🧬 Types of Hybrids

This lesson covers core principles and exam-focused points from this topic in plant breeding.

---

They are the first generation from crosses between two pure lines, inbreds, open

pollinated varieties of other populations that are genetically not similar.

Pure line hybrids: Tomato.

Inbred hybrids: Maize, bajra.

---

Kinds of hybrids

---

Single cross hybrids

AxB

Crossing two inbreeds or pure lines.

---

---

Three way cross hybrid

(A x B) x C

A cross between a single cross hybrid and an inbred.

---

---

Double cross hybrid

(A x B) x (C x D)

cross between two Fls.

---

---

Double Top Cross hybrid

Double Cross hybrid crossed with open pollinated variety.

Operation in production of hybrids.

In production of hybrids inbreds are preferred rather than open pollinated varieties

for the following reasons.

1. Inbreds can be maintained without a change in the genotype. Whereas open pollinated

variety cannot be maintained pure. They may alter genotypically due to natural selection

etc.

2. The hybrids derived from inbreds will be uniform where as it may not be in case, of open

pollinated variety.

3. The inbreds are homogenous and their performance can be predicted where as open

pollinated variety are heterogenous and their prediction in performance cannot be made.

---

Development of inbreds

1. By inbreeding, selfing etc.

2. Development of inbreds from haploids - rice, sorghum, maize.

---

---

Evaluation of inbreds

a) Phenotypic evaluation

Based on phenotypic performance. Highly suitable for characters with high heritability. .

b) Top cross test

Top cross test provides a reliable estimate of GCA. The selected inbreds will be crossed

to a tester parent with wide genetic base i.e. open pollinated variety. The cross progenies will

be evaluated in replicated progeny rows. Based on results better inbreds can be selected.

c) Single cross evaluation

The developed inbreds can be crossed and the single crosses can be estimated in

replicated trial. Out standing hybrids tested over years in different locations, then released.

d) Prediction of double cross performance

'The predicted performance of any double cross is the average performance of the four

non parental single crosses involving the four parental inbreds".

Inbreds : A, B, C, D.

6 possible single crosses = A x B, A x C, A x D, B x C, B x D, C x D.

From these 3 double crosses produced = (A x B) x (C x D)

(A xC) x (B x D)

(A x D) x (B x C)

The performance of these anyone double cross can be predicted from performance of the four

single crosses not involved in producing that particular hybrid.

(A x B) x (C x D)

A x C

A x D

B x C

B x D

---

Average

---

---

Production of Hybrids

---

Methods

I. Hand emasculation and dusting - Cotton, Tomato, Chillies, Bhendi

2. Use of male sterile lines

a) Cytoplasmic male sterility - ornamentals

b) Genic male sterility - Redgram, Castor.

c) Cytoplasmic - genic male sterility Jowar, Bajra, Rice

3. Use of self in compatibility

By planning cross compatible lines hybrids are produced. Here both are hybrids.

E.g. Brassicas.

---

---

Success of hybrids

a) Easy hand emasculation

b) Abundant seed set to compensate cost of hand emasculation.

c) Stable male sterile lines. .

d) Effective restorers.

e) Effective pollen dispersal.

---

---

Synthetic Varieties

A synthetic variety is produced by crossing in all combinations a number of inbreds (4-6)

that combine well with each other. The inbreds are tested for their GCA. Once synthesised, a

synthetic is maintained by open pollination. The lines that make up a synthetic may be usually

inbred line but open pollinated variety, or other population tested for general combining ability

are also be used.

Synthetic varieties are common in grasses, clover, maize and sugar beets. The normal

procedure is equal amounts of seeds from parental lines (Syn0) is mixed and planted in isolation.

Open pollination is allowed. The progeny obtained is Synl. This is distributed as synthetic variety

or it may be grown in isolation for one more season and Syn2 is distributed.

---

Merits

1. Less costly compared to hybrids.

2. Farmer can maintain his synthetic variety for more seasons which is not possible in

hybrids.

3. Because of wider genetic base the synthetics are more stable over years and environments.

4. Seed production is more skilled operation in hybrids where as it is not so in synthetics.

---

---

Demerits

1. Performance is little bit lower compared to hybrids because synthetics exploit only

GCA while hybrids exploit both GCA and SCA.

2. The performance may not be good when lines having low GCA are used.

---

---

Composites

It is produced by mixing seeds of phenotypically outstanding lines and encouraging open

pollination to produce crosses in all possible combinations among mixed lines. The lines used to

produce a composite are rarely tested for combining' ability. So the yield of composite varieties

cannot be predicted easily. Like synthetics, composites are commercial varieties and are

maintained by open pollination.

Composites were released in maize - Amber, Jawahar, Kissan.

Synthetic Composite

Parental components are generally inbreds

tested for their GCA

No of parental lines are limited to 4 - 6

inbreds

Synthetic produced with inbreds can be

reconstituted

It is not so in composite. The linesare not

tested for their GCA.

No such limit

It is not possible

Yield performance can be predicted Cannot be predicted

---

Poly Cross Test

This is done to estimate the GCA in crops where production of inbred is not possible.

This is followed generally in grasses. Poly cross test is based on seeds obtained by random

mating among the clones. Each clone is planted at different date to facilitate random mating.

Polycrosses are generally not perfect since mating may not be at random.

---

---

Combining ability

Ability of a strain to produce superior progeny when crossed with other strains.

---

---

General combining ability

Average performance of a strain in a series of cross combinations. The GCA is estimated

from the performance of F1 S from the crosses. The tester will have a broad genetic base.

---

---

Specific combining ability

Deviation in performance of a cross combination from that predicted on the basis of

general combining ability of the parents involved in the cross. The testing will be on inbred.

---

---

---

Summary Cheat Sheet

Quick Recall Points

• This lesson focuses on key plant breeding concepts, terminology, and exam-relevant applications.
• Review major definitions, classifications, and method-wise distinctions from the sections above.
• Revise tables and examples from this lesson for fast pre-exam recall.

Exam Traps

• Do not confuse similarly named breeding methods without checking their core selection logic.
• Pay attention to crop-specific examples because the same principle can behave differently by species.

References

1 source • [1]

[1]

Standard Plant Breeding Class Notes (GPBR211)

Book