Shut up,

Darwin

So much for

genius!

All of the inheritance patterns we have learned about so far, such as the one’s from Mendel’s experiments, are often referred to as simple Mendelian inheritance.

This would include any inheritance which is controlled by a single pair of alleles, one dominant, and one recessive.

Many inheritance patterns are far more complex than those studied by Mendel.

Some of these types of inheritance don’t even involve chromosomes!

Polygenic Inheritance

When inheritance follows the rule of dominance, then the homozygous dominant, and the heterozygous individuals all have the same phenotype.

TT=

Tt =

tt =

The allele for tall = T;

the allele for short = ttall

tall

short

When traits are inherited in a pattern where the heterozygous individual is intermediate between those of the two homozygotes, then the pattern is called Incomplete Dominance.

TT = tall

tt = short

Tt = Average height

Red Snapdragons

X

White Snapdragons

=

Pink Snapdragons

When showing incomplete dominance, the alleles look like this: R=redR1=whiteNeither is dominant to the other, so no lower case letter appears.

Can you predict what happens when you cross two pink snapdragons?Do a punnett square crossing RR1 x RR1

R

R1

R R1

RR RR1

RR1 R1R1

The snapdragons appear in the F2 generation in a 1:2:1 phenotypic ratioThe genotypic ratio is also 1:2:1

This result supports Mendel’s law of segregation, meaning that the

alleles combine during fertilization completely randomly in four possible

ways.

Codominance is apparent in a breed of chicken, where black feathers are B, and white feathers are W. Both are dominant, so both are expressed with a capital letter.

X =

When codominant alleles are expressed, both phenotypes are present equally.

BB WW BW

The proteins in our blood cell’s structure is controlled by three alleles; i, IA and IB.

Humans can have one of four possible blood types:

A; B; O; and ABHuman blood is separated into different classifications because of the varying proteins contained in each blood type's red blood cells. These proteins are there to identify whether or not the blood in the individual's body is it's own and not something the immune system should destroy.

If the recessive allele i is paired with IB or IA, it's phenotypic expression is hidden. When the IB and IA are together in a pair, both proteins A and B are present and expressed.

The i allele is the only recessive allele IA and IB are both codominant

IAIA=Type AIBIB=Type Bii=Type OIAIB=Type AB

Question:

What are the genotypes of a man and woman, each with type A, and B blood respectively, who produce offspring with types A, B, O and AB?

AB B

A O

Let’s not forget to show the offspring’s actual genotypes! Now it becomes obvious what their genotypes were.

IAIB IBi

IAi ii

IB

i

IA i

RH, or Rhesus factor is inherited separately from the blood type, and follows the rules of polygenic inheritance. About 85% of humans globally contain the Rhesus positive phenotype, which means they contain the Rh+ antigen on the surface of their red blood cells.

A woman with Type O blood and a man who is Type AB are expecting a child.  What are the possible blood types of the offspring? 

i i

IA

IB

IAi

IBi

IAi

IBi

This couple can only have offspring that are types A, and B.

Can a person with type AB blood ever produce a child with type O blood?NO!!!Blood groupings of humans not only follow the rules of codominance, but also are determined through multiple alleles. IA; IB; i

While many traits are controlled by a single pair of alleles, some traits can be controlled by more than two alleles. These traits are said to be carried and expressed by multiple alleles.In the case of rabbit fur color, there are four different alleles which determine different phenotypes.

Chinchilla rabbitDark-gray coloredC=dominant to all other

alleles

cch= dominant to Himalayan and to

white

Himalayan rabbitch =

dominant to white

White rabbitc =

completely recessive

Try predicting how many possible genotypes would code for chinchilla rabbits.Remember, while there may be multiple alleles in a population that determine a trait, each individual only inherits a pair of these alleles…one from mother, and one from father! Human blood types are not only codominant, but multiple allele!

Traits controlled by genes located on the sex chromosomes are called sex-linked traits. In humans, there are 23 pairs of chromosomes in all autosomal cells within the body. The 23rd pair are sex chromosomes.

The presence of the Y

chromosome in one of these karyotypes,

makes one of these individuals a male. When there are two

X’s, then it is a female

The male genotype is XY, the female genotype is XX

The alleles for sex-linked traits are written as superscripts of the X or Y chromosome…depending upon which chromosome they are associated with.Because males only inherit one X chromosome, when a recessive trait is inherited on the X chromosome there is no corresponding trait to mask that trait on the Y chromosome…so the recessive trait is always expressedIn fruit flies, the allele for eye color is on the X chromosome.

XR=red eyes

Xr=white eyes

What would the genotype of a white-eyed female look like?XrXr

What would the genotype of a red-eyed heterozygous female look like?XRXr

What would the genotype of a red-eyed homozygous dominant female look like? XRXR

Predict what the genotype of a white-eyed male would look like. XrY

Create a punnett square showing the mating results of a homozygous dominant female fruit fly for eye-color, and a male fruit fly with the recessive trait. Because there is no second X chromosome in the male, it cannot be homozygous. Instead, it is called hemizygous.

Xr YXR

XR

XRXr XRY

XRXr XRY

What color eyes will the males have?What color eyes will the females have?

Predict what genotypic combinations would create white-eyed male flies.

Thomas Hunt Morgan discovered that traits were linked to sex chromosomes, through his study of fruit flies.

Hemophilia is characterized by inability to form blood clots because of deficiency in a clotting factor in the blood. The frequency of hemophiliacs is about 1 in 10,000 males. Affected individuals suffer from severe bleeding following a relatively small injury like a cut or a bruise.

Duchenne Muscular Dystrophy affects young boys. It is characterized by muscular degeneration and weakness beginning at the age of 3 to 5 years and progressing rapidly so that affected individuals are wheel-chair bound by the time they are in their teens and die in their early twenties because of severe involvement of their respiratory muscles. Red-green color blindness is the inability to distinguish red from green.

An 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.

How can you tell quickly, that this must be a sex-linked trait?

Only the males have it!

Some traits, such as skin, eye color and height in humans vary over a wide range.

These traits are governed by many genes, or polygenic inheritance.

Skin Color: A Polygenic TraitScientists have found that when light-skinned people mate with dark-skinned people, their offspring have intermediate skin colors, similar to what you might expect with incomplete dominance.When intermediate skin-colored children mate with each other, the resulting skin color range from the light-skin color to the dark-skin color of the original parents.

=X

Gametes contributed by parents ABC ABc AbC Abc aBC aBc abC abc

ABC 6 5 5 4 5 4 4 3

ABc 5 4 4 3 4 3 3 2

AbC 5 4 4 3 4 3 3 2

Abc 4 3 3 2 3 2 2 1

aBC 5 4 4 3 4 3 3 2

aBc 4 3 3 2 3 2 2 1

abC 4 3 3 2 3 2 2 1

abc 3 2 2 1c 2 1 1 0

ABC = dark pigmentationabc = light pigmentation

As you can see, this individual offspring would inherit the darkest pigmentation, whereas this offspring would inherit the lightest pigmentation.

This makes it easier to understand how two offspring of biracial ancestry can actually pass on the darkest, or the lightest pigmentation to their offspring (even though the odds are against it! )

AaBbCc x AaBbCcWhat are the odds of this couple producing an aabbcc individual? AABBCC?1/64 1/64

Sometimes the genetic makeup of an organism determines only that organism’s potential to develop and function in a particular way.As the organism develops, many factors can influence how the gene is expressed, or even whether the gene is expressed at all.Influence of external environment:Temperature,

nutrition, light, chemicals, and infectious diseases can influence gene expression.

Arctic Fox

Influence of internal environment:

Hormones can influence horn size in mountain sheep.