Heterosis Breeding: Frequently Asked Questions | Methods | Plant Breeding

Everything you need to know about heterosis breeding !

Q. 1. What is heterosis?

Ans. The superiority of F₁ hybrid in one or more characters over its parents is called heterosis; also known as hybrid vigour.

Q. 2. Who coined the term heterosis?

Ans. The term heterosis was coined by Shull in 1914 working with maize crop.

Q. 3. What is luxuriance?

Ans. The increase of F₁ over parents in vegetative growth, but not in yield and adaptation is called luxuriance.

Q. 4. Who coined the term luxuriance?

Ans. The term luxuriance was coined by Renner in 1929.

Q. 5. What are main features of heterosis?

Ans. The main features of heterosis are listed below:

(i) The hybrid F₁ exhibits superiority over both parents in yield, adaptation, quality, maturity etc.

(ii) The superiority is confined to F₁ generation.

(iii) The superiority (heterosis) is under genetic control.

(iv) The superiority is reproducible in specific environment.

(v) The heterosis has association with specific combining ability and heterozygosity.

(vi) In hybrids, recessive deleterious genes are concealed in heterozygous condition.

(vii) The frequency of desirable heterosis is very low.

Q. 6. What are genetic bases of heterosis?

Ans. There are three possible genetic causes of heterosis, viz. dominance, over-dominance and epistasis. In other words, there are three genetic explanations of heterosis.

Q. 7. Explain dominance theory of heterosis.

Ans. According to dominance hypothesis, heterosis is the result of accumulation of dominant genes in F₁ from both the parents. Suppose the genetic constitution of female parent is AABBccdd and that of male parent as aabbCCDD. A hybrid between these two parents will have four dominant genes (Aa Bb Cc Dd) and exhibit superiority over both the parents having two dominant genes each.

Q. 8. Who proposed the dominance hypothesis of heterosis?

Ans. The dominant hypothesis of heterosis was proposed by Davenport (1910), Bruce (1910) and Keeble and Pellow (1910).

Q. 9. What are objections to dominant theory of heterosis?

Ans. There are two objections to dominance hypothesis of heterosis as follows:

(i) If the dominance hypothesis is true, it should be possible to obtain pure heterotic individuals in F₂ which are homozygous for all the dominant genes. However, such individuals are never obtained.

(ii) If the heterosis is due to dominance, the F₂ curve should be skewed towards dominant genes, but the curve of F₂ is found always smooth and symmetrical.

Q. 10. Why homozygous heterotic individuals with all dominant genes are not recovered in F₂?

Ans. Jones (1917) suggested that due to linkage between some dominant and recessive genes, it is not possible to obtain true breeding individuals with all dominant genes in F₂ generation.

Q. 11. Why the curve of F₂ generation of a hybrid is always smooth and symmetrical?

Ans. Collins (1921) provided explanation for this. He suggested that trait like yield is governed by large number of genes or polygenes which exhibit continuous variation resulting in symmetrical distribution of genes.

Q. 12. What is over-dominance hypothesis of heterosis?

Ans. According to this hypothesis, heterosis is due to superiority of heterozygote over its both homozygous parents.

The superiority of heterozygote over both homozygotes may arise due to following reasons:

(i) Production of superior hybrid substance in heterozygote which is completely different from either of the homozygous products or due to.

(ii) Greater buffering capacity in the heterozygote resulting from cumulative action/stimulation of divergent alleles.

Q. 13. Who proposed over dominance hypothesis of heterosis?

Ans. The over dominance hypothesis of heterosis was independently proposed in 1908 by Shull and East and it was supported by East (1936) and Shull (1945).

Q. 14. Who coined the term over dominance?

Ans. The term over dominance was coined by Hull in 1945 working with maize crop.

Q. 15. What is the main difference between dominance and over dominance theories of heterosis?

Ans. According to dominance hypothesis, heterosis results due to masking effect of dominant desirable alleles over harmful recessive alleles. According to over dominance, heterosis results due to complementation between divergent alleles.

Q. 16. What is the role of epistasis in heterosis?

Ans. Epistasis is of three types, viz. additive x additive, additive x dominance and dominance x dominance. The last type of epistasis (dominance x dominance) may contribute to heterosis.

Q. 17. Which is the most widely accepted theory of heterosis?

Ans. Dominance hypothesis is the most widely accepted genetic explanation of heterosis.

Q. 18. What are different methods of fixation of heterosis in crop plants?

Ans. In crop plants, heterosis can be conserved in following three ways:

(i) By asexual reproduction. This method can be used for conserving heterosis in vegetatively propagated crops such as sugarcane, potato, sweet potato, banana etc.

(ii) By apomixis. The fixation of heterosis by apomixis is common in citrus fruits, black berries, roses, blue grasses and many other flowering plants.

(iii) By polyplody. This method can be used for fixation of heterosis in interspecific and intergeneric hybrids.

Q. 19. How heterosis is conserved in Oenothera?

Ans. In evening primerose (Oenothera spp.), the heterosis is conserved by balanced lethal system. Homozygotes are lethal, hence they die. Only heterozygotes survive.

Q. 20. List genetic factors which affect, level of heterosis in crop plants.

Ans. In crop plants, the level of heterosis has been reported to be associated with genetic diversity, genetic base and adaptability of parental lines involved in hybridization programme. Moreover, the level of heterosis is generally higher in cross pollinated species than in self-pollinated crops.

Q. 21. How will you estimate heterosis?

Ans. Heterosis is estimated in three ways, viz., over the average value (mean value) of both the parents, over better parent and over the value of commercial cultivar/hybrid.

Q. 22. How would you classify heterosis?

Ans. The heterosis can be classified as follows:

(i) On the basis of estimation, heterosis is of three types, viz. average heterosis, heterobeltiosis and useful heterosis or economic heterosis.

(ii) On the basis of direction, heterosis is of two types viz. positive heterosis and negative heterosis.

(iii) Heterosis is also classified as real heterosis and pseudo-heterosis (Luxuriance).

Q. 23. How would you estimate average heterosis?

Ans. The average heterosis is estimated over the mean value of both the parents as follows:

Average heterosis = F₁ – MP/MP x 100 where F₁ is the mean value of F₁ hybrid and MP is the mean value of two parents involved in the hybrid or cross.

Q. 24. Who coined the term useful heterosis?

Ans. The term useful heterosis was coined by Meredith and Bridge in 1972 working with cotton crop.

Q. 25. How would you estimate useful heterosis?

Ans. Useful heterosis also known as economic heterosis is estimated over the standard commercial check variety of a region as follows:

Useful heterosis = F₁ – CC/CC x 100 where CC is the mean value of commercial cultivar.

Q. 26. How heterobeltiosis is estimated?

Ans. Heterobeltiosis is estimated over the better parent of the hybrid as follows:

Heterobeltiosis = F1 x BP/BP x 100

There BP is the value of better parent.

Q. 27. What are main effects of heterosis?

Ans. Heterosis leads to increase in yield, reproductive ability, adaptability, general vigour, quality etc.

Q. 28. What are the traits for which negative heterosis is desirable?

Ans. Negative heterosis is desirable for some characters such as earliness, height in cereals, micronaire value in cotton and toxic substances like neurotoxin in Lathyrus sativus.

Q. 29. What are the factors which help in commercial exploitation of heterosis?

Ans. In crop plants, the following factors help in the commercial exploitation of heterosis:

(i) Enough magnitude of heterosis.

(ii) High percentage of outcrossing.

(iii) Floral biology emanable for large scale production of hybrid seed with less expenditure.

(iv) Availability of male sterility and self incompatibility systems for easy hybrid seed production.

Q. 30. What is hybrid?

Ans. The progeny of a cross between genetically different (dissimilar) plants is called hybrid.

Q. 31. What is hybrid variety?

Ans. The F₁ population which is used for commercial cultivation is called hybrid variety or hybrid cultivar.

Q. 32. What are main features of hybrids?

Ans. The main features of hybrid cultivars are as follows:

(i) Hybrid varieties are vigorous and highly productive due to full exploitation of hybrid vigour.

(ii) Hybrid varieties are heterozygous but homogeneous populations.

(iii) Hybrids have wider adaptability than inbreds and purelines due to high inherent buffering capacity.

(iv) Hybrids are more tolerant to biotic and abiotic stresses than inbreds and purelines.

Q. 33. What are various types of hybrids?

Ans. Commercially cultivated hybrids are of two types, viz. intraspecific hybrids and interspecific hybrids.

Q. 34. What is intraspecific hybrid?

Ans. A hybrid between genetically different genotypes of the same species is called intraspecific or inter-varietal hybrid. Such hybrids are commonly used for commercial cultivation in several cross pollinated crops and some self pollinated crops. Intraspecific hybrids are always fertile.

Based on type of cross, intraspecific hybrids are of three types, viz.:

(a) Single cross hybrid,

(b) Three way cross hybrid, and

(c) Double cross hybrid.

Q. 35. What is single cross hybrid?

Ans. A hybrid between two inbreds or purelines is called single cross hybrid. In cross pollinated crops, such hybrids are developed from a cross between two inbred lines, whereas in self pollinated species such hybrids are developed from a cross between two purelines.

Q. 36. What are uses of single crosses?

Ans. Single crosses are useful in following ways:

(i) In developing single cross hybrid.

(ii) In developing three-way and double cross hybrids.

(iii) In predicting the performance of double cross hybrids.

(iv) In developing varieties in self pollinated species from segregating generations.

Q. 37. What is three way cross hybrid?

Ans. The hybrid progeny between a single cross and an inbred line is called three-way cross hybrid, the single cross is used as female and inbred as male. Such hybrids are developed when only three good inbreds are available.

Q. 38. What is double cross hybrid?

Ans. The hybrid progeny from a cross between two single crosses is called double cross hybrid. Such hybrids are commonly used in maize and sugar beet.

Q. 39. How many single crosses can be made using n inbred lines?

Ans. The following formula is used to decide number of all possible single crosses among n inbred lines:

All possible single crosses = n(n-1)/2

Q. 40. What is the formula used to decide all possible three-way crosses among n inbreds?

Ans. The number of all possible three-way crosses among n parents is determined by the following formula:

All possible three-way crosses = (n-1)(n-2)/2

Q. 41. How many double crosses can be made using n inbreds?

Ans. The number of all possible double crosses among n inbreds is determined by the following formula:

All possible double crosses = n (n – 1) (n – 2) (n – 3)/8

Q. 42. What is top cross?

Ans. A cross between an inbred line and an open pollinated variety is known as top cross or inbred variety cross. Top crosses are generally made for testing the combining ability of inbreds.

Q. 43. What is double top cross?

Ans. The hybrid progeny between a single cross and an open pollinated variety is called double top cross.

It is represented as follows:

Double top cross = (A x B) x open pollinated variety.

Q. 44. What is multiple cross?

Ans. A cross involving more than four inbred lines is called multiple cross or composite cross. Such crosses are made to combine monogenic characters from different sources into a single genotype. Multiple crosses generally have wider adaptation.

Q. 45. What is polycross?

Ans. Polycross refers to open pollination of a group of selected genotypes in isolation from other compatible genotypes to promote random mating among selected genotypes.

Q. 46. How the performance of a double cross hybrid is predicted?

Ans. Single crosses are used to predict the yield performance of a double cross hybrid. The mean yield of the four non- parental single crosses is used to predict the yield performance of the double cross hybrid.

Predicted yield of the double cross (A x B) x (C x D)

= (A x C) + (A x D) + (B x C) + (B x D)/4.

Q. 47. List the important hybrids of maize.

Ans. Important hybrids of maize include Ganga 2, Ganga 5, Ganga 11, Deccan 103, Makka 1, VL 42 etc.

Q. 48. Give names of some important hybrids of pearl-millet.

Ans. Important hybrids of pearl-millet include HB 3, BJ 104, MBH 110, BK 560, MH 179 etc.

Q. 49. List the important hybrids of sunflower.

Ans. Important hybrids of sunflower are BSH 1, HSFH 1, MSFH 10, KBSH 1 and KBSH 11.

Q. 50. Name the important hybrids of castor.

Ans. Important castor hybrids are GCH 2, GCH 3, Gauch 1 and GCH 4.

Q. 51. List the important hybrids of sorghum.

Ans. Important sorghum hybrids are CSH 1, CSH 5, CSH 6, CSH, CSH 10 and CSH 11 etc.

Q. 52. Give names of important intra-hirsutum cotton hybrids.

Ans. Important intra-hirsutum cotton hybrids include H 4, H 6, H 8, H 10, JKHy 1, JKHy 2, PKVHy 2, PKVHy 3, PKVHy 4, PKVHy 5, NHH 44, CICRHH 1, Savita, Surya, DHH 11, Fateh, Maruvikas, Dhanlaxmi, LHH 144, Omshankar etc.

Q. 53. What are interspecific tetraploid cotton hybrids?

Ans. Important interspecific tetraploid cotton hybrids include Varalaxmi, DCH 32, HB 224, TCHB 213, DHB 105, Sruthi etc.

Q. 54. Give names of diploid cotton hybrids.

Ans. Diploid cotton hybrids are of two types viz., intra arboreum and interspecific. Intra-arboreum hybrids include LDH 11, AAH 1, AKDH 7 and G. Cot MDH 11. Interspecific diploids hybrids include DH 7, DH 9, DDH 2 and Pha 46.

Q. 55. What are male sterility based diploid cotton hybrids?

Ans. The male sterility based diploid cotton hybrids include AAH 1, AKDH 7, and G. Cot MDH 11. All these hybrids have been developed using genetic male sterility.

Q. 56. What are male sterility based hybrids of upland cotton?

Ans. The male sterility based hybrids of upland cotton include Suguna, PKVHy 3, DKVHy 4, PKVHy 5 and MECH 11. The first hybrid is genetic male sterility based and rests are cytoplasmic genie male sterility based.

Q. 57. What are advantages of hybrid varieties?

Ans. Hybrid varieties are more productive, more uniform and attractive than synthetics, composites and open pollinated varieties.

Q. 58. What are demerits of hybrid varieties?

Ans. The main demerits of hybrid cultivars in comparison to synthetics, composites and open pollinated cultivars are as follows:

(i) The seed is expensive.

(ii) Fresh seed is to be purchased by farmers every year.

(iii) The cultivation requires more inputs.

Q. 59. What is inbreeding?

Ans. Mating between closely related individuals such as sefling and sib mating is called inbreeding. Inbreeding is required to maintain the genetic purity of the inbreds.

Q. 60. What is inbreeding depression?

Ans. The decline in fitness and vigour due to inbreeding is called inbreeding depression.

Q. 61. What is inbred?

Ans. In cross pollinated species, a true breeding line obtained by continuous inbreeding is called inbred.

Q. 62. Compare dominance and over-dominance theories of heterosis.

Ans. Comparison of dominance and over-dominance theories of heterosis is presented below in Table 16.1.