Genetics: Exceptions to Mendel's Rules
April 15, 2002
Genetics:
Exceptions to Mendel's Rules
April 15, 2002
"Heredity provides for the modification of its own machinery" --James Mark Baldwin, 1896
Do Mendel's Laws Always Apply?
We know today that there are many exceptions to Mendel's laws (ie not every gene has alleles that are strictly dominant or recessive). Does this mean that Mendel was "wrong"? NO, it means that we know more today about disesase, genes, and heredity than we did 150 years ago! Four of the most common exceptionss (we will work problems only with Blood Types):
I. Incomplete dominance / Codominance: (example: blood type)
One example of a three-allele gene: the ABO blood types of humans: In this case BOTH the IA (A) and IB (B) gene are dominant and are expressed in the phenotype. The recessive gene i is only seen in the phenotype (Type O) if the parents are heterozygotes or are Type O themselves.
Note that the IA and IB alleles two alleles are both dominant to i - but one isn't dominant over another. This is called Incomplete Dominance or Codominance
Real life question: Kathy and Jim both have type A blood. Their daughter Julia has type O blood.
(a) What blood type alleles do both Kathy and Jim have?? ________________
(b) Their son Ian has never had his blood typed. What are the possible blood type alleles he might have? ____________________
Knowing blood types of the parents and the baby can aid in paternity suits. (Worksheet)
II. Pleiotropy: One gene defect results in many widespread effects on the body because the gene product (protein) is something that used throughout the body
Example: Sickle cell disease - one defect in the hemoglobin gene - a single amino acid change in the protein - has many widespread and devistating effects.
1.pain episodes
2.strokes
3.increased infections
4.leg ulcers
5.bone damage
6.yellow eyes or jaundice
7.early gallstones
8.lung blockage
9.kidney damage and loss of body water in urine
10.painful erections in men (priapism)
11.blood blockage in the spleen or liver (sequestration)
12.eye damage
13.low red blood cell counts (anemia)
Over 2.5 million Americans are carriers (heterozygotes) for the disease, and 70,000 have sickle cell disease. The disease is very common in people of recent African descent, because in areas of the world where malaria in prevalent, being a carrier for SSD provides a selective advantage against developing malaria!
"T" (Tionne Watkins), of the grammy-award winning group TLC, has sickle cell disease and is the National Celebrity Spokeswoman for the Sickle Cell Disease Association of America, (SCDAA)
Another example: Marfan's syndrome - individuals have defects in skeletal system, eyes, and cardiovascular system. All these defects are due to one underlying defect - the body's ability to make connective tissue - one defect has widespread implications in the body
III. Epistatic ("covering up") genes : An epistatic gene interferes with the expression of another, perfectly functional, gene (example: Coat color in Laborador Retreivers, Albinism).
The gene E is involved the ability of the coat to contain pigments like black or brown. If the dogs are EE or Ee, the coat will appear black (BB or Bb) or brown ['chocolate'] (bb). BUT, if this gene is found in the homozygous recessive condition, ee, there will be no coat color, regardless of what the B alleles are! (BB, Bb, or bb). The result is a 'yellow' or 'golden lab:
Golden lab: eeBB, eeBb, or eebb
Black lab: EEBB, EeBB, EEBb, EeBb
Chocolate lab: EEbb or Eebb
Thus, it doesn't matter what the black or brown coat color alleles are, the E gene is epistatic to this gene and can cover up the expression of these traits if heterozygous recessive. (We will work this problem out in class)
Another example - Albinism in humans. Individuals with this trait lack pigment (melanin) in all parts of their body, even though they have inherited alleles for skin color, eye color, and hair color. elanin is a dark brown pigment normally present in the human skin, hair, and eyes. Albinism arises from a genetic
defect resulting in the tyrosinase gene, which produces an enzyme necessary for the melanin synthesis. production of melanin. Generalized albinism occurs in all races in about one in 20,000 persons.
If a person inherits a double dose of the allele for albinism (aa), they will be unable to produce the pigment melanin, regardless what color their alleles for eye, skin, and hair color are.
IV. Continuous Variation: Mendel studied "either-or" traits (purple vs white), but many characters such as human height and skin color vary as a continuum in populations (bell shaped curve)
1. Modifier genes: (example: eye color) Eye color is not a simple trait controlled by different alleles of one gene. Rather, eye color in controlled by at least 3 genes: EYCL, the Green/blue eye color gene (probably located on chromosome 19); EYCL2, the central brown eye color gene (possibly located on chromosome 15), and EYCL3 , the Brown/blue eye color gene located on chromosome 15. These three genes contribute to the typical patterns of inheritance of brown, green, and blue eye colors
For EYCL3, B = brown or b = blue eyes. However,grey eye color, hazel eye color, and multiple shades of blue, brown, green, and grey come from the ccombined action of all three genes PLUS the action of modifier genes that control how much pigment is deposited in the iris of the eye.
Cool eye color website : predict your kid's eye colors! (just for fun).
More info: Eye color at birth is often blue, and later turns to a darker color. Why eye color can change after birth not known (it is most likely due to changes in gene expression, but why?!) An additional gene for green is also postulated, and there are reports of blue eyed parents producing brown eyed children, which the three known genes can't easily explain. Mutations, modifier genes that supress brown, and additional brown genes are all potential explanations....so don't get worried if your eye color doesn't seem to fit with the rest of your family!!!
2. Polygenic Inheritance: (example: human height) more than one gene controls a particular phenotype
In polygenic inheritance, more than one gene controls a particular phenotype. ie skin color is thought to be controlled by 3 alleles (A, B, C).
A person with the alleles AABBCC is very dark skinned, a person with aabbcc alleles is very light skinned, and a person with AaBbCc (or any combo) has an intermediate skin color.
Different "units" produce different shades (AAbbCc, aaBbcC, etc...)
Objectives 4/15/02:
