Mahlon Engineer

A brief note: while we give you the phenotypic ratios in this review, it is probably not an efficient use of your time to memorize these ratios. Instead, use the tables to test your understanding of the inheritance mechanisms. If you understand the mechanisms well, you should be able to generate the ratios quickly on test day.

Autosomal recessive traits require two copies of the recessive allele to be expressed.

Autosomal Recessive

Autosomal recessive pedigrees can look differently based on the genotype of the parents. Here is an example pedigree:

One trick for identifying a recessive trait is that if a trait skips a generation in a pedigree, it is often an autosomal recessive trait (although a trait can be autosomal recessive and not skip generations).

These traits appear with equal frequency in both sexes.

An autosomal dominant trait will result in the dominant phenotype if one or more copies of the dominant allele are present.

An autosomal dominant trait will result in the same ratios of dominant to recessive phenotype as seen above in the autosomal recessive chart!

In a pedigree this phenotype will appear with equal frequency in both sexes but it will not skip generations.

Here is an example of an autosomal dominant recessive pedigree:

X-linked recessive traits are carried on the X chromosome. Because male offspring receive only one copy of the X chromosome, the trait is expressed phenotypically in all men with the X-linked recessive allele. Female offspring can also express an X-linked recessive trait although only if they inherit two X-linked chromosomes (one from each parent) containing the recessive allele.

As you can see above, this trait is more likely to appear in males than females so the pedigree may contain more affected males than females

This trait is never passed from father to son, because a father carries the allele on his X chromosome, but always passes his Y chromosome on to any sons.

Here is a sample X-linked recessive pedigree:

Above are the three most common inheritance patterns that will appear in an introduction to inheritance patterns. Below is a shorter overview of two, more unusual, inheritance patterns that you may come across in your studies:

This kind of inheritance is less common than X-linked recessive. It occurs when a dominant gene is carried on the X chromosome. Unlike X-linked recessive traits, only one copy of the dominant X-linked allele is required to result in the phenotype in both male and female offspring.

This is a rare type of inheritance, where the gene of interest is located on the Y-chromosome. Because, at most, one copy of the Y chromosome is inherited, dominant and recessive don’t really apply here.

Because only males have a Y chromosome, female offspring of an affected father cannot express the trait

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