Advanced Genetic Principles

Inheritance & Gene Expression
Advanced Principles Information

   Summary Outline of Advanced Info:

     A. Multiple Alleles
           1. Definition
           2. Human ABO Blood System
           3. Human Cross Example

     B. Polygenic Characters
           1. Definition
           2. Human Example

     C. Sex Determination
           1. Humans & Mammals
           2. Birds & Reptiles
           3. Default Sex

     D. Sex Linkage
           1. Definition
           2. X-linked
           3. Y-linked
           4. Human Cross Example

     E. Pedigree
           1. Purpose
           2. Symbols

     F. Sex-Influenced Genes
           1. Definition
           2. Human Example

     G. Expression of Genes
           1. Modifying Factors
           2. Nondisjunction
           3. Translocation

A. Multiple Alleles
           1. Definition - situation in which many alleles are present for some genes as a result of multiple mutations to the DNA over time; therefore, the result of these multiple alleles are multiple phenotypes for one trait

           2. Human ABO Blood System
                a. Alleles Involved:
                     1) A (I^A)
                     2) B (I^B)
                     3) O (I^O)
                b. Genetic Principles Involved:
                     1) A and B alleles are codominant
                     2) O allele is recessive to both A and B
                c. Important Terminology:
                     1) Antigen - protein on the surface of a specific red blood cell; acts as an identification marker

2) Antibody - protein produced by body's immune system to combat foreign substances

d. Summary of ABO Blood Types:

                e. Summary of ABO Blood Type Compatibility:
                            Donor - Listed in the Top Row
                            Recipient - Listed in the Left Column

                f. Universal Donor - O - The O alleles do not code for the production of any antigens on the surface of the O blood cells; therefore, these cells will be accepted by the immune systems of every other blood type.

                g. Universal Recipient - AB - The presence of both an A and B allele together codes for production of both antigens. Therefore, A and B blood cells can be received without rejection. O blood cells can also be received by AB individuals because there are not antigens present on these O blood cells.

           3. Human Cross Example - If a male has blood type B and a female has blood type A, what are the possible blood types in the offspring?
                Male Genotype = B_ (B and O possible)
                Female Genotype = A_ (A and O possible)

                          Possibility #1

Answer - All blood types are possible in the offspring. To get a more definitive answer, the parents' genotypes (not simply phenotypes) would need to be determined.

     B. Polygenic Characters
           1. Definition - a single phenotypic trait which is controlled by more than one pair of genes
                Examples = Height, Hair Color, Skin Color, Eye Color

           2. Human Example - Three genes are involved in the phenotypic determination of Human Skin Color.
                a. Homozygous Dominant Extreme = AABBCC - produces the maximum amount of melanin possible at each of three gene loci (dark skin)
                b. Homozygous Recessive Extreme = aabbcc - produces the minimum amount of melanin possible at each of three gene loci (light skin)

     C. Sex Determination
           1. Humans & Mammals - One pair of sex chromosomes determines the sex of the individual; therefore, the chances of having a son OR a daughter with each pregnancy is 50/50.
                a. Male = XY (heterozygous)
                b. Female = XX (homozygous)

           2. Birds & Reptiles - Sex determination in these organisms is still controlled by one pair of sex chromosomes; however, the male is homozygous.
                a. Male = ZZ (homozygous)
                b. Female = ZW (heterozygous)

           3. Default Sex - In humans, female is the default sex. The Y chromosome must be present to cause the embryo to develop male sexual characteristics. Therefore, Turner's Syndrome, 45 X, produces a female.

     D. Sex Linkage
           1. Definition - Situation dealing with genes that are found on one of the sex chromosomes, but not the other (Up to this point, all of the genes and traits we discussed were found on the autosomes.)

           2. X-linked - Genes only found on the X chromosome; therefore, males will express a recessive condition with only one allele

           3. Y-linked - Genes only found on the Y chromosome; therefore, females will never be affected

           4. Human Cross Example - Some forms of Hemophilia are X-linked Recessive conditions
                a. Alleles Involved:
                     1) X^H - Normal Blood Clotting Protein
                     2) X^h - Nonfunctional Blood Clotting Protein
                b. What are the offspring phenotypes from a carrier female and a hemophiliac male?
                        Male Genotype = X^hY
                        Female Genotype = X^H X^h

      Answer - The phenotypes of the offspring would be as follows:
                          25% Normal (Carrier) Females
                          25% Hemophiliac Females
                          25% Normal Males
                          25% Hemophiliac Males

     E. Pedigree
           1. Purpose - Diagram used to help trace genetic traits or diseases through families.

           2. Symbols:
                a. Open Square - Unaffected (Healthy) Male
                b. Colored Square - Affected Male
                c. Open Circle - Unaffected (Healthy) Female
                d. Colored Circle - Affected Female

     F. Sex-Influenced Genes
           1. Definition - Genes that are expressed to a greater or lesser extent as a result of the level of sex hormones present in an individual's body. These genes are usually located on autosomes.

           2. Human Example - Male patterned baldness
                a. Males will be affected with only one allele due to the compounding effects of testosterone.
                b. Females must have both alleles to be affected.

     G. Expression of Genes
           1. Modifying Factors:
                a. Age
                b. External Environment
                c. Diet
                d. Exposure to Light
                e. Temperature
                f. Modifier Genes - genes that produce proteins which lessen or strengthen the effects of other genes determining phenotypic traits

           2. Nondisjunction - Failure of chromosomes to separate properly during meiosis; Turner's Syndrome, Down's Syndrome, and Klinefelter's Syndrome are all results of nondisjunction



           3. Translocation - Attachment of all or part of a chromosome to a nonhomologous chromosome.

To test your knowledge about Advanced Genetic Principles, click on the Advanced Questions Link at the top of this page. After you answer the questions, be sure to check your responses by clicking on the Advanced Answers Link.

Blood-Type	Genotypes	Antigens	Antibodies
A	AA/AO	A	Anti-B
B	BB/BO	B	Anti-A
O	OO	None	Anti-A and Anti-B
AB	AB	A and B	None

	A	B	O	AB
A	Yes	No	Yes	No
B	No	Yes	Yes	No
O	No	No	Yes	No
AB	Yes	Yes	Yes	Yes