Pedigrees: Tools to study inheritance

Biology Index

Where are we going with this? The information on this page should increase understanding related to this standard:  Identify patterns of inheritance to predict genotype/phenotype and solve punnett square problems.

Article includes ideas, images, and content from Troy Smigielski (2022-01)

Pedigrees: Tools to study inheritance

(Like the thing with dogs to prove they are a purebred?)

Source, 2022

In human genetics, a pedigree is a diagram that shows family history. It is "a diagram of family history that uses [somewhat] standardized symbols. A pedigree shows relationships between family members and indicates which individuals have certain genetic pathogenic variants, traits, and diseases within a family as well as vital status. A pedigree can be used to determine disease inheritance patterns within a family" (Source, 2022).

Biologists use pedigrees to help track family genetics.

A pedigree is a diagram that tracks a certain trait through a family.

It also shows biological relationships between an organism and its family.

Example of a pedigree diagram…

• A male is a square.
• A female is a circle.
• If a shape is colored in, that person has the trait.
• If a shape is half-colored in, that person is a carrier for the trait.
• If a shape is not colored in, that person does not have the trait.

Most pedigrees do not use the half-shaded shapes to denote a heterozygote.

The main purpose of a pedigree is to look at the presence of the traits and "reverse engineer" the genotypes of the people depicted.

In this Punnett Square, two
heterozygous parents are crossed
to produce offspring.

Whereas with a Punnett Square, the genotypes are used as a way to predict the possible phenotypes of the offspring, a pedigree is pretty much the opposite!

A pedigree attempts to determine the genotypes of the individuals involved by looking at the observable phenotypes.

It is very important to remember that the shaded in shapes are the ones that represent people who HAVE the trait. Shaded or unshaded do not directly relate to being dominant or recessive. They show IF a trait is present. The same people coded for different traits will be shaded differently.

The very fancy image above shows the same seven people. On the left, they are shaded in to show who has blue eyes. On the right, they are shaded in to show who has brown eyes. The result is that the two pedigrees, since tracking different traits, are shaded in differently. Also, the shading will usually not be color-synced to the trait.

Working from the pedigree diagram of traits (phenotypes), it is possible to conclude certain things about the genotypes of the individuals represented.

It is a bit of a game, albeit perhaps not a fun game. Not in the sense of, say Candy Crush is fun (if you think that game is fun). Probably, the better way to describe it is to say it is like solving a puzzle.

In genetics, there are rules about dominant and recessive traits that set up strict possible outcomes (recall Punnett Squares.) Using these rules, the puzzle of the pedigree can sometimes be solved fairly easily.

Depending on what traits are passed from the parents to the children, geneticists can make decisions regarding if a trait is dominant or recessive.

In many, many, cases, the key to "cracking the code" is to find in the pedigree a case where parents that are the same (both have a trait, or both don't have the trait) have a child that is their opposite. (See below).

Of the six possible Punnett squares, there is only ONE case where that happens!

The only case where
parents have the same phenotype AND the child has a different phenotype
is in the case of Aa x Aa (Top row, center).

Think about it…

If the parents are the same and the child is different, then one of two things occurs, and—here's the good news—it occurs because the parents are heterozygous AND the child is homozygous recessive.

So… Say a trait (let's use B for dominant and b for recessive) is dominant and both parents have it. In the pedigree, their boxes would be filled in (left image below). For a child to NOT have it, both parents would have to provide the "b" version of the gene (the recessive). So, in the image on the left, the parents are Bb and the child is bb.

Because the trait is dominant, when the child gets the bb genotype, the trait is not expressed. The shape is filled in.

NOW, suppose the trait is recessive and NEITHER parent is showing it. BUT! A child has it. This can only happen if the parents are heterozygous and if they both provide the "b" version of the gene. In this case, the shaded boxes are showing a recessive trait.

Because the trait is recessive, when the child gets the bb genotype, the trait is expressed. The shape is filled in. 

Solving the puzzle of the pedigree begins, however, with the shaded boxes and circles. Based on how they are arranged, we reverse-think the genotypes.

The key, to repeat, is to find same parents with a different child. If you do that, you KNOW that the parents are heterozygous.

• If both parents have the trait, but one of their children does not, then the trait is dominant.
  
• If neither parent has the trait, but one of their children does, then the trait is recessive.

Process:

(Let's use B for dominant and b for recessive again.)

1. Find like parents with a different child. 

THIS ASSURES you have heterozygous parents ( Bb ) AND a homozygous recessive ( bb ) child. Let's call this the different child the First-Found. 

2. Do the parents or child have the trait?

3. IF the parents have the trait, it is dominant. IF the child has the trait, it is recessive. 

4. If the First-Found is shaded, then ALL of the shaded boxes are homozygous recessive. If the First-Found is NOT shaded, then ALL of the NOT shaded boxes are homozygous recessive.  Whatever the First-Found is, everything with that shading is homozygous recessive ( bb ). 

5. Now, for all of the boxes that have shading that is opposite the First-Found, you can put at least ONE of the dominant genes in the shape ( B_ ).

6. You cannot prove that something is homozygous dominant. Some shapes will have just the one gene shone  ( B_ ).

7. Considering that the bb shapes can ONLY provide a b, figuring out the genotype of the rest of the shapes is fairly obvious. Any parent of a bb child must have at least one b!

As an example, here is a pedigree with the parents and three generations of offspring:

It could be a pedigree for any trait. We'll call it the "B" trait (B for dominant and b for recessive). 

Using this pedigree, it is possible to deduce both genotypes and whether or not the trait is dominant or recessive.

Let's just pick out some things this pedigree shows…

There are 10 males in this family line.

Three (3) of them are affected by the trait.

Since the trait appears in the offspring of non-affected parents, it must be recessive. Hence the genotype of everyone in shaded boxes is bb.

Therefore, the non-affected parents must both pass the recessive trait to the child and, thus, must be heterozygous (e.g. Bb).

Looking at the II generation, the female #1 (who is not a descendent of the parents) crosses with a double recessive (homozygous recessive, bb) male. Since the produce three affected offspring, then it can be concluded that Gen II Female 1 is heterozygous (Bb) for the trait.

Looking at the II generation, male #2 must be double recessive (homozygous recessive, bb), since the recessive trait appears (when crossed with heterozygous, Bb female #2).

If a trait appears in individuals of both typical sexes (XX, XY), then the trait is autosomal and not a sex-linked trait. 

In the pedigree below , for a recessive trait, any individual showing the trait (colored boxes) must have the double recessive (homozygous recessive, bb) genotype.

Similarly, any individual NOT showing the trait must have at least one B in their genotype.

Pedigree of a autosomal recessive trait.

The "puzzle buster" is found in the children of parents 8 and 9. 

Since child 17 shows the trait that the parents (8 and 9) don't show, then the trait MUST be recessive AND the parents MUST be heterozygous.

17 is bb.

8 and 9 are Bb.

Then, ALL of the shaded boxes are bb. 

Then, ALL of the unshaded boxes have at least 1 dominant gene ( B_ ).

Then, any parent of a bb child has at least 1 b.

Let's have a look at another pedigree and think about a few more things…

Both 1 and 2 have the trait. Of their children, one of them has a different trait. Since 7 has a different trait from the parents…

1. The parents gave a gene to the child different from the one they are expressing, and that trait showed up. Therefore the trait the parents are showing is dominant.

2. Since 7 shows the other trait (which must be recessive) they must have received the recessive trait from both parents.

3. Therefore the parents are heterozygous (Bb) and 7 is homozygous recessive (double recessive, bb).

4. Since both 7 and 8 are expressing the recessive trait, they both must be homozygous recessive (bb).

5. Since 5, 6, and 9 have the trait and include both male and female individuals, the trait must NOT be sex-linked which makes it autosomal dominant.

In a dominant trait, the shaded in individuals must have at least one “B” while the unshaded individuals must be “bb”.

Here is a pedigree showing a sex linked trait:

Whereas neither Generation I, 1 nor 2 show the trait, but it appears in generation II means that it is recessive. That it appears only in males suggests that it is sex-linked. Thus, females I-2, II-1, II-5, and III-2 must carry the trait.

When determining whether the trait is autosomal or sex-linked, look at the gender of those affected. As a general trend, males are more commonly affected by sex-linked genetic traits than females.

Another way to know it is sex-linked is if the mother has the trait and so do all of her sons.

However, just because a trait is sex-linked does not mean that females can never get it too.