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Genetics: Beyond Mendel. IB Biology. Mendelian Genetics. This is the term used to describe the basic principles of inheritance for traits that are not inherited in complicated ways. There are many exceptions to the principles we have learned with basic Mendelian genetics. Incomplete Dominance.

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mendelian genetics
Mendelian Genetics
  • This is the term used to describe the basic principles of inheritance for traits that are not inherited in complicated ways.
  • There are many exceptions to the principles we have learned with basic Mendelian genetics.
incomplete dominance
Incomplete Dominance
  • Sometimes, neither allele for a trait is dominant, so there is a blending of phenotypes
  • Example:
    • Homozygous red carnations are crossed with Homozygous white carnations and produce 100% pink carnation offspring

RR x rr = Rr




  • Sometimes, both alleles for a trait are dominant, resulting in offspring with both phenotypes.
  • Example:

Crossing a homozygous white horse with a homozygous red horse produces a roan horse with a coat of red AND white hairs.

multiple alleles
Multiple Alleles
  • This term describes traits for which there are more than 2 alleles
  • Example
    • There are three alleles for blood type: A (IA), B (IB) (codominant), and O (i) (recessive), allowing for 6 possible genotypes:
    • IA IA, IAi (type A phenotype)
    • IBIB, IBi (type B phenotype)
    • IA IB (type AB phenotype)
    • ii (type O phenotype)
blood types
Blood Types
  • Type A blood produces the A type glycoprotein on blood cell membranes (Type B produces B glycoprotein, type O produces a carbohydrate with no effect)
  • Giving someone with type A blood a type B transfusion will cause their immune system to recognize the type A glycoproteins as foreign antigens, attack them, and cause clotting and usually death for the patient
blood types1
Blood Types
  • Those with type AB blood produce both glycoproteins (codominant), so they can receive A or B transfusions without an immune response
  • Those with type O blood are universal donors because the carbohydrate on their cell surfaces do not trigger an immune response, however, those with type O cannot receive any other form of blood
  • This occurs when one gene affects the phenotypic expression of a second gene.
  • Frequently occurs in the expression of pigmentation
    • One gene turns on (or off) the production of pigment, while a second gene controls either the amount of pigment produced, or the color of the pigment
  • Example: in mice, one gene codes for pigmentation, and another for the color of the pigment…
    • CC or Cc genotypes produce pigments, cc produces no pigments
    • BB or Bb makes the pigments black, bb makes the pigments brown (if present)
    • What color would a ccBb mouse be?
  • This occurs when a single gene has more than one phenotypic expression
  • Example:
    • The gene in pea plants that expresses the round or wrinkled texture of seeds also influences the phenotypic expressions of starch metabolism and water absorption
    • This is like killing 2 (or more) birds with one stone
pleiotropy example
Pleiotropy Example
  • Sickle-cell anemia is an example of a pleiotropic human blood disease.
  • It is caused by an allele that incorrectly codes for hemoglobin, causing normally round red blood cells to become sickle-shaped – leading to a painful death when homozygous recessive with that allele
  • Heterozygous individuals with that allele are more resistant to the mosquito-born pathogen malaria
polygenic inheritance
Polygenic Inheritance
  • Many traits are not expressed in just 2 or 3 varieties, such as yellow and green pea seeds or A, B, O blood types
  • Your height, for example, is usually not just short or tall, but can be one of a nearly infinite continuum of possibilities within a certain range
  • Many genes are required to shape single complex phenotypes like height.
linked genes
Linked Genes
  • The law of independent assortment only works for genes on different chromosomes
  • Linked genes are genes that reside on the same chromosome and cannot therefore segregate independently
  • Example:
    • Body color and wing structure genes in fruit flies are linked
linked genes1
Linked Genes
  • If the normal fruit fly body color is gray (B), while the mutant allele is expressed as black (b); and normal wings are full (V), while the mutant shriveled wings are vestigial (v)…
  • A dihybrid cross would typically reveal the following cross between this gray, normal-winged male (BbVv) and a black, vestigial-winged female (bbvv):
Normally: B’s and V’s sort independently

Probabilities: ¼ BbVv, ¼ Bbvv, ¼ bbVv, and ¼ bbvv

Linked: B’s and V’s do not sort so if B is on the same chromosome as V, they do not mix with b or v

Probabilities: ½ BbVv, ½ bbvv

Experimental Probabilities: 41/100 BbVv, 41/100 bbvv, 9/100 Bbvv, 9/100 bbVv

(41:41:9:9 ratio) . . . How?

Crossing Over:

Since genes cross over homologous chromosomes in prophase I of meiosis, in this case, about 18% of the time, we do see some of the unexpected combinations above (9% for each = 18% crossover rate). 82% of the time, normal linked combinations are revealed (41% for each expected result = 82%).

linked genes2
Linked Genes
  • The greater the distance between two genes on a chromosome, the more places between the genes that the chromosomes can break and thus the more likely the two genes will cross over during prophase I of meiosis.
  • So, we can think of every 1% of crossover rate as 1 map unit of distance separating the genes on a chromosome
  • This can help us to visualize the arrangement on a chromosome:
linked genes3
Linked Genes
  • Suppose you knew that for a fly with a genotype BBVVAA (where A is the apterous, or wingless mutant) the crossover frequency between B and V was 18%, between A and V was 12%, and between B and A was 6%. In what order do the genes lie on the chromosome, and how far apart are they?
linked genes4
Linked Genes
  • Hint: think of the 18% crossover rate between B and V as 18 map units apart (making these two the farthest apart from each other)
  • Draw out a possible solution:
sex linkage
  • The one pair of homologous chromosomes in animals that does not have exactly the same genes are X and Y (sex chromosomes)
  • Traits whose genes are located on X (usually) or Y are determined, in part, by the sex of the offspring
sex linkage1
  • Example: red-green colorblindness (bb) is due to a gene on the X chromosome, normal sight will be represented by (BB or Bb):

Normal Female

Normal Male

Carrier Female (normal)

Colorblind Male

Because their Y doesn’t have the colorblindness gene at all, males only need one copy to have the recessive phenotype (disorder); so they are much more likely to inherit sex-linked traits.

x inactivation
  • During fetal development in females, one of the two X chromosomes will be randomly inactivated
    • It is formed into a Barr body and its genes are not expressed
  • Daughter cells from a cell which has inactivated one X will also inactivate the same X.
x inactivation1
  • When X-inactivation begins, some cells will inactivate one X, and others will inactive the other X
  • It is unlikely that the same X will be inactivated in all initial embryo cells, but if it happens, then females can be subject to sex-linked disorders like hemophilia with only one copy of the mutated gene (like men)
  • If chromosomes do not separate correctly in meiosis, a parent can donate too many or too few chromosomes to their offspring:
  • Examples:
    • Down Syndrome: caused by an extra 21st chromosome (trisomy 21). Causes mental retardation, heart defects, respiratory problems, deformities, etc…
    • Turner Syndrome: caused by a missing X chromosome (X0). Results in a female with some physical abnormalities and sterility