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CSEC Biology: Genetic Diagrams, Punnet Squares and Pedigree Charts

There are several diagrams and charts that are used to show the inheritance of certain alleles through generations.

The first is called the genetic diagram. For these diagrams, we will use the dominant allele stimulating melanin production (N) and the recessive allele for albinism (n).

Here, you see a homozygous dominant parent and a homozygous recessive parent cross. The offspring all have normal pigmentation as they all have a dominant N allele. However, they are all heterozygous and therefore, carriers of the recessive albinism gene.

Genetic Diagram- Genotype Parent Cross NN x nn
Genetic Diagram- Genotype Parent Cross NN x nn

Here is a heterozygous parent and homozygous recessive parent cross. Half of the offspring have normal pigmentation (dominant trait), and the other half have albinism (recessive trait), giving a ratio of 1:1. 50% of the offspring are heterozygous and the other 50% are homozygous recessive.

Genetic Diagram- Genotype Parent Cross NN x nn
Genetic Diagram- Genotype Parent Cross Nn x nn

In this genetic diagram, there are two heterozygous parents (carriers). 75% of the offspring are heterozygous with normal pigmentation and 25% are homozygous recessive with albinism. This gives a ratio of 3:1.

Genetic Diagram- Genotype Parent Cross Nn x Nn
Genetic Diagram- Genotype Parent Cross Nn x Nn

Another diagram used to represent and predict the results of crosses is the Punnet Square. It is likely that you are already familiar with these. In the following Punnet Squares, we will use the allele for the production of normal Haemoglobin A (HbA) and the allele for the production of abnormal Haemoglobin S (HbS) as discussed in our previous lesson. Remember that both of these alleles are dominant and therefore exhibit codominance.

This diagram shows the predicted phenotypes of the offspring of two parents with sickle cell trait.

Punnet Square
Punnet Square- HbA+HbS x HbA+HbS cross

The final diagram that will be shown in this lesson is the Pedigree Chart. These are most commonly used with humans, race horses and show dogs. A pedigree is a chart of the genetic history of a family over several generations. To best understand a pedigree chart, you should know the following:

  • Males are represented as squares, while females are represented as circles

  • Shaded symbols mean an individual is affected by a condition, while an unshaded symbol means they are unaffected

  • A horizontal line between man and woman represents mating and resulting children are shown as offshoots to this line

  • Generations are labeled with roman numerals and individuals are numbered according to age (oldest on the left)

(The following is based on a lesson on Khan Academy)

For example, the following pedigree chart shows the inheritance of freckles:

The allele for freckles (F) is dominant to the allele for no freckles (f).

From this information, we can infer quite a bit of information:

  • Since the allele for no freckles is recessive, individuals I-1, II-1, II-2, II-4, II-6, III-1, III-2, and III-6 all must be homozygous recessive (ff).

  • Individual I-2 must be heterozygous or homozygous dominant, since they have freckles (the dominant trait), meaning that they have at least one of the dominant (F) allele. However, we can narrow it down even further. Since one of her children (II-2) is homozygous recessive, and her partner is also homozygous recessive, she must also have a recessive allele (f). Hence, individual I-2 must be heterozygous.

  • Individuals II-3 and II-5 must also be heterozygous since one of their parents was homozygous recessive.

  • Individuals III-3, III-4 and III-5 must also be heterozygous.

Likewise, with other pedigree charts, you can infer lots of information about the genotypes of the individuals. For a more detailed instructional on reading pedigree charts, you can look at this pdf of Pedigree Analysis.

Additional Reading: Scitable Lesson on Inheritance

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