Sometimes, things don’t always

follow Mendel’s Laws

I. Incomplete Dominance

• The production of a phenotype that is INTERMEDIATE to that of the two homozygous parents.

• Neither allele seems DOMINANT or recessive—both traits are partially expressed.

Examples

• Four o’clock flowers—A red flower crossed with a white flower makes a PINK flower (not red or white, as in normal dominance/recessiveness)

• With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. 

• I remember Incomplete Dominance in the form of an example like so:

• RED Flower x WHITE Flower ---> PINK Flower

Another Similar Example…

Cattle Color: “Red” cattle and “white” cattle are both dominant and when crossed—both colors are partially expressed as in “roan”.

• With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together. 

• I remember codominance in the form of an example like so:

• red x white ---> red & white spotted

This can occur in other animals as well…

• Mendel did not observe this because it does not occur in peas. It was discovered when crossing other organisms (the four o’clock flowers)

• In a Punnett Square or genotype, the dominant parents are expressed with 2 capital letters (one for each dominant allele). (The letters are different)

Snapdragon ExampleIncomplete Dominance i

n Humans & Plants

II. Multiple Alleles

• Sometimes, there are more than two forms a gene can take.

• Example: There are three possible alleles for blood type (A, B, and O alleles). Any person still only can inherit two, one from mom and one from dad.

Examples

**Blood Type is a special example because it both demonstrates

Incomplete/Co-dominance

AND

multiple alleles.

Examples

• Blood Type ABo Blood Types A, B are both

dominant alleleso The O allele is recessiveo If a person gets both alleles, then

both are expressed and the person has AB blood type.

Examples

o If a person has blood type A, then either they inherited two A alleles (AA) or an A and an O (AO).

o If a person has blood type B, then either they inherited two B alleles (BB) or a B and an O (BO).

o If a person has blood type O, what would be their genotype?

o AB blood type would be

OO

AB

oRemember the difference between genotype and phenotype!

Multiple Alleles & Co Dominance in Blood Type

III. Multiple Genes

• More than one gene can control a trait. This is called POLYGENIC INHERITANCE

• The effect of each allele by itself is small, but the combination of alleles can produce a wide variety.

• Example: Eye color– One allele may control pigment production and how

this one interacts with the color gene will vary in individuals. That is why there is such a variety in eye colors.

• Traits controlled in this way show much VARIETY in a population.Other examples of traits controlled by polygenic inheritance are:

• Fingerprints• height• weight• body build• shape of eyes/ears/lips• There are other traits in other

organisms that are controlled by mulitple genes, too.

Sometimes mutations in genes can cause an unwanted trait. There are several diseases that can be caused by mutations in the alleles controlling a particular trait. Not all mutations produce harmful results.

Recessive Genetic Disorders

• Person must get TWO copies of the recessive gene to get the disease.

Recessive Genetic Disorders

• Sickle Cell Anemia--red blood cells are sickle-shaped instead of disc-shaped

– Can’t carry enough oxygen– Don’t move through blood vessels

easily– Treated by transfusions of normal

cells or drugs to increase oxygen capacity

Cells are sickle-shaped instead of disc-shaped.

https://search.yahoo.com/yhs/search?p=sickle+cell+disease%2C+video&ei=UTF-8&hspart=mozilla&hsimp=yhs-002

• Cystic Fibrosis--thick mucus instead of thin fluid in lungs and intestines

– Most common genetic disorder among Caucasians

– 1 in 20 white people carry a recessive allele for the disorder

– Four babies born in U.S. with this disease everyday

• http://www.wsj.com/video/living-with-cystic-fibrosis/C5749CA6-AC78-42C2-8EEE-F13862F52D9B.html

Cystic Fibrosis affects the lungs!

People who have it tend not to live very long lives

.

Sex Determination

• Sex or gender in many organisms is determined by X and Y chromosomes

• These chromosomes differ from one another in shape and size. The Y chromosome is very short.

• Two X chromosomes will produce a female and one X and one Y will produce a male.

23rd Chromosome Pair

Is it a boy or a girl?

A girl !

Sex-linked Disorders

• Some genetic disorders are “linked” to the X and Y chromosome and are therefore called “sex-linked”

• An allele inherited on a sex chromosome is called a sex-linked gene

• Traits controlled by genes on the X chromosome show up most often in males (they inherit the trait from their mothers)

• Males are not carriers for sex-linked traits…(Females are!)

• Some examples: Colorblindness and hemophilia

Color Blindness “tests”

The test to the left is simpler.

The individual with normal color vision will see a 5 revealed in the dot pattern.

An individual with Red/Green (the most common) color blindness will see a 2 revealed in the dots.

The test below is simpler. The individual with normal color vision will see a 5 revealed in the dot pattern. An individual with Red/Green (the most common) color blindness will see a 2 revealed in the dots.

Pedigrees• A tool used to trace a trait through

a family is called a pedigree

• In pedigrees, a square represents a male and a female is represented by a circle.

Pedigrees• If the square or circle is filled in or

darkened, the person has the trait or condition.

• If it is half colored or half darkened, the person is a carrier (has one allele for the trait, but doesn’t show it)

Sample Pedigree:

Shows how the cystic fibrosis gene (recessive) got passed in this family

Genetic Engineering

• Chemical or biological methods to change the DNA sequence that makes up a gene

• Already used to treat diabetes, improve crop production for different plants

Genetic Engineering Clip

Manipulating Genes

Types of Selective Breeding

• Inbreeding- have alleles that are very similar to their parents.

Ex. Increases the probability that organisms may inherit alleles that lead to genetic disorders

Types of Selective Breeding (cont)

• Hybridization- receive the best traits form both parents

• Cloning- exactly like the parent.

Ex. Dolly the Sheep• https://search.yahoo.com/yhs/search?p=doll

y+the+sheep%2C+cloning+video&ei=UTF-8&hspart=mozilla&hsimp=yhs-002