Incomplete Dominance or Co-Dominance-Incomplete dominance is used to describe a pair of alleles in which neither allele is dominant. Heterozygous alleles show a blending of the phenotypes seen in individuals homozygous (AA or aa) for the alleles.

Sickle Cell Anemia-This disease affects the hemoglobin. Hemoglobin is an important protein found in erythrocytes that is responsible for transporting oxygen to the cells. In individuals suffering from this disorder, many of the erythrocytes have a sickle or irregular shape. The disorder interferes with the ability of hemoglobin to transport oxygen and symptoms include severe anemia and increased susceptibility to heart failure, pneumonia, kidney failure, and spleen enlargement. Prior to the advent of modern medical treatment most individuals with this disorder died as children. Even today, 31% die by the age of 15. This disorder is most common in individuals from central Africa (s allele frequency = 12%) and is also common in African Americans (5.4%) as compared to white Americans (0.1%). The disorder is caused by a single error in the gene that replaces the amino acid glutamic acid with valine on the surface of the hemoglobin molecule. The valine is non-polar unlike glutamic acid and forms a sticky spot on the hemoglobin molecule. The non-polar sections of hemoglobin molecules stick together forming long chains that lead to deformed erythrocytes. When oxygen is bound to the hemoglobin it shields the sticky area. Sickling crises are exacerbated by low oxygen conditions such as high altitude or exercise. The disorder was first described in Chicago in 1904 in a West Indian Black student. Only those individuals who are homozygous for the recessive "s" allele actually have sickle cell anemia. Carriers (heterozygotes--Ss) may develop symptoms when oxygen levels are low. Carriers are also resistant to malaria. Malaria is a leading cause of death and illness in central Africa. The selective advantage this affords carriers is responsible for the high frequency of the "s" allele in individuals from this area. An additional factor contributing to the high frequency is the currently undescribeed greater fertility of women who are carriers. There is no such advantage for African Americans in the U.S. and the frequency of the "s" allele is therefore decreasing. This disorder is usually considered to be caused by a recessive allele (s) present in the homozygous (ss) condition. However because heterozygotes (carriers, Ss) exhibit mild symptoms, it may also be considered an ex of incomplete dominance. This is because the recessive allele is not completely hidden in the heterozygous condition.

Use the information to answer the following problems. A man who is a carrier for Sickle cell anemia marries a woman who is also a carrier. Using a Punnett square and your formulas for probability will help you solve the problems.

14.
What is the genotype of the man?
A) ss
B) SS
C) Ss
D) cannot be determined
15.
What is the genotype of the woman?
A) ss
B) SS
C) Ss
D) cannot be determined
16.
What are the phenotypes of the man and the woman?
A) abnormal appearance; strong symptoms 
B) normal appearance; may have mild symptoms
C) abnormal appearance; no symptomss
D) normal appearance: strong symptoms
E) cannot be determined
17.
What is the probability that they will produce a child with sickle-cell anemia?

A) zero
B) 25% (1/4)
C) 50% (1/2)
D) 75% (3/4)
E) 100%
18.
What is the probability that they will produce a child who does not have sickle cell anemia and who does not carry the gene (recessive allele) for sickle cell anemia (genotype = SS)?
A) zero
B) 25% (1/4)
C) 50% (1/2)
D) 75% (3/4)
E) 100%
19.
What is the probability that they will produce a child who is a carrier for sickle cell anemia, but does not have the disease?
A) zero
B) 25% (1/4)
C) 50% (1/2)
D) 75% (3/4)
E) 100%
Sex-linked Crosses-These are special types of monohybrid crosses in which the alleles controlling a certain trait are located on a sex chromosome. In X-linked crosses the determining allele is located on the X chromosome and there is no corresponding allele on the Y chromosome. Therefore, whatever allele a male inherits on the X chromosome is expressed. In females these traits are determined by the interaction between the alleles on the two X chromosomes. More simply stated, males have no second-or backup---allele for insurance. For this reason disorders determined by X-linked genes are much more likely to occur in males. These include hemophilia (1 in 10,000 males born with the disorder versus 1 in 50 million females), color blindness, androgen insensitivity, and Duchenne Muscular Dystrophy. According to the National Institute of Health (NIH September 2009), there are estimated 900 to 1400 genes on the X chromosome and only approximately 70 to 200 genes on the male chromosome (Y). 

Y-linked inheritance is less common, but does occur. That is, there are some genes on the Y chromosome that have no counterpart on the female chromosome. These include the SRY gene necessary for proper male development.

It is vital that you keep track of the chromosomes on which the specified alleles are located when working with sex-linked traits. Therefore instead of just writing a gene pair as AA, Aa, or aa, the alleles should appear as superscripts on the chromosomes they are attached to. For ex XAXA of XAXa or XAYO. Note that a O is used as a superscript in X-linked crosses to indicate that there is no allele present on the chromosome for the specific trait being studied. 

Hemophilia-Hemophilia is an X-linked recessive genetic disorder in which the blood clots slowly or not at all. The failure of the blood to clot is caused by a missing protein (factor) that is needed for blood clotting. Some forms of hemophilia are caused by alleles present on the autosomes while other forms are caused by alleles present on the sex chromosomes. Individuals with hemophilia are in constant danger of bleeding to death. Today, hemophiliacs are supplied with the missing factor by transfusions. These transfusions enable hemophiliacs to live a relatively normal life. Royal Hemophilia is the most famous hemophilia and it is caused by a sex-linked recessive allele. It is called Royal Hemophilia because of its appearance in many of royal lineages. The disorder was apparently inherited by Queen Victoria as a new mutation in one of her parents. There is no previous history of the disease in the British Royal Family. Queen Victoria's husband (Prince Albert) did not have the disorder. Queen Victoria did not have the disorder, but carried it on one of her X chromosomes. Her offspring married into the royal houses of Germany, Prussia, Russia, and Spain and introduced hemophilia into the their royal houses. Her son Edward (King Edward VII) eventually became King. He apparently did not have the disorder since the current royal family does not carry the disorder and they are his direct descendants. Xh = recessive allele on the X chromosome; XH = dominant allele on the X chromosome; YO = no allele present on the Y chromosome.

Use this information to answer the following quesitons. Using a Punnett square and your formulas for probability will help you solve the problems. 

20.
What was Queen Victoria's genotype?
A) XHXH
B) XHXh
C) XHYO
D) XhYO
21.
What was Prince Albert's genotype?

A) XHXH
B) XHXh
C) XHYO
D) XhYO
22.
What was King Edward's probable genotype?

A) XHXH
B) XHXh
C) XHYO
D) XhYO
23.
What was the probability that Queen Victoria would produce a male heir without hemophilia? Note that you have two probabilites here that the heir would be male and that he would not have hemophilia (remember from your PowerPoint presentation the ace and the heart!).
A) zero
B) 25% (1/4)
C) 50% (1/2)
D) 75% (3/4)
E) 100%
24.
What was the probability that one of Queen Victoria's daughters would have hemophilia?

A) zero
B) 25% (1/4)
C) 50% (1/2)
D) 75% (3/4)
E) 100%
25.
Can a male child ever inherit an X-linked trait from his father?
A) Yes because all males inheirt one X chromosome and one Y chromosome.
B) Yes because males inherit their sex chromosomes from their fathers only.
C) No because males only inheirt a Y chromosome from their father and an X chromosome from their mother.
D) No because males do not carry X chromosomes. They only inherit a Y chromosome.