Complex Inheritance

What is complex inheritance?

There are two main types of complex inheritance:

1) Multifactorial inheritance - multiple genes are involved along with an environmental influence

2) Polygenic inheritance - multiple genes are involved with no environmental influence

Types of complex traits:

1) Discrete characters (or discontinuous traits) - these are traits that you either have or you don't. For example: colour mutations, birth defects, and common behavioral disorders. The threshold model is used to explain discontinuous traits. It simply states that the underlying variable has a continuous distribution (as shown by the graph), but the disorder does not appear until a certain threshold is reached (the dotted line).

                  clip_clip_image003

With this form of complex inheritance, the more affected relatives you have, the increased number of liability factors you possess, and the increased severity of the disease or trait.

2) Continuous traits - these are traits that everyone has and they vary from person to person. For example: height, weight, intelligence. These traits always have a normal distribution.

                       clip_clip_image005

3) Genetic Heterogeneity - the genes causing a disorder in one family may not be the genes causing the disorder in another family. For example: epilepsy. It may be caused by single gene disorders, multifactorial inheritance, chromosomal disorders, or environmental factors such as brain injuries.


What are colour mutations?

• Mutations are generally thought of as autosomal recessive (or sex-linked recessive or autosomal dominant in rare cases). While this may be true for some mutations, it may not be the case for others. Certain mutations, such as Blue in Pacific Parrotlets and Lineolated Parakeets, have a tremendous range of shades and hues which are not explained by the simple autosomal recessive model. There are a few possible modes of inheritance that may explain this occurrence:

1) Autosomal recessive inheritance with multiple alleles (alternative forms of a gene). For example: Green (Wild type form), Light Blue, Medium Blue, and Dark Blue. This model would give you a range of colours, but does not account for the colour variation seen on different parts of the body. This phenomenon may be responsible for body colour, but other genes have to be involved to determine the colour for other parts of the body (e.g. back colour, face colour, etc.).

2) Multiple linked genes inherited in an autosomal recessive manner (this would be a form of polygenic inheritance). Linked (or Linkage) means that all of the genes are very close to each other on the chromosome and are always inherited together. The most likely manner for this form of inheritance to occur in is the following manner: Imagine that there is one main gene which is responsible for the main colour mutation and is inherited in an autosomal recessive fashion (the same as the conventional autosomal recessive Blue mutation), but this gene has other genes linked to it that have an effect on colour as well. For example: call the main colour mutation gene "Gene A" and the genes that are linked to it "Genes B, C, D, & E". Gene A is responsible for the main colour mutation (being either Green or Blue), Gene B is responsible for face colour, Gene C is responsible for back colour, Gene D is responsible for rump colour, and Gene E is responsible for tail colour. Each of the different genes (B, C, D, & E) would have 2 or more alleles to exert an effect on the colour of the bird. This model allows for the great variation seen from parrotlet to parrotlet.

 Gene A - main colour mutation

AA = Green, Aa = Split, aa = Blue

Gene B - face colour 

BB = bright blue face, Bb = medium blue face, bb = light blue face

Gene C - back colour

CC = wild type (no grey wash), Cc = light grey wash, cc = smoke grey wash

Gene D - rump colour

DD & Dd = Dark (deep cobalt in males, bright blue in females), dd = light (cobalt in males, turquoise in females)

Gene E - tail colour

EE & Ee = Wild type (regular colour), ee = bright turquoise-green

• This way you can get all of the different colour variations present in green and blue parrotlets. For example: a Blue bird with a medium blue face, a light grey washed back, a dark rump, and a turquoise green tail.

• This model may involve 1, 2, 3, 4 (this example), 5, or more different linked genes that alter the effect of the main colour gene. I just chose 4 different linked genes (B, C, D, & E) to correspond to the main parts of the body for the purposes of explaining this possible model.

3) More than one mutation present. For example: white birds which consist of both the yellow and blue mutations. If this were the case, one would expect to see odd Blue birds (like the one outlined in #2) to occur at a MUCH lower frequency than what they do.

• Why is it apparent that there is more than one gene responsible for the blue mutation?

1) great colour variation between birds.

2) differences in body colouration. E.g. We have birds that are entirely turquoise-grey or entirely bright turquoise. We also have a bird that has a medium turquoise face, a light turquoise body, a light grey back, and a turquoise-green tail and wing feathers and another that has a bright turquoise face, a smoky grey back, a grey-turquoise body, and a turquoise-green tail and wing feathers. If the blue mutation was the result of only one gene controlled by autosomal recessive inheritance, the first two colour types would be all that you would see and the last two would never exist. Since some birds have no real colour variation throughout their body while others have a lot, it leads one to conclude that there are multiple genes involved in regulating the colour of different parts of the body.  To try to determine what method of inheritance that all of the shades of blue follow, breeders would have to keep a detailed record of the different shades that they breed and the colour shades of all of the resultant offspring. But to do this, we would have to come up with names of convention for each shade so that the data is usable.

• We will never know what exactly is behind any of these mutations until the entire genome of these birds is sequenced and the function of each and every gene is determined.

• Since parrotlets are not on the list of model organisms frequently studied by humans this is not likely to occur any time soon or even at all. 

Contact Krissy Bird - all pictures are copyrighted to me unless otherwise noted - April 12, 2011