Incomplete dominance
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Incomplete Dominance. This is a special type of inheritance in which there are no forms of a particular gene which are totally dominant nor totally recessive.

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Incomplete Dominance

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  • This is a special type of inheritance in which there are no forms of a particular gene which are totally dominant nor totally recessive.

  • An analogy: if you place a piece of blue glass over a piece of yellow glass and hold it up to the light, you see green glass – an color intermediate between the two.

Incomplete Dominance

  • In traits that follow incomplete dominance, we see something unusual in heterozygous individuals.

  • Heterozygous = a blending of the two different traits.

    • ex. snapdragon flower color:

    • Red – RR

    • White - WW

    • Pink – RW (white and red = pink)

Incomplete Dominance

  • In order to show that neither form of the gene is totally dominant, we do not use any lowercase letters. Instead, we can use two different capital letters.



In snapdragons, flower color

shows incomplete dominance.

There are genes for red color or

white color. Show a cross of a

pink-flowered snapdragon with

a red-flowered snapdragon.







Incomplete Dominance

  • Another way to write the gene letters for incomplete dominance is using two of the same capital letter, with a ’ (prime) mark to distinguish the two.



Red blood cells can come in two shapes: normal round (R) or sickled (R’). Show a cross of someone with sickled cells with someone with round cells.







Sickle-Cell Anemia

  • An inherited disorder in which the red blood cells are shaped like crescent moons (sickle shape). Sickled cells cannot carry oxygen properly so body tissues may be damaged. Sickled cells don’t move easily through blood capillaries because of their shape. Symptoms: fatigue, pain, may have shortened life span.

  • Seen mainly in African American populations.

  • Trait show incomplete dominance.

    • RR = all round RBC

    • RR’ = some sickled, some round RBC

    • R’R’ = all sickled RBC

Normal RBC vs. Sickled RBC

Co-dominanceHuman Blood Types

  • Four types: A, B, AB, O

  • Determined by certain proteins which can be found on the surface of the cell membrane of red blood cells.

    • A protein: type A blood

    • B protein: type B blood

    • both A and B proteins: type AB blood

    • neither A nor B proteins: type O blood

Human Blood Types

  • Genes for blood type A and B show co- dominance with each other. But both A and B are totally dominant over O.

  • Type O blood: genes OO

  • Type AB blood: genes AB

  • Type B blood: genes BB or BO

  • Type A blood: genes AA or AO

Sex Linked Genes – Genes on the X chromosome

  • Remember, every chromosome has sections on it called genes. Each gene contains a genetic code for the production of a protein for one particular trait.

  • Every chromosome of every pair, from chromosome number 1 through number 23, has genes.

image from University of Washington, Department of Pathology (

Genes on the X chromosome

  • So even the X chromosome (of the 23rd pair) has genes on it.

  • We know some of the genes found on the X chromosome:

    • colorblindness

    • male pattern baldness

    • hemophilia

  • Genes found on the X chromosome are called X-linked genes. We say they code for X-linked traits.

So why are there more bald men than women?

  • X-linked traits are seen more commonly in males than in females.

  • This is because females have two X chromosomes and therefore have 2 of every gene on the X chromosome. If a female inherits one “bad” copy of a recessive X-linked gene, she often will have a “good” dominant copy on her matching X chromosome. Therefore she would show the dominant trait and would be “normal” for that trait. In order to show the “bad” trait, she would have to inherit two “bad” copies of the gene, which is much less likely.




normal dad, normal (carrier) mom; all female children normal; but half of sons will have baldness.














bald dad, normal mom; half of daughters and half of sons will have baldness.







Bald father, normal mother. All children normal hair.





  • Colorblindness is also caused by a recessive X-linked gene. What would happen if a woman with Cc (XCXc) and man with a c gene on his X chromosome (XcY) had children?









Genetic Disorders

  • Each day, nearly 600 babies are born in the U.S. with some type of disorder. Some of these disorders are inherited.

  • Errors in chromosome number

    • sometimes people are born with more or fewer than 46 chromosomes.

    • this happens when the sperm or egg cell does not have exactly 23 chromosomes.

    • Addition or deletion of more than 1 chromosome is usually fatal (but there are exceptions to this).

Trisomy 21

  • Trisomy 21 is a genetic disorder in which the individual inherited an extra 21st chromosome.

  • The resulting syndrome is called Down syndrome. Symptoms include some degree of retardation, heart problems and certain physical characteristics.

image from University of Washington, Department of Pathology (

Klinefelter’s syndrome

  • This is a disorder in which males have inherited extra X chromosomes.

  • The picture shows a person with 49XXXXY.

  • Symptoms of the disorder include tall stature, infertility, abnormal sexual structures.

image from University of Washington, Department of Pathology (


  • Males who inherited an extra Y chromosome

  • Symptoms can include tall stature and acne.

image from University of Washington, Department of Pathology (

Genetic Disorders and Sex Chromosomes

  • A certain recessive gene on the X chromosome can cause a rare disorder called hemophilia.

  • Hemophilia is a disorder in which a person’s blood does not clot properly.

  • Bleeding from a cut or bruise may take hours to stop. Special medication is available to help stop bleeding.

Genetic Disorders and Autosomes

  • Genetic disorders can also be caused by genes found on the autosomes

    • Dyslexia – genetic disorder (possibly on chromosome 6 and/or 15) involving word blindness; people may have trouble writing/reading certain letters or parts of words. Caused by a dominant gene.

    • PKU – phenylketonuria. Recessive genetic disorder on chromosome 12 which prevents the normal use of protein in food. Chemical byproducts from abnormal breakdown can harm brain cells. Today babies are tested for PKU soon after birth. Treated with dietary modifications (ex. a diet low in phenylalanine).

More genetic disorders

  • Cystic fibrosis (CF) – a recessive genetic disorder found on chromosome 7 which, in its classic and most severe form, involves the production of large amounts of sticky mucus in the lungs; pancreas problems; salty sweat.

  • Huntington’sdisease – a dominant genetic disorder found on chromosome 4. This is a brain disorder which usually doesn’t strike until a patient is in his 30’s or 40’s. Symptoms can include difficulties with movement, thinking, behavior and emotions.

Genetic Counseling

  • “It runs in the family” – genetic disorders can run in families, if a particular gene is passed on through generations.

  • Genetic Counseling involves people meeting with a counselor who is specially trained to help couples know their chances of having a child with a certain disorder.

    • can involve discussions, Punnett squares, family histories, blood tests, etc.

Example of Genetic Counseling

  • The baby of a young couple died when it was very young, from cystic fibrosis.

  • The couple went to a genetic counselor to help them understand the genetics behind their baby’s condition and their chances of having another child with the same disorder.

  • If both parents are Ff, there is a 25% chance of having a child with cystic fibrosis.

  • Genetic counseling allows families the opportunity to make certain choices in their family planning.

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