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Basic Genetics of Heritable Eye Disorders

 

Most heritable ocular disorders are transmitted as autosomal recessive genes. The GENOTYPE of an animal represents its genetic makeup relative to a given trait i.e.: normal, carrier or affected, while the PHENOTYPE represents the physical makeup relative to that trait i.e. normal or affected. An animal with a normal PHENOTYPE, may be genetically normal, or may be a carrier. In certain diseases where the onset of the disease is not noted until later in life, the animal may have a normal phenotype before onset of the disease, but is genetically affected.

For diseases with autosomal recessive mode of inheritance:

RR represents a genetically normal animal
Rr represents a carrier animal
rr represents an affected animal.

How to read these tables:

one parent
other parent offspring 1 offspring 2
offspring 3 offspring 4







Table 1: Normal X Carrier

R R
R RR RR
r Rr Rr







The result of this breeding is 50% normal (RR), 50% carriers (Rr)

Table 2: Normal X Affected

R R
r Rr Rr
r Rr Rr







The result of this breeding is 100% carriers (Rr)

Table 3: Carrier X Carrier

R r
R RR Rr
r Rr rr







The result of this breeding is 25% genetically normal (RR), 50% carriers (Rr), and 25% affected (rr)

Table 4: Carrier X Affected

R r
r Rr rr
r Rr rr







The result of this breeding is 50% carriers (Rr), 50% affected (rr)

Table 5: Affected X Affected

r r
r rr rr
r rr rr

The result of this breeding is 100% affected (rr).

For diseases with autosomal dominant mode of inheritance:

In dominantly transmitted genetics, heterozygous affected animals may be affected to a lesser degree than homozygous animals. There are a few factors which may confuse the distribution numbers of the condition in the progeny. Some heterozygotes may appear phenotypically normal under a phenomenon known as "incomplete penetrance". In some conditions, homozygotes die in utero, while heterozygotes survive.
dd represents a genetically normal animal
Dd represents a heterozygous affected animal
DD represents a homozygous affected animal.

Table 6: Normal X Heterozygous Affected

d d
D Dd Dd
d dd dd







The result of this breeding is 50% normal (dd), 50% heterozygous affected (Dd)

Table 7: Normal X Homozygous Affected

d d
D Dd Dd
D Dd Dd







The result of this breeding is 100% heterozygous affected (Dd)

Table 8: Heterozygous Affected X Heterozygous Affected

D d
D DD Dd
d Dd dd







The result of this breeding is 25% genetically normal (dd), 50% heterozygous affected (Dd), and 25% homozygous affected (DD)

Table 9: Heterozygous Affected X Homozygous Affected

D d
D DD Dd
D DD Dd







The result of this breeding is 50% heterozygous affected (Dd), 50% homozygous affected (DD)

Table 10: Homozygous Affected X Homozygous Affected

D D
D DD DD
D DD DD







The result of this breeding is 100% homozygous affected (DD).