Types of Inheritance

1. Types of Inheritance Dominant, Recessive, X-linked

2. Chromosome Review • Every cell has 23 pairs of chromosomes (46 chromosomes total). • We inherit one member of each pair from our mother and the other member from our father. • In females all 23 pairs of chromosomes look similar. • In males, 22 pairs of chromosomes look similar but the 23rd pair is mismatched.

3. The 23rd pair of chromosomes are the sex determining chromosomes. • In the cells of females, both members of the 23rd pair are X chromosomes. • In the cells of males, both members of the 23rd pair are mismatched with two unlike chromosomes called X and Y. QUESTION: Based on what you have learned, can you identify which partner, mom or dad, is responsible for the sex of the child? EXPLAIN.

4. Definitions: • Gene = tiny packets of information that contain instructions for how our bodies develop and function. Genes are a part of chromosomes. • Autosomal = not sex-linked • Karyotype = a photograph of the entire set of chromosomes with the images of the individual chromosomes arranged by shape and size in an orderly manner. http://learn.genetics.utah.edu/content/begin/traits/karyotype/

5. Homologous chromosomes = two chromosomes of a pair. (same length, same pattern of light and dark bands when stained, and carry the genes for the same inherited characteristics) • Alleles = different versions of the same gene • An individual with two genes the same for a trait is said to homozygous for the trait. • An individual with two different alleles for the same trait is heterozygous for the trait.

6. Autosomal Dominant Inheritance • There is a 50% chance the dominant trait (or disorder) will be passed on to a child (male or female). • A child gets this dominant trait (or disorder) from one parent. • This dominant trait (or disorder) will be visible/present in the parent who is passing the trait (disorder) on to a child.

7. Examples of autosomal dominant disorders: • Achondroplasia • Familial high cholesterol • Huntington’s disease • Some forms of glaucoma • Polydactyly • Marfan syndrome

8. Autosomal Recessive Inheritance • Carrier = a person with one recessive gene for a trait or disorder. • A carrier is not affected by the trait (or disorder), but can pass it on to his/her children. • Both parents must be “carriers” of the autosomal recessive trait (or disorder). • A child with a recessive trait(disorder) must receive the trait(disorder) from BOTH parents. • If both parents are carriers, there is 25% chance that any child (male or female) will inherit the trait (or disorder). There is a 50% chance that any child (male or female) will be an unaffected carrier just like the parents. There is a 25% chance that any child will be neither affected or a carrier.

9. Autosomal recessive disorders: • Sickle cell anemia • Tay-Sachs disease • Cystic fibrosis • Phenylketonuria (PKU) • Thalassemia (Cooley’s anemia)

10. X-linked Recessive Inheritance • Mother is a carrier of a recessive trait (or disorder) on her 23rd pair of chromosomes. She is unaffected because the trait (or disorder) is recessive. • Dad is not a carrier of the recessive trait (or disorder. He has no genes for this trait (or disorder) and is unaffected.

11. Mother has a 50% chance of passing this recessive trait (or disorder) on to a child. • A female child will be a carrier of the trait (or disorder) because she has a normal gene on the paired 23rd chromosome. • A male child who inherits the affected X chromosome from his mother does not have a matching X in the 23rd pair to off-set the disorder. His 23rd pair is mismatched (XY). He will have the trait (or disorder) even though it is transferred from only one parent.

12. X-linked disorders are always passed from mothers to sons. • The rule that says recessive disorders must be inherited from both parents does not apply because the males 23rd chromosomes are mismatched.

13. X-linked recessive disorders: • Hemophilia • Red-green colorblindness • Duchenne muscular dystrophy

14. Carrier tests for some recessive disorders are available. • Hemophilia • Thalassemia (Cooley’s anemia) • Sickle cell anemia • Tay-Sachs disease • PKU • Duchene muscular dystrophy • Cystic fibrosis

15. Question—Why is this true? • Some genetic disorders tend to be more common in certain groups of people. • Cystic fibrosis—more common in Caucasian people of Northern European descent • Tay-Sachs disease—more common in people of Central or Eastern European Jewish heritage. • Thalassemia (Cooley’s anemia)—more common in people of Greek, Italian, or Middle Eastern descent. • PKU (Phenylketonuria)—more common in Caucasians of Northern European descent. • Sickle cell anemia—more common in people of African American and Hispanic descent.