gen III-4,5.
• Since the gene is heterozygous, 50% of the offspring are affected (Gen III-1,3).
• Some examples of autosomal dominant pedigree are Huntington’s disease, Marfan syndrome, achondroplasia, Noonan syndrome, familial breast cancer, and retinoblastoma.

X-linked Recessive Pedigree

• Marriage between a male with X-linked recessive traits (bX) and a normal woman (BB)
• With the X chromosome from the father and the B allele from the mother, all chromosomes have bX
• With the b allele from the father and the B allele from the mother, all the daughters are heterozygous carriers represented by dots
• The daughter with bB will not show any traits but can pass the traits to her son
• BX and BX males are both homozygous as they have only one allele per locus
• If the heterozygous daughter (Ba; carrier) marries an unaffected man (BX), they can produce four genotypes

1. Half of the sons show the trait (bX)
2. Half of the sons do not show the trait (BX)
3. Half of the daughters will be carriers (Bb)
4. Half of the daughters will not be carriers (BB)

Autosomal Recessive Pedigree

If both the parents are unaffected and the children are affected, alleles are recessive. Therefore, the parents are considered heterozygous carriers.

Parents must be heterozygous. Alleles cannot be dominant as two unaffected parents could not affect offspring.

Conclusion

Pedigree analysis is a scientific approach to studying the inheritance of human genes. Pedigree analysis can be used to easily understand family history and the inheritance of dominant and recessive traits. With the help of a pedigree chart, one can easily identify dominant and recessive traits and also the carriers of these traits.