1. Lecture 6: Inbreeding and Heterosis

2. Inbreeding Inbreeding = mating of related individuals
Often results in a change in the mean of a trait
Inbreeding is intentionally practiced to:
create genetic uniformity of laboratory stocks
produce stocks for crossing (animal and plant breeding)
Inbreeding is unintentionally generated:
by keeping small populations (such as is found at zoos)
during selection

3. Genotype frequencies under inbreeding
The inbreeding coefficient, F
F = Prob(the two alleles within an individual are IBD) -- identical by descent
Hence, with probability F both alleles in an individual are identical, and hence a homozygote
With probability 1-F, the alleles are combined at random

4. Alleles IBD Random mating Genotype Alleles IBD Alleles not IBD
frequency
A1A1 Fp
(1-F)p2
p2 + Fpq
A2A1
(1-F)2pq
A2A2 Fq
(1-F)q2
q2 + Fpq

5. Changes in the mean under inbreeding
Genotypes A1A1 A1A2 A2A2
0 a+d 2a
freq(A1) = p, freq(A2) = q
Using the genotypic frequencies under inbreeding, the
population mean mF under a level of inbreeding F is
related to the mean m0 under random mating by
mF = m0 - 2Fpqd

6. For k loci, the change in mean is
Here B is the reduction in mean under
complete inbreeding (F=1) , where
• There will be a change of mean value dominance is present (d not zero)
• For a single locus, if d > 0, inbreeding will decrease the mean value of the trait. If d < 0, inbreeding will increase the mean
• For multiple loci, a decrease (inbreeding depression) requires
directional dominance dominance effects di tending to be positive.
• The magnitude of the change of mean on inbreeding depends on gene
frequency, and is greatest when p = q = 0.5

7. Inbreeding Depression and Fitness traits
Inbred
Outbred

8. Define ID = 1-mF/m0 = 1-(m0-B)/m0 = B/m0
Drosophila Trait
Lab-measured ID = B/m0
Viability
0.442 (0.66, 0.57, 0.48, 0.44, 0.06)
Female fertility
0.417 (0.81, 0.35, 0.18)
Female reproductive rate
0.603 (0.96, 0.57, 0.56, 0.32)
Male mating ability
0.773 (0.92, 0.76, 0.52)
Competitive ability
0.905 (0.97, 0.84)
Male fertility
0.11 (0.22, 0)
Male longevity
0.18
Male weight
0.085 (0.1, 0.07)
Female weight
-0.10
Abdominal bristles
0.077 (0.06, 0.05, 0)
Sternopleural bristles
-.005 (-0.001, 0)
Wing length
0.02 (0.03, 0.01)
Thorax length
0.02

9. Why do traits associated with fitness show inbreeding depression?
Two competing hypotheses:
Overdominance Hypothesis: Genetic variance for fitness is caused by loci at which heterozygotes are more fit than both homozygotes. Inbreeding decreases the frequency of heterozygotes, increases the frequency of homozygotes, so fitness is reduced.
Dominance Hypothesis: Genetic variance for fitness is caused by rare deleterious alleles that are recessive or partly recessive; such alleles persist in populations because of recurrent mutation. Most copies of deleterious alleles in the base population are in heterozygotes. Inbreeding increases the frequency of homozygotes for deleterious alleles, so fitness is reduced.

10. Estimating B
In many cases, lines cannot be completely inbred due to
either time constraints and/or because in many species
lines near complete inbreeding are nonviable
In such cases, estimate B from the regression of mF on F,
mF = m0 - BF m0 mF F 1
m0 - B
If epistasis is present, this regression is non-linear,
with CkFk for k-th order epistasis

11. Minimizing the Rate of Inbreeding
Avoid mating of relatives
Maximize effective population size Ne
Ne maximized with equal representation
Ne decreases as the variance of contributed offspring increases
Contribution (number of sibs) from each parent as equal as possible
Sex ratio as close to 1:1 as possible
When sex ratio skewed (r dams/sires ), every male should contribute (exactly) one son and r daughters, while every female should leave one daughter and also with probability 1/r contribute a son

12. Line Crosses: Heterosis
When inbred lines are crossed, the progeny show an increase in mean
for characters that previously suffered a reduction from inbreeding.
This increase in the mean over the average value of the
parents is called hybrid vigor or heterosis
A cross is said to show heterosis if H > 0, so that the
F1 mean is average than the average of both parents.

13. Expected levels of heterosis
If pi denotes the frequency of Qi in line 1, let pi + di denote
the frequency of Qi in line 2.
The expected amount of heterosis becomes
• Heterosis depends on dominance: d = 0 = no inbreeding depression and no.
heterosis as with inbreeding depression, directional dominance is required for heterosis.
• H is proportional to the square of the difference in gene frequency
Between populations. H is greatest when alleles are fixed in one population and
lost in the other (so that | di| = 1). H = 0 if d = 0.
• H is specific to each particular cross. H must be determined empirically,
since we do not know the relevant loci nor their gene frequencies.

14. Heterosis declines in the F2
In the F1, all offspring are heterozygotes. In the F2,
random mating has occurred, reducing the frequency
of heterozygotes.
As a result, there is a reduction of the amount of
heterosis in the F2 relative to the F1,
Since random mating occurs in the F2 and subsequent
generations, the level of heterosis stays at the F2 level.

15. Crossing Schemes to Reduce the Loss of Heterosis: Synthetics
Take n lines and construct an F1 population by
making all pairwise crosses
Allow random mating from the F2 on to produce a
synthetic population
H/n ( )
Only 1/n of heterosis
lost vs. 1/2

16. Schemes to Reduce the Loss of Heterosis: Rotational Crossbreeding
Suppose we have three “pure” lines, A, B, C
Originally suggested for pig populations
Dam
Sire A B
AxB C
Each generation, cross
a crossbred dam with a sire
from the next line in the sequence
AxBxC A

17. b R = S C ° P 2 1 * n For a two-line rotation
H/3 vs. H/2 for synthetics
General expression for n-line rotation
Subset of all pair-wise crosses (non-adjacent
Crossed lines)
Average of parental lines b R n = S C P 2 1 *
Average of all pair-wise crosses
Can show the expected mean is greater than the
corresponding n-line synthetic

18. Individual vs. Maternal Heterosis
Individual heterosis
enhanced performance in a hybrid individual
Maternal heterosis
enhanced maternal performance (such as increased litter size and higher survival rates of offspring)
Use of crossbred dams
Maternal heterosis is often comparable, and can be greater than, individual heterosis

19. Individual vs. Maternal Heterosis in Sheep traits
Individual H
Maternal H
total
Birth weight
3.2%
5.1%
8.3%
Weaning weight
5.0%
6.3%
11.3%
Birth-weaning survival
9.8%
2.7%
12.5%
Lambs reared per ewe
15.2%
14.7%
29.9%
Total weight lambs/ewe
17.8%
18.0%
35.8%
Prolificacy
2.5%
5.7%

20. Agricultural importance of heterosis
Crosses often show high-parent heterosis, wherein the
F1 not only beats the average of the two parents
(mid-parent heterosis), it exceeds the best parent.
Crop
% planted as hybrids
% yield advantage
Annual added yield: %
Annual added yield: tons
Annual land savings
Maize 65 15 10
55 x 106
13 x 106 ha
Sorghum 48 40 19
13 x 106
9 x 106 ha
Sunflower 60 50 30
7 x 106
6 x 106 ha
Rice 12 4
15 x 106

21. Variance Changes Under Inbreeding
Inbreeding increases the variation between populations
(i.e., variation in the means of the populations)
Inbreeding reduces variation within each population
F = 1
F = 3/4
F = 0
F = 1/4

22. Variance Changes Under Inbreeding
General
F = 1
F = 0
Between lines
2FVA
2VA
Within Lines
(1-F) VA VA
Total
(1+F) VA

23. Mutation and Inbreeding
• Eventually these two forces balance, leading to
an equilibrium level of genetic variance reflecting
the balance between loss from drift, gain from mutation
• As lines lose genetic variation from drift, mutation
introduces new variation
VM = new mutation variation each generation, typically
VM = 10-3 VE
Assuming:
Strictly neutral mutations
Strictly additive mutations
Symmetrical distribution of mutational effects

24. Between-line Divergence
The between-line variance in the mean (VB) in generation
t is
For large t, the asymptotic rate is 2VMt
Implications: Two identical lines will have their
difference in means eventually (approximately)
following a normal distribution with mean 0 and
variance 2VMt, e.g., m(1) - m(2) ~ N(0, 2VMt)