Beyond Mendelian Inheritance
Dec 18, 2015
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.
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 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.