Pedigrees

What Are Pedigrees?

Pedigrees represent a family tree, but more importantly, they include a particular trait. Every pedigree shows a lineage (family of parents and descendants) and the prevalence of a given trait (which individuals have it).

Another way to look at pedigrees is that they are a quick way to observe patterns in the phenotype of a family in order to assess how that trait is passed on.

In this pedigree, you can see that the males are represented by squares and females by circles. Individuals who have children together are linked by a horizontal line with the children following underneath. The separate rows, therefore, represent different generations of the family.

If an individual is shaded in, that means that they are 'affected', or have the trait in question. So, if for this pedigree, we are looking at disease, the father from generation had the disease, as well as 2 individuals in both generations II and III. Sometimes individuals will have their names underneath their symbol as well.

Assigning Genotypes to Pedigrees

The purpose of a pedigree is to visually represent data about the given family so that an individual can begin to notice some patterns of inheritance. These patterns will allow you to assign genotypes to individuals when all you know is their phenotype. In other words, it allows you to figure out if individuals are heterozygous, homozygous dominant, or homozygous recessive for the given trait. This is incredibly useful for real-world situations because how often do you have the list of genetic alleles for somebody?

Let's take a look at another pedigree. This time, we know some genotypes, so let's just investigate what we can see to ensure that we understand what is occurring. In this pedigree, the female in generation I is homozygous recessive and is showing the trait. However, none of her offspring show the trait. How is this possible? Well, if the trait is recessive (which we know it is given her genotype of ff), then the offspring all need two recessive alleles to have the trait. Mom will always pass a recessive allele. But dad is not affected by this trait. This means he is not ff. He could be FF or he could be Ff. We actually don't know for sure, so his genotype is labeled as F?. Note that the mother is always going to pass that recessive allele because that is all that she has. It is like she has a coin with both sides containing tails. Because none of her offspring are affected, we know they HAVE to be heterozygotes (Ff) given that mom gave them f.

Now let's take another look at that same pedigree, this time focusing on generation III. In generation III, we can be certain of the male's genotype as ff because he is affected. The other children in this generation, however, are mysteries. They are unaffected, so could be either Ff or FF.

Their parents in generation II MUST be heterozygotes, however. How can you tell? Well, they have a child who is affected (ff). This means that both mom and dad had to have an f to give. So right off the bat, the parents in generation II can only be Ff or ff. But let's look at their phenotypes. They are unaffected, so can only be Ff. They MUST be heterozygotes by the process of elimination.

You don't need to be experts in this right now, we are going to practice quite a bit in class. Sometimes you won't be able to say with 100% certainty what everyone's genotype is, but you should be able to narrow down some options.

Autosomal Dominant

 (need one dominant allele to have the disease) AA or Aa

Characteristics

1. Males and Females  are affected equally

2. Does not skip generations (you have it or you don’t and can not pass on)

3. No carriers

4. Often homozygous dominant is lethal

5. If one parent has it child has 50% chance of inheriting

Huntington’s, Achondroplasia, Polydactyl

Autosomal Recessive

 (need both recessive alleles to express this condition) (aa)

Characteristics

1. Males and females are equally affected

2. Carriers are heterozygous (If both parents are heterozygous, you have a 25% chance of a child with the condition)

3. Can skip several generations

Ex. Xermodermopigmentosa (XP),  Ichthyosis, ,Sickle Cell *, Tay-Sach, Cystic Fibrosis

X -Linked Recessive

 X aXa     or Xa Y

Characteristics :

1. Males are more likely to have it

2. Carried on the X chromosome

3. The only way for the  female to have it is if Dad has it and mom has it or is a carrier)

Color Blindness, Hemophilia A, Duchenne’s Muscular Dystrophy

Mitochondrial Inheritance

Recall that Mitochondria are only inherited from the mother.

• If a female has the trait all of her offspring will have it

• If a male has a trait none of his offspring will inherit it

• Remember that mitochondria have a single circular gene so there is no dominant /recessive