Extrachromosomal Inheritance

 Inheritance is the transfer of genetic information or traits from one cell or individual to another. While most inherited traits follow patterns of chromosomal inheritance, where genes on the chromosomes control the traits, there are some traits that do not follow this conventional pattern. These traits are caused by extrachromosomal inheritance.

Extrachromosomal inheritance, also known as cytoplasmic or extranuclear inheritance, refers to the inheritance of traits that are not controlled by chromosome genes. Instead, they are determined by genetic materials located outside the chromosomes. This form of inheritance occurs in the cytoplasm of cells and involves genes present in cytoplasmic organelles like mitochondria and plastids. The extrachromosomal hereditary factors have the ability to self-replicate and can be transmitted sexually or asexually. It is important to study these non-chromosomal factors to gain a comprehensive understanding of heredity. 

The early recognition of extrachromosomal inheritance started with the demonstrations by Carl Correns, who observed that heredity is not solely governed by the nucleus. Correns demonstrated that hereditary factors can also be present in the cytoplasm, not just the nucleus. Over time, extrachromosomal inheritance was observed in many cases in plants and animals. 

Characteristics of Extrachromosomal Inheritance

There are several characteristics associated with extrachromosomal inheritance:

Extrachromosomal inheritance does not follow the typical Mendelian inheritance patterns. 

The inheritance of extrachromosomal factors is independent of genes located within the cell nucleus. 

In some cases, extrachromosomal traits are inherited exclusively from the mother. This is because the egg contributes more cytoplasm to the zygote compared to the male parent. 

Extrachromosomal inheritance can lead to characteristic phenotypic changes that are not inherited in a Mendelian pattern. 

Extrachromosomal genes can exhibit vegetative (somatic) segregation which is rare in nuclear genes.

Types of Extrachromosomal Inheritance

There are two main types of extrachromosomal inheritance that are briefly discussed below:

1. Chloroplast inheritance

Chloroplasts are organelles located in plant cells that play a vital role in photosynthesis. They possess their own DNA, known as chloroplast DNA (cpDNA), which is distinct from nuclear DNA. 

The inheritance of chloroplast genes was first discovered by Carl Correns and Erwin Baur in 1909. 

Correns conducted a study on Mirabilis jalapa, commonly known as the four o’clock plant, where he observed that the transmission of leaf color was strictly maternal, determined by the color of the ovule’s source.

Baur found in his experiment in geranium (Pelargonium zonale) that chloroplast genes can also be inherited from both parents or from the male parent only, resulting in variegated plants. 

Recent research conducted at the Max Planck Institute of Molecular Plant Physiology with tobacco plants presents new evidence that challenges the commonly held belief that chloroplasts are solely inherited from the mother plant. The researchers discovered that under specific environmental conditions, chloroplasts from the father can also be passed on to the offspring.

2. Mitochondrial inheritance

Mitochondria are cellular structures present in eukaryotic cells that are responsible for generating energy. They also contain their own unique DNA, known as mitochondrial DNA (mtDNA). 

mtDNA is the main form of extrachromosomal inheritance in animals. mtDNA is circular and encodes 37 genes on 16.5 kb of DNA.

Margit and Sylvan Nass discovered the DNA in mitochondria in 1963.

Mitochondria are primarily inherited uniparentally, mostly maternally. The zygote receives mitochondria exclusively from the mother, while the paternal contribution of mitochondria is minimal or negligible. 

In 2018, a controversial claim suggested that children can inherit mtDNA from their fathers. However, subsequent research found that in cases of biparental inheritance, mitochondrial DNA fragments can migrate into the nucleus and integrate with the chromosomes. These mitochondrial DNA fragments are inherited alongside the nuclear chromosomes but the primary inheritance of mitochondrial DNA still occurs from the mother. This research confirms that the concept of maternal inheritance is still true.

MtDNA exhibits a higher rate of mutational change compared to nuclear DNA. Mutations in mtDNA can have significant effects and are associated with various diseases.