Quantitative Inheritance in Plant Breeding

1. Quantitative Inheritance in Plant Breeding

3. Transgressive segregation • Plants with traits that arise by segregation of genes for a quantitative character that fall outside the range of the parents are known as transgressive segregates • Transgressive segregation results when progeny plants contain new combinations of multiple genes with more positive effects, or more negative effects, for a quantitative character than were present in either parent. • The accuracy with which the superior transgressive segregates can be identified will be increased if environmental variations affecting the expression of the quantitative character in the population are small in relation to the genetic variations.

4. Heritability • If two plants selected at random from a mixed population differ in yield, the yield difference may be due to: • Hereditary differences in the plants. • Differences in the environments in which the plants were grown. • A combination of both. • The effectiveness of selecting for plants with high yield within a mixed population will depend upon: • The extent to which the variability in yield of individual plants in the population is the result of genetic factors. • How much the variability in yield among the plants is due to the environment in which the different plants are grown.

5. Selection of plants for high yield would be ineffective if the environmental variation was so great that it masked the genetic variation. • Heritability: • Capability of being inherited. • If the genetic variation in a progeny is large in relation to the environmental variation, then heritability will be high; or if genetic variation is small in relation to the environmental variation, then heritability will be low. • Selection is more effective when genetic variation in relation to environmental variation is high than when it is low.

6. Heritability Estimates • The statistic that is important here is the variance V. • The variance calculated from the observed variations in the quantitative character constitutes the phenotypic varianceVP. • The phenotypic variance may in turn be divided into three components: • Genetic variance VG. • Nongenetic or environmental variance VE. • Interaction between genotype and environment VGE. It follows then that VP = VG+VE + VGE

7. The genetic variance (VG) is composed of three major components: • Additive genetic variance (VA). • Dominance variance (VD). • Nonallelic interactions or epistasis variance (VI). • This may be written • VG= VA + VD + VI. • The resemblance between parents and offspring is largely the result of additive genetic. • The dominance component represents the deviation of the heterozygote from the mid-parent or average of the homozygous parents. • The interaction variance results from deviations caused by epistatic effects of nonallelic genes.

8. VP = VG + VE + VGE VA VNA VD VI So now: VP = VA + VD + VI + VE + VGE

9. Types of Heritability Estimates. Heritability may be given by H = VG /VP = VG/(VG+VE+VGE). H = (VG /VP) x 100 • is referred to as heritability in the broad sense (H), because it estimates heritability on the basis of all genetic effects • A more restrictive and often more useful estimate, is obtained if heritability is expressed as a percentage of the additive variance, • h2 = (VA/VP) x 100. The result is represented as heritability in the narrow sense (h2), Narrow-sense heritability cannot exceed, and is usually less than, broad-sense heritability.

10. Heritability and Selection • Quantitatively inherited characters differ in heritability. • Characters not greatly influenced by environment usually have a high heritability. • This may influence the choice of selection procedure used by the plant breeder. • Selection in the F2 of a cross between homozygous parents (F1 if parents were heterozygous) will not be very effective for characters that have low heritability. • Selection in the F2 is more effective if it is limited to characters that have a high heritability.

11. Selection for characters with low heritability may be made more effectively if based on F2 progeny performance. • Examples of characters with relatively high heritability over a range of environments are heading date and kernel size in wheat; ear height, flowering date, and ear characters in corn; and maturity date in soybean. • Yield, lodging resistance, winter survival, and protein content generally have low heritability estimates.

12. The principal uses of heritability estimates are: • To determine the relative importance of genetic effects which could be transferred from parent to offspring. • To determine which selection method would be most useful to improve the character, • To predict gain from selection (genetic advance).

13. If a plant breeder is working with clonally propagated crops such as sugarcane, bananas, or if the crop reproduces apomictically, estimates of broad-sense heritability would be appropriate because vegetative propagation and apomixis fix both additive and non-additive (dominance plus epistatic) gene action and transfer it from parent to offspring.

14. Selection Intensity and Genetic Advance • The plant breeder is continually evaluating mixed populations of breeding materials to identify individual plants or breeding lines with superior genetic potential. • The breeder is faced with choosing, based on phenotypic performance, the number that should be selected without risk of omitting a superior line that did not perform up to its potential because it grew in an unfavorable environment. • Also, it would be useful to the breeder to know the progress that can be made with different selection intensities in composite populations. • 10% of the population or breeding lines be kept, or can it be reached by keeping 5 % of the population?

15. Simply, the expected gain or genetic advance with one cycle of selection can be predicted from the population variance and heritability of the quantitative character being studied: Gs = (i)(√VP)(h2) • In this formula, • Gs is the predicted genetic advance. • i a constant based on selection intensity in standard deviation units. • √VP the square root of the phenotypic variance. • h2 the narrow-sense heritability of the quantitative character being evaluated. • H the broad-sense estimates are used for clonally or apomictically reproducing crops.

16. Values for i differ with selection intensities and are available from statistical tables. Representative values of i for some selection intensities commonly practiced by plant breeders are:

18. The equation for calculating genetic advance may be modified by inserting a parental control factor (c) that varies with the pollen source. Gs = (c)(i)(√VP)(h2). • If selection is based on the phenotype of the female without regard to pollen source, as in a field of open-pollinated corn, the parental control factor (c) is 0.5; if both parents are selected and intermated in isolation to constitute the next cycle of selection, the parental control factor is 1.0. • It is evident that the theoretical gain from selection is doubled if both paternal and maternal selection are practiced rather that only maternal selection.

19. The equation for calculating genetic advance may be modified if different information is desired. • For example, assuming one generation is completed per year, the breeder may wish to determine the expected gain per year, over the number of years required to complete a cycle of selection. If so, the gain per year (Gy), would be • Gy= [(c)(i)( √VP)]/y • Where y = number of years required to complete a cycle of selection.

20. Example • Magnesium concentration in random samples of the outcrossing species, tall fescue. • From variance components it was determined that: • The additive genetic variance was 0.114. • The genotype x location x year variance was 0.035. • The error variance was 0.022 for magnesium concentration. Therefore, the narrow-sense heritability percentage estimate was h2 = [0.114/(0.114 + 0.035 + 0.022)] = 0.114/0.171 = 0.67

21. After selecting the top 5% of the plants with the highest magnesium concentration and practicing maternal and paternal selection: • Gs = (2.063)(l)(√0.171)(0.67) = 0.57 • If the mean magnesium concentration is 4.5 mg/g on a dry matter basis in the unselected population, • The gain from selection expressed as a percentage of the mean is (0.57/4.5) x 100 = 12.6%; • This means that if the top 5% of the tall fescue plants are chosen, the magnesium concentration would be expected to increase by 12.6% after one cycle of selection.