Pedigree Creation and Analysis
Pedigrees can be used to keep records (they are similar to a family tree), track a colour mutation or genetic disorder through a particular family, and determine whether a colour mutation is autosomal (dominant or recessive) or sex-linked (dominant or recessive).
The diagrams on the side are those commonly used in pedigree analysis and are essential to understanding and interpreting a tree when you see one.
LEGEND
1. Symbol for a male
2. Symbol for a female
3. Symbol for when the sex of an individual is unknown
4. A line connecting the middle of two individuals symbolizes a mating
5. A line dropping down to individuals indicates that these are the children/offspring. Their order indicates their order of birth. Oldest on the left, youngest on the right)
4 & 5 together indicates a family (2 parents & 2 children)
6, 7, & 8 represent dizygotic (non-identical) twins
9 & 10 represent monozygotic (identical) twins
11. Symbol for an affected individual (may have colour mutation or genetic disorder)
12. Symbol for a carrier or heterozygote for an autosomal colour mutation or genetic disorder (equivalent to a Split)
13. Symbol for a carrier or heterozygote for a sex-linked colour mutation or genetic disorder
14. Symbol for a deceased individual
15. Symbol for the propositus or proband - the individual in which the mutation originates
16. Symbol for a consanguineous marriage - mating between closely related individuals (for example siblings, mother-son, father-daughter, and cousin-cousin)
17. The format of a typical pedigree. The Roman Numerals on the left hand side indicate the generation of the individuals. The normal numbers under each individual indicates their placement in the tree for each generation.
There are 4 basic patterns of single gene inheritance which are Autosomal Dominant, Autosomal Recessive, Sex-linked Dominant, & Sex- linked Recessive. Anything autosomal has to do with the non-sex chromosomes (anything other than X or Y chromosomes in humans). Anything Sex-linked has to do with the sex chromosomes (X and Y in humans or Z and W in Birds).
Autosomal Inheritance
Since I am not certain of particular parrotlet genetic disorders that are due to these 2 types of inheritance, I simply made these up to give you some examples. The Autosomal dominant colour mutation chart is made up too since there are no known dominant colour mutations in the Pacific. They are just designed to give you an example and to show the certain guide lines for each so you can recognize them if you ever run across them.
CRITERIA FOR AUTOSOMAL DOMINANT INHERITANCE - GENETIC DISORDER
(1) the phenotype appears in every generation, with each affected individual having an affected parent
(2) any child of an affected parent has a 50% risk of inheriting that trait
(3) Phenotypically normal family members do not transmit the phenotype to their children
(4) Males and females are equally likely to transmit the phenotype to children of either sex. in particular, male to male transmission can occur and males can have affected daughters (do not see this with Sex-linked inheritance)
LEGEND - Genetic Disorder is Long Beak (BB)
(1) Regular Bird (bb)
(2) Bird with long beak (Bb)
(3) Bird with long beak (BB)
*There are no carriers/ heterozygotes in an autosomal dominant genetic disorder. All heterozygotes (Bb) are affected with the disorder.
THE NUMBERING SYSTEM IN THESE CHARTS
The first black circle on the top left hand side of the tree is numbered as I. 1. The white square that is next to it is numbered as I. 2. The first black square on the left side, second row is II. 1. The white circle next to it is II. 2. and the black square to the right of II. 2. is II. 3. and so on....... #8 in the bottom row is.......? I f you said V. 8. then you are right! If not go through the explanation again to see the logic. What is the Black circle in the 4th row with the #4 under it? IV. 4.
Autosomal Dominant Genetic Disorder (Long Beak)
As you can see (1) the phenotype is in every generation (2) any child with an affected parent has a theoretical, not always practical chance of inheriting that trait (3) normal family members do not transmit the trait (4) males and females are equally likely to transmit that trait to children of either sex.
I know most of you will not be tracing genetic disorders with charts (as I am sure there are not many known in parrotlets), but you may want to trace family trees/pedigrees to keep track of various information on each parrotlet that you own and breed. This is why I made a second chart for Autosomal Dominant, but it instead involves a colour mutation instead of a genetic disorder. This is a made up example, but it illustrates the concept very well.
This chart shows how you can trace a trait (such as a colour mutation) through a whole network of birds. Assume that all birds that are not shown to breed and give offspring were sold as pets or no further records are known. You could catalogue every bird that you breed and its characteristics in a pedigree like this. That way you will know which traits pass on and what type of inheritance they fall into. As this is a colour mutation, it does not follow the guide lines for the autosomal genetic disorders completely. Each affected bird may be of either sex and they can transmit the trait to offspring of either sex. But the phenotype does not have to be in every generation (this depends on the phenotype and genotype of the birds that you breed them too). Another important difference is that there are heterozygotes/ carriers/ Splits that may show a combination of both traits or may just appear like the dominant trait (in this example they are medium green, but they could also be dark green like "DD" birds). Also if 1 parent is DD and the other is dd, then the offspring have a 100% chance of being Dd, instead of a 50% chance of being affected (DD) like in genetic disorders.
Autosomal Dominant Colour Mutation
LEGEND: Example is a Dark Factor Colour Mutation (DD)
1) Regular Green Bird (dd)
2) Medium Dark Green Bird (Dd)
3) Dark Green Bird (DD)
CRITERIA FOR AUTOSOMAL RECESSIVE INHERITANCE - GENETIC DISORDER
(1) an autosomal recessive phenotype. If it appears in more than 1 member of the immediate family, it typically is seen only in the siblings of the propositus and not in the parents or offspring or other relatives. It skips generations.
(2) For most autosomal recessive traits, males and females are equally likely to be affected
(3) The parents of the affected person may be consanguineous (related). This is especially likely if the gene responsible for the condition is rare in a population.
(4) The recurrent risk for each sibling of the propositus is 1/4 (if both parents are unaffected)
WHAT THE CHART SHOWS:
(1) it is not seen in any immediate family members and it does skip generations
(2) males and females are equally affected
(3) the parents of the proband (IV. 4.) are related
(4) unknown with this chart.
*These types of disorders almost always occur from inbreeding!!!*
LEGEND: Genetic Disorder is No Toe Nails (nn)
(1) Regular Bird (All toes have toe nails = NN)
(2) Regular bird, but carries 1 copy of the "no toe nail" gene (It is a heterozygote/carrier = Nn)
(3) Bird with no toe nails (Affected = nn)
Autosomal Recessive Genetic Disorder
Autosomal Recessive is what most colour mutations (if not all) are so this is a great way to track them. Remember the Blue and Yellow mutations from the single gene mutation page. Go through this chart and label the genotype (AA, Aa, or aa) of each individual for practice. Here are a few to help you get started: V. 1. is AA and I.3. is Aa and I.1. is aa. These pedigrees do not have to be used to track a genetic disorder or a trait of any other kind. They can just be simply used to trace relatedness which is a major factor in breeding. Each circle or square can be used to house all known information on that bird so that all of your birds records are held in one convenient easy to read chart for quick access.
LEGEND: Autosomal Recessive Colour Mutation (Blue)
1) Green Bird (AA) = Regular
2) Green Bird (Aa) = Split to Blue
3) Blue Bird (aa) = Blue
PEDIGREE CHARTS FOR SEX-LINKED GENES
There are 2 types of sex-linked inheritance:
(1) X-linked recessive inheritance
(2) X-linked Dominant inheritance
These types of inheritance are much less common than the 2
types of autosomal inheritance (autosomal inheritance has 22 pairs of
chromosomes to work on while sex-linked only has 1 chromosome pair to work on).
Both types of Sex-linked inheritance are easy to distinguish from the 2 types
of autosomal inheritance.
SEX-LINKED DOMINANT INHERITANCE
The pedigree follows all of the following rules:
(1) affected females with normal mates have no affected
daughters and no normal sons
(2) affected females would have affected fathers
(3) both males and females have a 50% chance of being affect
if they have an affected father
(4) males are more likely to be affected than females
* The example for Sex-Linked Dominant Inheritance in Birds -
Big Head, is a completely fictitious example. I am not aware of any sex-linked
dominant disorders or colour mutations in parrotlets so I had to make up an
example to illustrate the point.
Try assigning a genotype to each individual. For example:
individual II.7 is Z with a big "H" and Z with a little "h"
and not both Z's with big "H"s because he has 1 unaffected daughter.
As you can see, you are either affected or not with this type of inheritance. There are no carriers because heterozygotes (only females in human example and only males in bird example) are all affected as bad as homozygotes (AA)....usually.
Sex-Linked Dominant Inheritance in Birds - Big Head
Sex-linked traits are not as common as autosomal traits, but
that does not make them any less important. For Example, some colour mutations
of various cage birds (e.g. yellow in Lineolated Parakeets) are sex-linked
which makes one wonder if there are any sex-linked colour mutations in the
parrotlet. There are many colour mutations, physical mutations, and disorders
yet to be discovered and some of them have to be sex-linked! Maybe one of you
will discover a new sex-linked mutation type thanks to what you have learned in
these lessons.
Just for fun, I've added the criteria for sex-linked
dominant inheritance in humans so you can compare the two. There are very few
genetic disorders with this type of inheritance in humans!
Criteria for Sex-linked Dominance in Humans:
(1) affected males with normal mates have no affected sons
and NO normal daughters (all affected)
(2) affected males have affected mothers
(3) both males and females have a 50% chance of being
affected if they have an affected mother (these offspring are almost always
heterozygotes "Aa"). This is the same as autosomal dominant
inheritance
(4) females are more likely to be affected than males
SEX-LINKED RECESSIVE INHERITANCE
Criteria:
(1) the trait appears in way more females than males.
(2) the gene responsible for the trait is transmitted from an
affected female through all of her sons. Any of her son's daughters have a 50%
chance of being affected. ALL AFFECTED FEMALES HAVE PHENOTYPICALLY NORMAL
OFFSPRING (if the male/mate is normal).
(3) the gene is never transmitted directly from a mother to
her daughter. However, the gene is transmitted to all of her sons in the
heterozygous or carrier form.
(4) the gene may be transmitted through a series of carrier
males (appear normal, but is actually a heterozygote eg. Aa). If this is true,
the affected females in a kindred (group of relatives) are related through
males. Affected females almost always have normal parents!
(5) Heterozygous (Aa) males are usually unaffected, but some
may show the condition in a lesser form than an afflicted individual.
Note: only females are affected and only males are carriers.
Although the offspring off of II.1 and II.2 may be an exception to the rule.
Since both parents carry 1 afflicted gene on their Z chromosome, they have a
25% chance of having a hemophiliac daughter, a 25% chance of having a normal
daughter, a 25% chance of having a carrier son, and a 25% chance of having a
hemophiliac son.
Now try assigning a genotype to each individual. E.g. II.2
is ZHZh.
Sex Linked Recessive Inheritance in Birds -
Hemophilia-like condition in Parrotlets (inability to clot
blood properly)
Normal = Normal
Carrier = Normal, but carries the Hemophilia gene
Affected = Has Hemophilia
Criteria for Sex-linked recessive inheritance in humans:
(1) the trait appears in way more males than females.
(2) the gene responsible for the trait is transmitted from an
affected males through all of his daughters. Any of his daughter's sons have a
50% chance of being affected. ALL AFFECTED MALES HAVE PHENOTYPICALLY NORMAL
OFFSPRING (if the female/mate is normal).
(3) the gene is never transmitted directly from a father to
his son. However, the gene is transmitted to all of his daughters in the
heterozygous or carrier form.
(4) the gene may be transmitted through a series of carrier
females (appear normal, but is actually a heterozygote eg. Aa). If this is true,
the affected males in a kindred (group of relatives) are related through
females. Affected males almost always have normal parents!
(5) Heterozygous (Aa) females are usually unaffected, but
some may show the condition in a lesser form than an afflicted individual.
Hemophilia is the best example of recessive sex-linked inheritance in humans. Only males are affected (unless a female has an affected father and a carrier mother) and all females from an affected male are carriers/ heterozygotes.