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

==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== occurs when two dominant alleles are equally expressed.
Some examples include AB blood type, roan cattle and horses, and some flower cultivars.

==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== 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== 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== 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).