Appendix E - INHERITANCE OF SEX-LINKED COLOUR BLINDNESS

The inheritance patterns shown below for red-green colour blindness can equally well apply to other sex-linked genes such as haemophilia. Since most sex-linked genes are usually only carried on the large X chromosome, a dash ( - ) signifies the presence of the relatively inert Y chromosome. The colour blindness gene is recessive to the gene for normal sight. Because the males have only one X chromosome and, therefore, only a single sex-linked gene at each locus, they are said to be hemizygous. The females, however, with two sex-linked genes, can be either homozygous or heterozygous.

Key to Symbols

Let	C	=	gene for normal sight (non-affected), and
	c	=	gene for red-green colour blindness (affected).
Then	CC	=	genotype of non-affected female
	C-	=	genotype of non-affected male
	Cc	=	genotype of non-affected carrier female
	c-	=	genotype of affected male
	cc	=	genotype of affected female

There are 6 possible types of matings between the above genotypes. The following section shows the expected results of each mating:

1 Affected male x Normal female

2 Normal male x affected female

3 Normal male x non-affected carrier female

4 Affected male x non-affected carrier female

5 Affected male x affected female

6 Normal male x Normal female

Affected females can only come from matings no. 4 & 5. Mating no. 2 is the type used commercially in chickens to sex day-old chicks using colour genes and genes affecting early wing feather growth.

Expected Frequencies of Individuals with Sex-linked Colour Blindness

According to Stern (1960), the frequency of the recessive gene (c) for sex-linked colour blindness in various white populations varies between .05 and .09. If we take .07 as an average figure then:

frequency of c = q = .07

and frequency of C = (1 - q) = p = .93

Using the Hardy-Weinberg law (see Falconer, 1989) the expected frequencies of the 5 genotypes in a random mating population will be as follows:

Table 34 Expected Proportions of Genotypes with Colour Blindness

 

Therefore 7.00% of males will be affected,

0.49% of females will be affected and

13.02% of females will be non-affected carriers.

The rest of the population, i.e. 93% of males and 86.5% of females, will be free of the condition.

See below for estimates of the expected proportions of the six possible mating types

Expected Proportions of the Mating Types Involving Sex-linked Colour Blindness

The Hardy-Weinberg law will also enable the frequencies of the different mating types to be predicted:

Table 35 Details of Six Mating Types

	Expected Frequencies
Mating Types	Proportions			%	Per Million Pairs of Parents
c - x  CC	p2q	=	.060 543	6.05	60,543
C - x  cc	pq2	=	.004 557	0.46	4,557
C - x  Cc	2p2q	=	.121 086	12.11	121,086
c -x  Cc	2pq2	=	.009 114	0.91	9,114
c -x  cc	q3	=	.000 343	0.03	343
C -x  CC	p3	=	.804 357	80.44	804,357
Totals			1.000 000	100.00	1,000,000

Of the first 5 mating types involving the defective gene (c), the commonest is number 3 (C - x Cc) where the parents, before marriage, will usually be unaware that they carry the condition. The rarest mating type is number 5(c - x cc), which is to be expected because of the low numbers of affected females. However, 4/5 of the matings will be between parents who are completely free of the harmful gene (C - x CC).

With sex-linked haemophilia, where the gene is much rarer, i.e. q =1/5000:

Proportion of affected males (q)=1/5000

Proportion of affected females (q²)=1/25,000,000

Proportion of non-affected (carrier) females (2pq)=1/2500 (approx).