THE GENETICS OF CROP EAR

This article will attempt to give a description of how the genes work that give rise to crop eared animals and to postulate some explanations as to discrepancies on the subject. The important genetic facts in this explanation are taken from the scientific paper referred to at the end of this article. The writers used information gathered from 548 stud book Highlands on 15 different farms in Germany.

General Concepts

 This research has suggested that this ear defect is inherited by a single autosomal dominant gene, however there is incomplete dominance. ‘Autosomal’ means not sex-linked and ‘incomplete dominance’ refers to the fact that animals with the gene will show varying degrees to which they are affected. Some will be so severe that the external ear is barely recognisable and some so mild that the notch in the ear is barely noticeable.

 The nomenclature that will be used in this article is the standard nomenclature that is used in all genetics. In this article we will call the gene for crop ear ‘C’ and the small letter ‘c’ will be the gene for normal ears. All animals will have two genes for ear shape - one derived from their dam and one derived from their sire. This is the same as all autosomal genes.

 Only animals with two genes for normal ears (‘cc’) will have normal ears. An animal with one or two genes for crop ear (‘CC’ or ‘Cc’) will have crop ear. This is the case with all dominant genes. Crop ear is dominant over normal ears.

The Possible Genetic Combinations

 CC
·  These animals will have crop ear (the most severe form).
· These animals will pass the gene on for crop ear to their offspring - guaranteed.  All of their calves should have some degree of crop ear.
· These animals MUST have received one dominant gene (C) from their sire and one from their dam. (i.e. both of their parents must have had crop ear).

 Cc
· These animals should also have crop ear but these are the ones that account for the variation in severity. Some will be severe (although none quite as severe as CC animals) and some mild.
· These animals have a 50:50 chance of passing the dominant gene for crop ear (C) onto their offspring.
· These animals must receive the dominant gene (C) from one parent, and the recessive gene (c) from the other parent.

 cc 
· These animals CAN NOT have crop ear. All animals in Australia should be like this.
· Because these animals only carry the recessive gene 'c', they can not possibly give rise to an animal with crop ear when bred to each other.
 

 An interesting way to look at it is with all the possible matings involving these genes.

Table - Genetic Combinations of Possible Matings

	CC	Cc	cc
CC	100% CC	50% CC 50% Cc	100% Cc
Cc	50% CC 50% Cc	25% CC 50% Cc 25% cc	50% Cc 50% cc
cc	100% Cc	50% Cc 50% cc	100% cc

CC bred with CC	- all offspring should have crop ear - 100% of offspring will be CC (affected with severe crop ear)
CC bred with Cc	- all offspring should have crop ear - 50% will be CC (severe crop ear) and 50% will be Cc (still crop ear but variable degrees)
CC bred with cc	- all offspring should have crop ear - 100% will be Cc (variable degrees of crop ear, but none should be clear of it)
Cc bred with Cc	- ¾ of offspring should have crop ear (25% CC, 50% Cc), and ¼ will be free of it (25%cc)
Cc bred with cc	- ½ of offspring should have crop ear - 50% Cc (variable crop ear), 50% cc (no crop ear)
cc bred with cc	- all offspring will be free of crop ear.

 

Facts We can Derive

According to this data:

►	You should never get crop ear in a calf bred from two animals that are genetically free of crop ear.
►	If a calf is born with crop ear, then at least one of the parents MUST have had a gene for crop ear (‘C’).

 As I have just said, a crop-eared calf must have at least one parent with at least one crop ear gene (Cc or CC). We can not say that an animal with a crop ear gene will always have visible crop ear though.

 

 Possible explanations for crop eared calves that come from supposed crop ear free animals include:

(i)       One of the parents has mild crop ear (a very small notch) that was not detected. Highlands with very mild crop ear will not be picked up purely on a visual basis. You need to palpate the ears very carefully or even clip the ears to be sure.

(ii)    One parent has ears that are smaller than normal but they have no notches (this may be another variation of the Cc animals). This has been reported anecdotally to occur in animals that have crop-eared offspring.

(iii)     There is a very small percentage of animals with one gene for crop ear (Cc) that have normal ears (no notch and a normal size overall).

(iv)     Incorrect parentage identification. Meaning that the recorded parents are not the real parents of the calf.

(v)      A mutation in the calf’s genetic make up. This would be possible but would be an extremely rare event.
 

 

Reference
 ‘Inheritance of notched ears in Highland cattle’
by A. Scheider, P. Schmidt and O. Distl,
Berl Munch Tierarztl Wochenschr. 1994 Oct; 107(10): 348-352