Types of Inheritance

Codominance and Sex-Linked

Codominance Inheritance

Codominance is a pattern of inheritance in which both alleles of a gene are always expressed.

Our human blood type is a good example of codominance. Look at the table below that shows the four human blood types, distinguished by their red blood cells. A person with AB blood has both A and B antigens on their red blood cells, because with codominance, both alleles expressed.

If you have blood type O, you do not make the proteins that code for the antigens on the red blood cell surface. Persons with blood type A or B would only make A or B antigens, respectively.


The gene for human blood type is also an example of a multiple-allele trait. This means that three or more alleles are possible for that gene. The three alleles for blood type are represented as follows: A, B and O

An individual’s blood type (or phenotype) is determined by two of the three alleles as follows:

Phenotype (Blood Type) Genotypes
A AA or AO
B BB or BO

Look at the table above. If you have blood type B, you either have two B alleles (BB) or you have a B and and O allele (BO).

Sample Codominance Problem: A mother has blood type O and her husband has blood type AB. They have a child who has blood type A. Is this possible?


Genotype ratio: 2AO: 2BO

Phenotype ratio: 2A: 2B

Yes, it is possible for an AB and O parent to have an offspring with an A blood type. All that is needed is for the AB parent to donate their A allele.

Sex-linked Inheritance

Sex-linked traits have genes located on a sex chromosome. Genes on the X chromosome are X-linked and genes on the Y chromosome are Y-linked.

  • Females receive two alleles for these genes (XX), but males only receive one, since they are XY.
  • When genes are X-linked, males are much more likely to show recessive traits since there is no complementary gene on the Y chromosome.
  • In humans, hemophilia, Duchenne muscular dystrophy and color blindness are X-linked traits.

Tips for Punnett squares of sex-linked traits:

  • Use large X’s and Y’s to denote male and female, use superscript letters to designate the alleles.
  • Male genotypes/phenotypes are determined by the allele on their one X chromosome. For instance, XbY is a color blind male, since he only has one X allele and colorblindness is recessive. A normal male must then be XBY
  • A female can be XBXB – normal, XBXb – carrier, or XbXb – colorblind. Did you notice that when females are heterozygous they are called “carriers?”

Sample Sex-Linked Problem: A man has normal color vision and is married to a woman who is colorblind. What are the chances that they will have children that are color blind like the mother?

Colorblindness is carried on the X chromosome and colorblindness is recessive (see above for this information). Since the man has normal vision, he will have a dominant allele on his X chromosome: XBY

The woman is colorblind. This means she must have two recessive alleles – one on each X chromosome. Her genotype is XbXb.


Genotype ratio: 2XBXb : 2XbY

Phenotype ratio: 2 normal females : 2 colorblind males

There is no chance that daughters would be colorblind. 100% of all sons would be colorblind. This is because they can only inherit one X (which must come from mom) and both of mom’s X chromosomes have the recessive allele.

Other Types of Inheritance

Polygenic inheritance

To get our feet wet with Genetics, so far we have been using traits that are controlled by one gene in most of our practice problems.The reality, however, is that most human characteristics are controlled by several genes, which is polygenic inheritance.  

Skin and eye color, both polygenic traits, are each the product of additive effects of 3-6 genes. Each gene would lay down a certain amount of a pigment called melanin. The more of the genes that are expressed, the darker the color produced. Human height is also a polygenic trait.



Pleiotropy , on the other hand, is quite the opposite of polygenic inheritance. In pleiotropy, onegene results in multiple (seemingly unrelated) phenotypes. Often these are congenital (present at birth) diseases, such as sickle cell anemia or phenylketonuria (or PKU). In the case of sickle cell anemia, one gene mutation can cause anemia, lack of blood supply to vital organs, pain, brain damage, and even paralysis, in addition to many other symptoms. A sickled red blood cell is misshapen, unable to do it’s job to carry oxygen, and is quickly broken down by the body.

Sex-Influenced Traits

Sex-influenced traits are located on autosomes, but express themselves differently in the sexes because of sex hormones. Pattern baldness is an example. Females may carry the genes but estrogen helps them avoid losing hair as early and completely as males. Sex-influenced traits should not be confused with sex-linked traits.