1. Genetics Part 3 Modes of Inheritance
A quick refresher for the teacher on important terms:
Autosomal gene- a gene found on any chromosome except for the sex chromosome. These are chromosomes numbered 1-22.
X-linked gene- a gene found on a chromosome designated as a sex chromosome (X or Y).
Dominant- Refers to an allele that is expressed phenotypically and masks any recessive counterpart.
Recessive- An allele that is not phenotypically expressed when its counterpart is dominant.

2. Remember… Humans have 23 pairs of chromosomes
Two types of chromosomes:
Autosomes: all chromosomes except sex chromosomes
Sex Chromosomes: X and Y
Inheritance can be categorized according to the type of chromosome from which the trait comes.

3. = = Do you remember.......? In humans (and fruit flies), X X X Y
(You don’t have to write this down).
Do you remember ?
In humans (and fruit flies), X X X Y = =
Female
Male
It is the presence of a Y chromosome that determines if an individual will be male or not.

4. Autosomal Dominant: trait controlled by a dominant allele located on one of the autosomes
Examples:
Huntington’s Disease (brain disorder that causes mental illness))
Dwarfism
Polydactyly (extra fingers or toes)
A person with an autosomal dominant disorder can have a HH or Hh genotype
Just because a trait is dominant does NOT mean it is more common in a population!

5. Examples of people from a variety of human races who exhibit the albino trait.
Autosomal Recessive: trait controlled by a recessive allele on one of the autosomes – Examples:
Albinism (no pigmentation)
Cystic Fibrosis (excessive mucus production in lungs)
Sickle cell anemia (red blood cells are shaped like a “C” instead of round)
A person with an autosomal recessive disorder must have a homozygous recessive genotype (hh).
Carrier: a heterozygous person who does not have the disorder, but carries the recessive allele so that it can be passed on to future generations (Hh)

6. Sex chromosomes Differ in appearance
(You don’t have to write this down).
Sex chromosomes
Differ in appearance
Only small parts are homologous (carrying same genes)
Sex-linked genes
Found only on one of the sex chromosomes
Only 20 genes on Y
Related to male characteristics.
1500 genes on X
Related to a variety of traits (few are gender-related).

7. Sex Linked Traits
Sex Linkage: inheritance from a gene located on one of the sex chromosomes
Y-Linked: inheritance from a gene on the Y chromosome – only affects males
The Y chromosome has very few genes that cause an observable phenotype so there is not a lot of y-linkage.
Ex: hypertrichosis pinnae (hairy ears)

8. Write this down! X-Linked: inheritance from a gene on the X chromosome
Most X-linked disorders that we know of are X-linked recessive – Examples:
Colorblindness
Hemophilia
Muscular Dystrophy
Pattern Baldness
Muscular dystrophy
Male baldness: XbY Female baldness: XbXb

9. Why is it more common for males to be affected by an X-linked disorder than a female?

10. ARE YOU COLORBLIND?

13. © Archimedes' Lab

14. How they are written:
Sex-Linked Recessive: Females who are XaXa and males who are XaY will express/show the recessive disease/trait being studied.
A female who is XAXa is a carrier for it, but will not express it.
Males have only one X and cannot be carriers.

15. Constructing Pedigrees
Genetics Part 4
Constructing Pedigrees

16. Scientists observe how traits are inherited by studying phenotypes of a certain species from generation to generation One tool scientists have developed to study inheritance is a pedigree.
Pedigree: A family tree diagram that shows how a trait is inherited over several generations.

17. Draw all of the symbols and their meanings!

18. Interpreting a Pedigree Diagram
Determine if the pedigree chart shows an autosomal or X-linked disease.
If most of the males in the pedigree are affected, then the disorder is X-linked
If it is a 50/50 ratio between men and women the disorder is autosomal.
When interpreting a pedigree chart of a family with a disease like muscular dystrophy, it is important to consider two steps. The first is to determine if the disorder is autosomal or X-linked.
If the disorder is X-linked most of the males will have the disorder because the Y-chromosome cannot mask the affects of an affected X-chromosome. A female can have the disorder, but it would be a very low percentage. For a female to be affected, she would have had to receive an affected gene from the mother and the father. This means that the father would have the disorder and the mother was a carrier.
In an autosomal disorder, the disorder is not found on the X or Y chromosome. It is found on the other 22 chromosomes in the human body. This means that men and women have an equal chance of having the disorder. The mother and father can be homozygous dominant, heterozygous, and homozygous recessive. If a person is homozygous dominant, the person has two of the same dominant genes. For example if someone is homozygous dominant for being tall it may be represented as TT. Capital letter always represent a dominant gene. If a person is heterozygous, this person would have a dominant trait and a recessive trait. It may be represent as Tt. The dominant gene will mask the recessive gene, so the person is still tall. If a person is homozygous recessive, the person has two of the same recessive genes. For example if someone is homozygous recessive for height, it may be represented as tt. The tt would mean the person is short.

19. Interpreting a Pedigree Chart
Determine whether the disorder is dominant or recessive.
If the disorder is dominant, one of the parents must have the disorder.
If the disorder is recessive, neither parent has to have the disorder because they can be heterozygous.
The second step is to determine if the disorder is dominant or recessive. It is important to find out if a disorder is dominant or recessive. For example, Huntington’s disease is a dominant disorder. If you have only one dominant gene you will have Huntington’s disease, which is a lethal disorder. The disorder does not show up until a person is in their middle ages such as 45. It will quickly decrease their motor skills and the brain will begin to deteriorate.
If a disorder is dominant, one parent must have the disorder (either homozygous dominant (TT) or heterozygous recessive (Tt). Both parents do not have to have the disorder. One parent might not have the disorder or be a carrier. If a disease is dominant, it does not skip a generation unless one parent is heterozygous dominant (Tt) and the other parent is homozygous recessive (tt). In this case the child has a chance of not receiving the dominant gene.
If the disorder is recessive, a parent does not have to have the disorder, but could still pass it to their offspring. This would happen when a parent is heterozygous recessive (Tt) and passes on the recessive (t) gene. This means this disorder can skip generations. An example of a recessive disorder would be sickle cell anemia.

20. Examples: Pedigree #1 Is it Autosomal or X-linked?
Take a minute and try to decide if this slide is autosomal or X-linked.

21. Answer:
Autosomal
Take a minute and try to decide if this slide is autosomal or X-linked.

22. Examples: Pedigree #1 Is the trait Dominant or Recessive?
Is this pedigree dominant or recessive?

23. Answer:
Dominant
Is this pedigree dominant or recessive?

24. Examples: Pedigree #2
Is the trait Autosomal or X-linked? Dominant or Recessive?
Is this pedigree dominant or recessive?

25. Answer:
Autosomal Recessive
Is this pedigree dominant or recessive?

26. Examples: Pedigree #3
Is the trait Autosomal or X-linked? Dominant or Recessive?
Is this pedigree dominant or recessive?

27. Answer:
X-linked Recessive
Is this pedigree dominant or recessive?