Patterns of inheritance!

Although we are SOOOO thankful for Mendel, there are many exceptions to Mendel’s Laws of Heredity….

Genetics is more complicated!

Incomplete dominance:

One allele is not completely dominant over another

Incomplete dominance

Heterozygous offspring have an appearance in between the phenotype of the parents!

Example: Snapdragons (flower)Red (RR) x White (rr)

Incomplete dominance

Heterozygous phenotype is between the dominant & recessive phenotype!

RR

RR

rrrr

Rr Rr

Rr Rr

All F2 generation Are Rr

Rr = PINK flowers!!!!!

Incomplete dominance

Incomplete dominance

Quick tip….

Codominance

BOTH alleles contribute to the phenotype!!!

Codominance

When heterozygous, BOTH alleles act dominant…BOTH alleles are seen!

Example: Chickens BB = black bb = white

BB = black feathersbb = white feathers

Codominance

Heterozygous offspring show both phenotypes!!

Example: Chickens BB = black feathers bb = white feathers

All F1 generation areBb …Bb = black and white speckles!

b b

B

B

Bb Bb

Bb Bb

Codominance in Chickens

P generation

Codominance is SOOO beautiful!

Quick tip…

Review:

Mendel Complete DominanceRed (RR)Red (Rr)White (rr)

Yellow #12Incomplete dominance-Long tails (L)-Short tails (S)

Multiple Alleles

Genes that have more than two

alleles

Color coat in rabbits is determined by a single gene that has at least four different alleles. Different combinations of these alleles result in the four colors you see here.

• In some rabbits, there are four alleles for fur color but each rabbit only has two alleles. Depending on which two alleles they have, they will be one of four colors!

Red is dominant over Tan, which is dominant over Black, which is dominant over Albino (R) (T) (B) (A)

Red > Tan > Black > Albino

• If a heterozygous black male rabbit mates with a homozygous tan rabbit, what will the babies look like?

Another example of multiple alleles…(Codominance, TOO!)

Human blood typeA B O

Three alleles for one trait!

Human Blood Type•Blood type is determined by an antigen found on the surface of the blood cell (protein that can stimulate the production of antibodies)

• A B O

•Everybody receives one allele for blood type from each parent!

•A and B are CODOMINANT!•O is recessive!

Blood Type Phenotype

AA Blood Type A

AB Blood Type AB

AO Blood Type A

BB Blood Type B

BO Blood Type B

OO Blood Type O

So…there are FOUR blood types A B AB O

Multiple Allele & Codominance Problem:

Homozygous male Type B (BB) XHeterozygous female Type A (AO)

½ = AB ½ = BO

SO, what are there actual blood types?

B B

A

O

AB AB

BO BO

If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?

Quick tip…

Polygenic Traits

Traits controlled by two or more genes

The wide range of skin color in humans is due to more than four different genes that control

this trait!

And eye color!

Quick Tip…

Polygenic traits are more complex than

our punnett squares!

Sex-Linked traits

Genes that are carried on the X or Y chromosomes(the sex chromosomes)!Ex: Colorblindness, Hemophilia and

Duchenne Muscular Dystrophy

First, remember how to determine gender…

Females have XX chromosomesMales have XY chromosomes

Sex-Linked traits

BUT…the X chromosome is BIGGER and holds WAY more genes than the Y

-The Y chromosome appears to only contain a few genes total

-Currently 100 x-linked genetic disorders have been mapped

Sex-Linked TraitsFEMALES:

XRXr XRXR

•MALES:

XRY XrY

Carriers

• Since females have two copies of the X chromosome, it is possible to have certain alleles “hidden” by a dominant allele. She is a healthy “CARRIER”

• However, because males only have one X chromosome, they either have it…or they don’t. They can NOT be carriers!

B: Healthyb: Muscular Disorder

FEMALEBb Healthy Phenotype “Carrier”

MALEb Muscular Disorder

Healthy Father, Carrier Mom

2 out of 4 children will have the hemophilia

allele.

-The female: Carrier

-The male : HAS hemophilia

The infected son has children with a healthy woman

•None of the children would have hemophilia.

ALL females: Carriers

•This is why sex-linked traits often disappear and reappear from generation to generation!

Quick Tip!

Sex-linked traits affect males and females

differently!!!

Genetics and Environment

• Traits are determined by a combination of genes and environment!!!

A sunflower gene for height or flower size

BUT, these are influenced by: sunlight, soil, water

Genes provide a plan for development…but how that plan unfolds depends on the environment!

Some more examples….-Blood Pressure

-Intelligence

-High yielding tomato plant

-Fair skin prone to cancer

-Woman baldness

Practice Problems

Incomplete dominance:In a particular species of bird, feather color shows incomplete dominance. A

cross between a blue bird (BB) & a white bird (bb) produces offspring that are silver (Bb). If two silver birds were to mate, what would the genotypes & phenotypes be of their babies?

Practice Problems

Codominance:Cow coloration is commonly an example of codominance. Homozygous cows

can be white or brown and heterozygous cows are called roan. Describe the appearance of the baby cows if a white cow crosses with a brown cow:

Practice ProblemsMultiple Alleles:In one kind of mouse, fur color is a multiple allele trait because there are

three alleles. Black (B), Tan (T), and white (W) are all alleles that mice can have but each mouse only has two alleles.

Black is dominant over Tan, which is dominant over White.Often written: Black > Tan > White

If a homozygous black mouse mates with a heterozygous tan mouse, what will the genotypes & phenotypes be of the offspring?

Practice ProblemsSex Linked Trait:The bison herd on Konza Prairie shows a sex linked genetic defect carried on the X

chromosomes. Some males have a malformed back leg that interferes with its normal motion. If a healthy male bull mates with a female cow that is a carrier, what are the chances of producing a normal son?

Male Genotype: Female Genotype:

If he mates with this cow every year, what percentage of their daughters will have normal knees?What percentage of their daughters will be carriers of this disease?