Patterns of Inheritance

Gregor Mendel – Father of Genetics-lived in a monastery in Austria-Taught math and science-tended the garden-did most of his discovery work using the garden pea, Pisum sativum-wasn’t credited with work until afterhe had died-most of Mendel’s original rules still hold true today

Mendel determined that organisms inherit “traits” from their parents.

This gives them both a similarity and difference from their parents.

http://1.bp.blogspot.com/-7AYWwKVU66Y/TiaAkw9x6hI/AAAAAAAACc8/ri5Y9IYU1jI/s1600/Gregor+Mendel1.gif

Genes (locus) actually consist of two different parts called ALLELES… 2 alleles make a gene and one gene can determine a trait.One allele comes from each parent on the gametes passed down during fertilization of an egg by a sperm.

Alleles will be either DOMINANT or RECESSIVE

dominant alleles always appear and mask out the recessive alleles. recessive alleles must be purely recessive in order to appear. http://www.bio.georgiasouthern.edu/bio-home/harvey/lect/images/alleles.gif

If recessive alleles must be pure in order to appear, then that means they are HOMOZYGOUS in their make-up. Homozygous genes are either purely dominant or purely recessive. We say this is a “true breeding” gene for a trait. Recessive traits can only show up this way because there are no dominant traits to mask them out if they are homozygous.

AA dominant

or aa recessive

Sometimes, a dominant trait will appear by will be HETEROZYGOUS dominant instead. This means the gene contains one dominant allele and one recessive allele. It appears dominant because the dominant allele masks out the recessive allele, however, the recessive allele may show up again in future generations.

Aa

Alleles will give an organism both their outward looks and their genetic make-up on the genes.

PHENOTYPE is the outward appearance of the organism. This is a result of the gene being translated into proteins or other molecules that end up becoming your hair color, skin type, eye color, length of your big toe and zillions of other traits that are uniquely you.

When you observe brown eyes, blonde hair or freckles on someone you are seeing the phenotype of that individual.

GENOTYPE is responsible for creating the phenotype. The genotype is the actual message on the gene (DNA) that tells the cells what type of protein they will be made of. This message results in eye color, hair color, etc.

You cannot see the genotype…it is inside the genes in your cells.

We can use information from these terms to predict an organism’s offspring outcome.

For instance…if a homozygous tall pea crosses with a homozygous short pea, what will the offspring look like?

Well…it depends on which is the dominant trait and which is the recessive trait.

When the dominant trait is determined, is it heterozygous dominant or homozygous dominant?

What about the recessive trait???

http://www.glencoe.com/qe/images/b136/q4315/ch10_0_h.gif

P1 represents the parent generation of pea plants. F1 represents the first generation and F2 the second generation. Based on the F1 generation being all tall plants, do you think the parents are heterozygous or homozygous for their traits?Still having a hard time?

At this point…a PUNNET SQUARE would be a nice tool to introduce. Created by R.C. Punnet who was a poultry geneticist.

Notice how the top and side of each square only lists a single parent’s gametes.WHY?

http://feistyhome.phpwebhosting.com/punnett.gif

Gamete 1 Gamete 2

Gam

ete

1G

amet

e 2

Remember that meiosis produces haploid numbers of chromosomes in the sperm and egg cells (gametes). That means that even though each gene contains 2 alleles, only one of those alleles will be passed on during reproduction by each parent. Thus, we separate the parental gametes in order to produce offspring that have the normal 2 alleles per gene. Each offspring has it’s own unique combination of alleles, so chance has no memory in genetics and all offspring have equal chances of getting all the traits.

http://anthro.palomar.edu/biobasis/images/meiosis.gif

Adding the gametes from our pea plant parents (P1 generation) gives us the following possible combinations for the F1 offspring…

http://feistyhome.phpwebhosting.com/punnett.gif

T T

t

t

Tt Tt

Tt Tt

What do the pea plant offspring look like? Are they tall or short?Homozygous or heterozygous?

Can you determine the ratios for the offspring?

What do the ratios tell you?

http://creationwiki.org/pool/images/thumb/3/3b/Punnett_square.JPG/200px-Punnett_square.JPG

This punnet square shows the resulting offspring from a cross between two of the F1 generation.They are called the F2 generation.What are the genotype and phenotype ratios?Are they the same as the F1 generation ratios?

http://www.exploringnature.org/graphics/anatomy/Punnetts_square.jpg

These punnet squares only represent MONOHYBRID or one trait crosses. In reality, it is never this simple. Most crosses deal with multiple genes requiring multiple squares on the punnet.

These crosses are called DIHYBRID, TRIHYBRID AND SO FORTH…

Notice that there isstill only one allelefor each trait fromeach parent.

Also notice there are16 possible offspring in this cross, not 4.

http://image.tutorvista.com/content/feed/u2044/Punnet%20square.gif

Can you work this dihybrid cross and determine the offspring genotype and phenotype ratios?

Just as we can have chromosomal disorders, we can also have genetic disorders. Instead of having an abnormal number, though, we deal with dominant and recessive alleles instead.

DOMINANT DISORDERS: (p. 151, Table 9.1)-tend to run in families and are very difficult to mask

out.-alleles can either be homo- or heterozygous.

Examples: Neurofibromatosis (NF)—

**1/2500**benign tumors on body**begin as coffee colored spots

in childhood

Huntington’s disease…strikes the nervous system causing uncontrollable movements and mental deterioration.

**1/25,000 **strikes in middle age.

Achondroplasia…dwarfism strikes 1/25,000

www.sciencemuseum.org.uk/exhibitions/genes/196.asp

Alzheimer’s disease (one type)

RECESSIVE DISORDERS: -very easy to mask out-must be homozygous to appear-tend to be found in races and

populations where close relatives marry

EXAMPLES:

Cystic Fibrosis…excess mucus in lungs, digestive tract and liver, prevents digestion, pneumonia in lungs; affects 1/1,800 European Americans

Sickle-cell disease…abnormally shaped red blood cells, cause damage to many tissues; affects 1/500 African Americans

Albinism—lack of pigment in skin, hair and eyes*true albinos have colorless eyes*white hair*highly susceptible to skin cancer due to sun exposure*1/22,000 occurrence

White moose seen in northern Michigan.

PKU (Phynylketonuria)…inability of the body to breakdown phenylalanine (aa); 1/10,000 US and Europesevere retardation in untreated cases, hospital test on newborns

IF ONLY….

GENETICS WERE ALWAYS THIS EASYTO UNDERSTAND???@@@###$$$

THERE ARE always EXCEPTIONS TO THE RULES!!

Beyond Mendel we have some genetic phenomenon that doesn’t play by the rules:

INCOMPLETE DOMINANCE—traits are both dominantand unable to mask each other out soresulting offspring have an appearance that is a “blending” of each parent. (NOT

blending theory) Red x White Snapdragons = Pink

offspring Hypercholesterolemia = high

blood pressure HH = normal bp, hh = severely

high bp (1/1mil) Hh = bp about 2x normal (1/500)

CO-DOMINANCE—offspring will show both dominantcharacteristics of parents. Traits are unable to mask each other out so they both show up. Black/White face cattle and A/B blood type

MULTIPLE ALLELES—some traits have more than 2(i.e. dom/rec) that code for them. Only 2 of themultiple alleles can be on a gene at a time.

Blood type in humans is a good example. (p. 156, F 9.20)Have 3 alleles for blood type:

A is dom, B is dom, “O” is rec or “zero”These code for several types of blood:

AA, AO are both type “A”BB, BO are both type “B”AB and OO (type O).

Blood can only be infused with a compatible type. Only A can infuse A, Only B can infuse B, Only AB can infuse AB and Only O can infuse O. Since O is recessive is considered a universal infuser. It can infuse all types.

PLEITROPHY—one gene can influence many inherited characteristics. Usually only one gene influences one characteristic. Examples:

Sickle-cell disease—gene to make a normal shape of red blood cell is missing. “Half moon” shaped red blood cells cause many problems; anemia, joint pain, spleen damage,heart damage, brain damage, weakness

Marfan’s Syndrome—gene to make normalconnective tissue in muscle is missing. Causes tallness, long fingers and toes, hearthas weak aorta wall and can rupture.

POLYGENETIC INHERITANCE—one trait is controlled by more than one gene.

Example: Skin color in humans: controlled by 3 genes

A, B, and CAABBCC is darkest skin coloraabbcc is albinismAaBbCc or any variation of ½ dom and ½ rec alleles is medium skin.

Environment can affect skin color (and many other traits) as much as genetics do. Some people with lighter skin genetics appear dark due to the sun or tanning beds.Height is affected by hormones, nutrition in early years.Heart disease is both genetic and environmental.

SOME DISORDERS ARE CARRIED ONLY ON THESEX CHROMOSOMES, X and Y. These disorders arecalled SEX-LINKED DISORDERS:

Red-green colorblindness good example.

Can you see the shapes in this diagram? If not, you may be color-blind. More men than women are affected because it is carried on the “X” chromosome and males cannot mask it out with a normal X like women can