Beyond Mendelian Inheritance

INCOMPLETE DOMINANCE

-NEITHER ALLELE IS COMPLETELY DOMINANT OVER THE OTHER

-THE HETEROZYGOUS PHENOTYPE IS A BLENDING OF THE TWO HOMOZYGOUS PHENOTYPES

Red flower = RR

Pink flower = Rr

White flower = rr

Sample problem:

Red snapdragon flowers (R) are incompletely dominant to white snapdragon flowers (r). A heterozygous flower is crossed with a white flower.What is the genotypic and phenotypic ratios of the offspring?

CODOMINANCE

-BOTH THE DOMINANT ALLELES ARE EXPRESSED IN THE HETEROZYGOUS INDIVIDUAL.

-USE 2 DIFFERENT CAPITAL LETTERS TO RERESENT THE DOMINANT ALLELES

Sample problem: Red and white coat color are co-dominant in cattle. Two heterozygous cows are crossed. What is the genotypic and phenotypic ratio of the offspring?

MULTIPLE ALLELESSeveral dominant alleles and/or recessive alleles can be combined to create multiple phenotypes.

Blood TypeIn human blood, type A blood and type B blood are codominant. However, there is a recessive allele for type O.

Type A and B show regular dominance over this recessive allele. Thus, the only way to be type O is to be homozygous recessive.

BLOOD TYPE CAN RECEIVE FROM: CAN DONATE TO:Type A    

 Type B    

 

Type AB     

Type O     

Human Blood Genotypes and Phenotypes

Phenotype Genotype(s)•1.

•2.

•3.

•4.

Sample Problem #1:

Adam Sandler is homozygous for type A blood. Megan Fox

is heterozygous for type B. If they have kids what will be the

genotypic and phenotypic ratios of the possible kids?

Sample Problem #2:

Jimmy Fallon has type O blood. Jennifer

Aniston has type AB blood. If they have kids

what will be the genotypic and phenotypic ratios

of the possible kids?

Sample Problem #3:

Channing Tatum has type A blood. Ms. Palmeri

has type B blood. They have a baby with type O

blood. How is this possible? Show your work!

Sex-Linked Traits

• Sex-linked traits are traits that are controlled by genes on the sex chromosomes – The X and Y chromosomes

Are they controlled by both sex chromosomes?

• Most sex-linked traits are controlled by genes on the X chromosome.

• This is because an X chromosome is much larger than a Y chromosome.

• A few traits are suspected to be controlled by genes on the Y chromosome, however there is less research about Y-linked traits.

Different Forms of Sex-linked Inheritance

• There are three different forms of sex-linked inheritance that we will be examining:– X-linked recessive inheritance– X-linked dominant inheritance– Y-linked inheritance

X-linked Recessive Inheritance

• X-linked recessive traits are traits resulting from a recessive allele on the X chromosome.

• There are over 100 different human conditions that are caused by recessive alleles found on the X chromosomes.

• X-linked recessive alleles are represented by a X , superscript lower case letter

X-linked Recessive Inheritance

• These traits tend to show up in males more than females. – Why?

X-linked Dominant Inheritance

• X-linked dominant traits are traits that result from the presence of a dominant allele on the X chromosome.

• Unlike X-linked recessive traits, females and males both require only ONE dominant allele in order to express the trait.

• X-linked dominant traits are represented by an X, superscript capital letter or a (+)

Polygenic Traits• Most of your traits are controlled by the

interaction of many genes.

• Multiple genes working together produce a continuous distribution in a “Bell Shape” curve of degrees.

Examples of Polygenic Traits

• Body Type• Height• Skin Color• Hair color• Eye color• Intelligence• We often see the

famous “Bell Curve”

Individual genes of a polygenic trait follow Mendel's laws but together do not produce Mendelian ratios.

The Environment

The Environment

Heavily Heavily

Influences

Influences

Polygenic traits

Polygenic traits

Recent studies show … • Hypertension

• Diabetes

• Cancers

• Allergies

• Cardiovascular diseases

• Behavioral traits (alcoholism and phobias)

…..have some genetic link but also environmental explanation.

Environmental Effects

Expression of some genes may be impacted by environment

Gene for pigment production expressed in cooler regions of body

Another example of environmental influence:

• Hydrangeas – same genotype, different environments different color flowers

Acid pH

Alkaline pH

• Epistasis–Action of genes at one loci modify

expression genes at another loci• Effects often complex

–Examples• Normal expression of ABO blood type

depends on functional fucosyltransferase 1 –Recessive = no expression/attachment of

antigen to blood cell

• Other examples:–Anthocynanin coloration in corn–Coat color in Labrador dogs

Epistasis

One gene affects the expression of a second gene

Example: H gene is epistatic to the ABO gene.•H protein attaches the A or B protein to the cell surface•hh genotype = no H protein•Without H protein the A or B antigens can not be attached to the cell•All hh genotypes have the phenotype of type O

•IAIA hh, IAi hh, IAIB hh, IBIB hh, IBi hh, and ii hh

Epistasis

•R.A.Emerson – 1918

•9:7 ratio Purple:white corn

•Progeny must have at least 1 copy of dominant allele to produce purple seed

• Epistasis in Labrador Dogs

• Bb or BB dark (black) pigment produced

• bb light (brown) pigment produced

• Ee or EE deposition of melanin

• ee deposition of pigment blocked

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eebb

Yellow fur;brown nose,lips, eye rims

eeB_

Yellow fur;black nose,lips, eye rims

ee

No dark pigment in fur

Yellow Lab

E_

Dark pigment in fur

E_bb

Chocolate LabBrown fur,nose, lips,eye rims

E_B_

Black LabBlack fur,nose, lips,eye rims

Pleiotropy

PleiotropyPleiotropy refers to an allele which has more than one effect on the phenotype.

This can be seen in human diseases such as cystic fibrosis or sickle cell anemia.

In these diseases, multiple symptoms can be traced back to one defective allele.

Pleiotropic effects

• Occurs when an allele has >1 effect on phenotype

• Examples are:– Sickle cell anemia– Cystic fibrosis