Genetics Part 3Modes of Inheritance

Remember…Remember…

Humans have 23 pairs of chromosomesHumans have 23 pairs of chromosomes Two types of chromosomes:Two types of chromosomes:

AutosomesAutosomes: all chromosomes : all chromosomes exceptexcept sex chromosomes sex chromosomes

Sex ChromosomesSex Chromosomes: X and Y: X and Y Inheritance can be categorized according Inheritance can be categorized according

to the type of chromosome from which the to the type of chromosome from which the trait comes.trait comes.

In humans (and fruit flies),

Female= = Male

It is the presence of a Y chromosome that determines if an individual will be male or not.

(You don’t have to write this down).

Autosomal DominantAutosomal Dominant:: trait trait controlled by a dominant allele controlled by a dominant allele located on one of the autosomeslocated on one of the autosomes

Examples:Examples: Huntington’s Disease (brain disorder that causes mental illness))Huntington’s Disease (brain disorder that causes mental illness)) DwarfismDwarfism Polydactyly (extra fingers or toes)Polydactyly (extra fingers or toes)

A person with an autosomal dominant disorder can A person with an autosomal dominant disorder can have a HH or Hh genotypehave a HH or Hh genotype

Just because a trait is dominant does NOT mean it is Just because a trait is dominant does NOT mean it is more common in a population!more common in a population!

Autosomal RecessiveAutosomal Recessive:: trait controlled by a recessive allele on trait controlled by a recessive allele on one of the autosomes – Examples:one of the autosomes – Examples:

AlbinismAlbinism (no pigmentation)(no pigmentation) Cystic FibrosisCystic Fibrosis (excessive mucus production (excessive mucus production

in lungs)in lungs) Sickle cell anemia Sickle cell anemia (red blood cells are shaped like a (red blood cells are shaped like a

“C” instead of round) “C” instead of round) A person with an autosomal recessive disorder must have a homozygous A person with an autosomal recessive disorder must have a homozygous

recessive genotype (hh).recessive genotype (hh). Carrier:Carrier: a heterozygous person who does not have the disorder, but carries 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)the recessive allele so that it can be passed on to future generations (Hh)

Examples of people from a variety of human races who exhibit the albino trait.

Sex chromosomesSex 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).

(You don’t have to write this down).

Sex LinkageSex Linkage:: inheritance from a gene inheritance from a gene located on one of the sex chromosomeslocated on one of the sex chromosomes

Y-Linked:Y-Linked: inheritance from a gene on the Y inheritance from a gene on the Y chromosome – only affects maleschromosome – only affects males

The Y chromosome has very few genes The Y chromosome has very few genes that cause an observable phenotype so that cause an observable phenotype so there is not a lot of y-linkage.there is not a lot of y-linkage.

– Ex: hypertrichosis pinnae (hairy ears)Ex: hypertrichosis pinnae (hairy ears)

X-LinkedX-Linked:: inheritance from a gene on the X inheritance from a gene on the X chromosomechromosome

Most X-linked disorders that we know of are X-Most X-linked disorders that we know of are X-linked recessive – Examples:linked recessive – Examples:

ColorblindnessColorblindnessHemophiliaHemophiliaMuscular DystrophyMuscular DystrophyPattern BaldnessPattern Baldness

Male baldness: XbY Female baldness: XbXb

Muscular dystrophy

Write this down!

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

http://www.toledo-bend.com/colorblind/Ishihara.html

ARE YOU COLORBLIND?

http://www.toledo-bend.com/colorblind/Ishihara.html

http://www.toledo-bend.com/colorblind/Ishihara.html

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Sex-Linked Recessive: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.

How they are written:

Genetics Part 4

Scientists observe how traits are inherited by studying phenotypes of a certain species from generation to generationOne tool scientists have developed to study inheritance is a pedigree.

Pedigree:Pedigree: A family tree diagram that shows A family tree diagram that shows how a trait is inherited over several how a trait is inherited over several generations.generations.

Draw all of the symbols and their meanings!

Interpreting a Pedigree Diagram

1) 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.

Interpreting a Pedigree Chart

2) 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.

Examples: Pedigree #1

Is it Autosomal or X-linked?

Answer:

Autosomal

Examples: Pedigree #1

Is the trait Dominant or Recessive?

Answer:

Dominant

Examples: Pedigree #2

Is the trait Autosomal or X-linked? Dominant or Recessive?

Answer:

Autosomal Recessive

Examples: Pedigree #3

Is the trait Autosomal or X-linked? Dominant or Recessive?

Answer:

X-linked Recessive