Biology 120 Notes (Part 19) Continuing the Discussion of Genetic Disorders

Complex Patterns of Inheritance

• Modern genetics has expanded the range of Mendelian genetics.

• Mendelian genetics is, "the study of the pattern of segregation of phenotypes under the control of genetic loci taken one at a time.

• Loci is "a plural of the word "locus" and "locus" is the position of a chromosome where a particular genetic trait presides." Sometimes used to describe multiple genes that affect the same function."

• (The link for the information of the words “loci” and “locus” are attached here: https://en.mimi.hu/biology/loci.html#:~:text=Locus%20%28pl.%20loci%20%29%3A%20The%20position%20on%20a,mean%20regions%20of%20DNA%20that%20are%20express%20ed. )

  (The link for the information obtained on the term “mendelian genetics” is attached here: https://medical-dictionary.thefreedictionary.com/mendelian+genetics )

• Most patterns of inheritance are more complex than previously thought.

• There are several variations of Mendelian genetics which include: Multiple alleles, Codominance, Incomplete Dominance, Pleiotropy, Polygenetic Inheritance, and X-Linked Inheritance.

  Multiple Alleles

• In explaining multiple alleles, it describes how several alleles for a given trait exist in a population.

• Though many alleles are available, each person will only possess two.

• For example, blood type.

• There are three possible allele combinations: IA (A antigen is on the red blood cells), IB (B antigen is on the red blood cells), and i (there are neither A nor B antigens on the red blood cells).

• There are also multiple blood type alleles.

• For the A phenotype, the genotype can be IAIA or IBi.

• For the B phenotype, the genotype can be either be IBIB or IBi.

• For the AB phenotype, the genotype can only be IAIB.

• For the O phenotype, the genotype can only be ii.

Codominance

• Codominance occurs when two dominant alleles are equally expressed.

• Some examples include AB blood type, roan cattle and horses, and some flower cultivars.

Incomplete Dominance

• Incomplete Dominance occurs when the heterozygous phenotype is intermediate between homozygous dominant and recessive phenotypes.

• Some examples include flower color in some species and Familial Hypercholesterolemia (FH) in humans.

Pleiotropy

• Pleiotropy occurs when a single mutant gene affects two or more distinct traits.

• Some examples include Cystic Fibrosis and Sickle Cell -- severe anemia, poor circulation, pain, high fever, spleen damage, paralysis.

Polygenic Inheritance

• Polygenic Inheritance occurs when a trait is governed by two or more sets of alleles, i.e., AaBbCc.

• Alleles can be on different chromosomes.

• Some examples include height, skin color, hair color, and eye color.

X-Linked Inheritance

• X-Linked Inheritance refers to genes that are located on the x chromosome but have nothing to do with gender.

• X-linked recessive disorders affect males disproportionality.

• Some examples include color blindness, hemophilia (causes internal bleeding), and Muscular Dystrophy (the wasting away of muscles).