Biology Reference
In-Depth Information
BOX 13.1 CALCULATING COEFFICIENTS OF RELATEDNESS,
r
,
IN HAPLODIPLOID SPECIES
General point
Males develop from unfertilized eggs and so are haploid; all of a male's sperm
are genetically identical so the probability of sharing a copy of a gene via the
father is one. Females develop from fertilized eggs and so are diploid; the
probability of sharing a copy of a gene via the mother is 0.5, because of meiosis.
Method
Draw out a pedigree, linking the two individuals through their recent common
ancestors. To determine the coefficient of relatedness between individual A and
individual B, draw arrows along the pathways, pointing from A to B. Indicate on
each link in the pathway the probability that a copy of a gene will be shared.
Examples
(a) Sister-sister
Half of a female's genes come from the father; the
probability that a copy of one of these is shared with
the sister is one. The other half come from the mother;
the probability that a copy of one of these is shared is
0.5.
+
½
½
½
1
+
+
Via mother
=
(0.5
×
0.5)
+
Via father
=
(0.5
×
1);
r
=
0.75
(b) Sister-brother
A female is linked to her brother only via her mother,
as her brother develops from an unfertilized egg. Half
of her genes come from her mother; the probability
that a copy of one of these is shared is 0.5. The other
half come from her father; the probability that a copy
of one of these is shared is zero.
+
½
½
½
+
Via mother (0.5
×
0.5)
+
Via father (0.5
×
0);
r
=
0.25
(c) Brother-brother
All of a male's genes come from his mother. There is a
0.5 chance of sharing a copy of a particular gene with
his brother.
+
½
1
Via mother (1
×
0.5);
r
=
0.5
(
d) Brother-sister
All of a male's genes come from his mother. There is a
0.5 chance of sharing a copy of a particular gene with
his sister.
+
½
+
Via mother (1
Note that haplodiploidy can lead to relatedness values
being asymmetric. For example, a brother is more
related to his sister (
r
×
0.5);
r
=
0.5
=
0.5) than she is to him
(
r
=
0.25) (compare b with d).
