Heterosis

• Heterosis may be defined as the superiority of an F, hybrid over both its parents in terms of yield or some other character.
• Generally, heterosis is manifested as an increase in vigour, size, growth rate, yield or some other characteristic.
• But in some cases, the hybrid may be inferior to the weaker parent. This is also regarded as heterosis; Often the superiority of F, is estimated over the average of the two parents, or the mid -parent.
• If the hybrid is superior to the mid -parent, it is regarded as heterosis (average heterosis or relative heterosis).
• Heterosis is estimated over the superior parent; such an estimate is sometimes referred to as heterobeltiosis.
• The term heterobeltiosis is not commonly used since most breeders regard this to be the only case of heterosis and refer to it as such i.e., heterosis.

Heterosis and Hybrid Vigour

• Hybrid vigour describes only the superiority of hybrids over their parents, while heterosis describes other situations as well.
• The few cases where F1 hybrids are inferior to their parents may also be regarded as cases of hybrid vigour in the negative directions.

Luxuriance

• Luxuriance is the increased vigor and size of interspecific hybrids.
• The principal difference between heterosis and luxuriance lies in the reproductive ability of the hybrids.
• Heterosis is accompanied with an increased fertility, while luxuriance is expressed by interspecific hybrids that are generally sterile or poorly fertile.
• In addition, luxuriance may not result from either masking of deleterious genes or from balanced gene combinations brought together into the hybrid.
• Therefore, luxuriance does not have any adaptive significance.