non mendelian inheritance

incomplete dominance

One allele is NOT always completely dominant over another allele. Sometimes an individual has a phenotype between the two parents because one allele is not dominant over another. This pattern of inheritance is called incomplete dominance. For example, snapdragon flowers show incomplete dominance. One of the genes for flower color in snapdragons has two alleles, one for red flowers and one for white flowers. A plant that is homozygous for the red allele (RR) will have red flowers, while a plant that is homozygous for the white allele will have white flowers (WW). But the heterozygote will have pink flowers (RW) ( Figure 1.1) as both alleles are expressed. Neither the red nor the white allele is dominant, so the phenotype of the offspring is a blend of the two parents. Pink snapdragons are an example of in- complete dominance. Another example of incomplete dominance is with sickle cell anemia, a disease in which a blood protein called hemoglobin is produced incorrectly. This causes the red blood cells to have a sickle shape, making it difficult for these misshapen cells to pass through the smallest blood vessels. A person that is homozygous recessive (ss) for the sickle cell trait will have red blood cells that all have the incorrect hemoglobin. A person who is homozygous dominant (SS) will have normal red blood cells. What type of blood cells do you think a person who is heterozygous (Ss) for the trait will have? They will have some misshapen cells and some normal cells ( Figure 1.2). Both the dominant and recessive alleles are expressed, so the result is a phenotype that is a combination of the recessive and dominant traits. Sickle cell anemia causes red blood cells to become misshapen and curved unlike normal, rounded red blood cells.



exceptions to mendels rules

In all of Mendels experiments, he worked with traits where a single gene controlled the trait. Each also had one allele that was always dominant over the recessive allele. But this is not always true. There are exceptions to Mendels rules, and these exceptions usually have something to do with the dominant allele. If you cross a homozygous red flower with a homozygous white flower, according to Mendels laws, what color flower should result from the cross? Either a completely red or completely white flower, depending on which allele is dominant. But since Mendels time, scientists have discovered this is not always the case.


Another exception to Mendels laws is a phenomenon called codominance. For example, our blood type shows codominance. Do you know what your blood type is? Are you A? O? AB? Those letters actually represent alleles. Unlike other traits, your blood type has three alleles, instead of two! The ABO blood types ( Figure 1.3) are named for the protein attached to the outside of the blood cell. In this case, two alleles are dominant and completely expressed (IA and IB ), while one allele is recessive (i). The IA allele encodes for red blood cells with the A antigen, while the IB allele encodes for red blood cells with the B antigen. The recessive allele (i) does not encode for any proteins. Therefore a person with two recessive alleles (ii) has type O blood. As no dominant (IA and IB ) allele is present, the person cannot have type A or type B blood. What are the genotypes of a person with type A or type B blood? An example of codominant inheritance is ABO blood types. There are two possible genotypes for type A blood, homozygous (IA IA ) and heterozygous (IA i), and two possible genotypes for type B blood, (IB IB and IB i). If a person is heterozygous for both the IA and IB alleles, they will express both and have type AB blood with both proteins on each red blood cell. This pattern of inheritance is significantly different than Mendels rules for inheritance, because both alleles are expressed completely, and one does not mask the other.


instructional diagrams

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every human trait has one dominant allele and one recessive allele.

a. true

-->  b. false

white flowers crossed with red flowers can sometimes make pink flowers.

-->  a. true

b. false

what will be the phenotype of an iai individual?

-->  a) type a blood

b) type b blood

c) type o blood

d) type i blood

what is the genotype of an individual with type o blood?

a) ioio

b) iiii

c) ioi

-->  d) ii

snapdragons have a dominant red allele and a dominant white allele for flower color. what is the genotype for flower color of a heterozygous flower?

a) rr

b) ww

-->  c) rw

d) rw

what will be the phenotype of the flower from question 8?

a) red

b) white

c) both red and white

-->  d) pink

two people with type o blood can have children with what blood types?

-->  a) type o blood only

b) types a and b blood

c) types a, b and o blood

d) cannot be determined from the information given.

diagram questions

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