APPLIED GENETICS

GENE-CHROMOSOME THEORY

* genes and chromosomes are made up of DNA, which is the genetic material for all life on earth (the structure of DNA will be talked about in detail next packet)
* a gene is a segment of DNA that codes for one type of molecule
* genes are found in specific places on chromosomes called LOCI
* chromosomes are condensed coils of DNA that contains thousands of genes--so they are responsible for THOUSANDS of molecules

* when a change happens in the genetic code (that is DNA--that is, a gene--that is, a chromosome), it is called a MUTATION

MUTATIONS
* a change in the genetic information (DNA)
* can happen in any cell of the body

IN BODY CELLS: normally, not too much damage because only a few cells are generally changed

IN SEX CELLS: can affect a new organism if the gametes are involved prior to fertilization

I. TYPES OF MUTATIONS

A) Chromosome Mutations

* because chromosomes are made of thousands of genes, these mutations can have a very large impact on the organism

1) Chromosome Number

* normally, there are 46 chromosomes in diploid human cells; and 23 chromosomes in monoploid human gametes
* now, remember MEIOSIS?

INTERPHASE -->SYNAPSIS -->A TETRAD -->CROSSING OVER -->DISJUNCTION
when the pair of homologous chromosomes separate

* when the chromosomes do not split during the process of meiosis, it is called NONDISJUNCTION
* in this case, gametes can have more or less than the n number of chromosomes

Down Syndrome is an example of NONDISJUCTION

  • varying degrees of retardation
  • disfigured features
  • occurs when you have nondisjuction of chromosome 21 (3 instead of 2); this
    is also called trisomy 21
  • total of 47 chromosomes
* if an entire set of chromosomes fail to separate, you have a polyploid organism (where the chromosome number is 3n or 4n instead of the diploid 2n condition
* polyploidy is common in plants, an example is the funky plump strawberries
* in animals, polyploidy is usually lethal: this is because so many genes are affected, and animals are much more complex than plants

2) Chromosome Structure

* when pieces of chromosomes are "broken" off or have other pieces added on
* sometimes can be very harmful, but depends on the number and type of genes that are affected

ADDITION: the gain of a portion of a homologous chromosome

DELETION: the loss of a portion of a chromosome

TRANSLOCATION: the gain of a portion of a non- homologous chromosome


B) Gene Mutations
* affects many less traits than chromosome mutations do
* results from a small change in DNA
* some are not harmful--albino or dwarf
* others are not seen
* MOST are RECESSIVE and HARMFUL; are called lethal alleles
* every human is a carrier for about 5 to 7 different lethal alleles
* a beneficial mutation would be one where the organism is better suited to fit in its environment--this is EVOLUTION

Applied Genetics

a comprehensive guide to all things involving genetics; download it as a stand alone demo here
see what is meant by nondisjunction
these video clips explain the chromosome mutations of additions, deletions, and translocations
See the different types of chromosome mutations, as also see to the right.

II. MUTAGENIC AGENTS

* things that cause or increase the amounts of mutations (changes) in an organism's DNA, examples:

1) RADIATION:
* cosmic rays, X-rays, UV (ultraviolet rays, radon, radioactivity, microwave radiation, cell phones?
* different amounts of radiation can cause small changes in DNA

2) CHEMICALS:
* benzene, formaldehyde, asbestos, many of the additives in tobacco products--all known carcinogens (cancer-causing chemicals)
* different amounts of these chemicals can cause changes in DNA

III. EFFECTS OF THE ENVIRONMENT

* besides genes controlling our characteristics, the environment also plays an important role in making organism what they are

many examples...

1) hair/skin color and sunlight
* some people's hair/skin changes colors with different amounts of sunlight
* the sunlight causes an increase in melanin production in skin

2) sunlight and chlorophyll
* plants will produce chlorophyll in sunlight, otherwise, it will only make yellow pigment

3) temperature and the Himalayan rabbit
* when the temperature is warm, the fur is white; when the temperature is colder, the fur is black
* this enables the rabbit to stay warm--black fur absorbs more sunlight!

4) identical twins
* are identical twins truly identical? NO!
* over time since birth, they probably have had changes in the DNA (mutations), but not the same!
* also, their social and physical environment help shape their personalities

IV. APPLICATIONS OF GENETIC THEORY

A) Artificial Selection (breeding):

* when you choose the parents because of their desirable traits
* ex. dogs, apples, horses --> mules

B) Inbreeding:

* "crossing" of 2 parents with very similar genetic backgrounds
* can lead to genetic defects by combination of similar lethal alleles

C) Outbreeding (hybridization):

* "crossing" of 2 parents of the same species with very different genetic backgrounds to minimize genetic defects

 

Applied Genetics

a comprehensive guide to all things involving genetics; download it as a stand alone demo here
see how dogs have been bred to the many different types of modern day dog
click on the 'Selective Breeding' animation to show how to breed a better crop through natural farming methods

V. HUMAN HEREDITY

all of the Mendelian principles that we learned last packet hold true for humans too...however, because humans are not suitable for experimentation, some knowledge is limited
A) Polygenic Inheritance
* when 2 or more genes influence a phenotype (many-genes)
* not to be confused with multiple alleles
* many human traits are caused by polygenic inheritance: eye color, skin color, and height

Say that the genes A and B control human height. The dominant allele would be for tall and the recessive allele will be for short. However, the total height depends on the amount of tall genes you have.

AABB

aABB, AaBB, AAbB, AABb

AaBb, aABb, AAbb, AabB, aABb, aaBB

Aabb, aAbb, aaBb, aabB

aabb

B) Oncogenes
* genes that are responsible for abnormal cell growth -- CANCER
* ALL CANCER IS GENETIC; IT MIGHT NOT BE ACTIVATED YET
* certain genes can cause cancer if they are changed--mutated
* this change can cause the cell to carry out repeated cell divisions and ignore signals to stop

C) Genetic Counseling

* by looking at the traits of your ancestors, you can learn more about your genotype and that of your offspring
* genetic counseling use techniques such as:

1) genetic screening
* the chemical analysis of body fluids such as blood, urine, semen

2) amniocentesis

* the removal of amniotic fluid for chemical and/or cellular analysis...used with

3) karyotyping
* the preparation of an enlarged photograph of paired homologous chromosomes
* can see chromosomal (gene) abnormalities like chromosome mutations, nondisjunction (Down's Syndrome), and polypolid sets

 

Applied Genetics

a comprehensive guide to all things involving genetics; download it as a stand alone demo here
learn a little about oncogenes; the genes that when mutated lead to cancer

VI. GENETIC DISORDERS IN HUMANS

A) PKU (Phenylketonuria)
* caused by a recessive allele (need 2 copies to express it)
* development of mental retardation
* occurs when one gene does not produce the enzyme (a protein) needed to break down phenylalanine (an amino acid)
* can "treat" with a diet low in phenylalanine

B) Tay-Sachs
* caused by a recessive lethal (fatal) allele
* nervous system shuts down at an early age; usually before 6-8 years
* fatty material accumulates in the nervous system because of an enzyme cannot be produced
* found most often in European Jewish heritage and Pennsylvania Dutch (Amish)

C) Cystic Fibrosis
* caused by a recessive allele on chromosome 7
* glands produce a thick mucus that clogs and damages the lungs
* found most often in Caucasians

D) Huntington's Disease
* caused by a DOMINANT ALLELE; those that have this allele will develop this genetic disease
* this disease causes a progressive breakdown of the brain cells followed by death
* however, symptoms do not develop until after the person is over 30-35+ years old

E) Sickle-Cell Anemia
* caused by a recessive allele but follows incomplete dominance; two copies--severe cases, one copy--moderate case
* the allele changes the structure of hemoglobin; under certain conditions, the abnormal hemoglobin cause the biconcave shape of the red blood cells to change
* blood vessels can become blocked (painful) and red blood cells do not carry enough oxygen (anemia)
* common in people of African decent


VII. PEDIGREES
* a way to track genes through a family
* symbols usually are the same

- circles are females
- half-filled circle means a female carrier
- filled circle means female with condition
- squares are males
- half-filled squares means a male carrier
- filled square means male with condition
- shape with a line through it means deceased

* sometimes these can get pretty complex



Try this pedigree out. Draw the 'symbols' for each person, then answer the questions at the end. This family treeinvolves two families and tongue rolling:

(R=DOMINANT, a roller; r=recessive, non-roller)

Place the persons genotype in their respective shapes (RR=homozyogus dominant, Rr= heterozygous, R_=if the person is a tongue roller but you do not know if they are homozygous or heterozygous, and rr=a non tongue roller)

Narly Story
Fred Narly, a tongue roller, broke with family tradition and married a non-tongue rolling person, Joan Fool in 1944. Fred and Joan proceeded to have two children: Fred Jr. and Mary. Fred Jr. carried on the Narly tradition of tongue rolling while Mary was often chastised and berated for her inability to roll her tongue.

Fred Jr. married a tongue rolling woman named Jean Gueeba, while Mary tried to redeem herself and marry a tongue rolling man named Steve Dorhoff.

Fred Jr. and Jean had two children: Nancy and Frank. Nancy was a true tongue rolling Narly. While Frank was almost disinherited from his dad's will because he could not roll his tongue.

Mary and Steve Dorhoff had a daughter, Sarah. Unfortunately, Sarah couldn't roll her tongue.

Frank eventually ran away from home because he didn't want to bring shame to the family. Nancy Narly settled down with Bill Quiff. After the wedding, the Narly's discovered that Bill could not roll his tongue.

Sarah Dorhoff married a man named Ted Goof. Ted was accepted into the family due to the fact that he could roll his tongue. Unfortunately, Ted died when he choked on a bad Chalupa. After Ted's death, Sarah never remarried and did charity work to help non-tongue rollers. But before Ted's death, Sarah became pregnant and eventually had Craig Goof. Much to everyone's relief, Craig could roll his tongue.

Meanwhile, Bill and Nancy Quiff also had a child named Candice Quiff. Again, everyone was relieved when Candice could roll her tongue.

At a party in college, Candice Quiff and Craig Goof met and fell in love.

Dorhoff Story
In 1945, Neil Dorhoff and Marie Shoo, both tongue rollers, had a whirlwind affair and married. They had three children, Steve, Maureen, and Tom. Steve was always the favorite son because he could roll his tongue, while Maureen and Tom could not. Unfortunately, Tom died in a bizarre gardening accident.

Steve Dorhoff married Mary Narly and had a child named Sarah.

Maureen met and married Bob Lee, a person who took his tongue rolling seriously. Bob and Maureen had a child named Clarence. Luckily for the Lee's, Clarence could roll his tongue.

Questions:
1) What type of 'relative are Candice and Craig? Can they marry legally in the US? Should they?

2) If Candice and Craig did have a child, what is the possibility that the child could roll his/her tongue? Show the Punnett square.

3) If Clarence marries a lady named Lucy Lavender who was not a tongue roller, what is the possibility that their child could roll his/her tongue? Show the Punnett square.

Applied Genetics

a comprehensive guide to all things involving genetics; download it as a stand alone demo here
PI: Pedigree Investigator, On the Case of Nicotine Addiction, from the Genetic Science Learning Center