Structural Biochemistry/Inheritance Patterns

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Mendelian Genetics/

Mendelian genetics hereditary patterns include the idea of dominant and recessive alleles that were determined using his monohybrid and dihybrid crosses. The basics of Mendelian inheritance is that the dominant genotype is expressed in individuals that are homozygous or heterozygous for the dominant allele. The recessive phenotype is only expressed in individuals homozygous for that allele.

Geneticist Gregor Mendel studied how pea plants inherited two different traits that had two different possibilities each: flower color (white or purple) and pea shape (smooth or wrinkled). When he crossbred a purple-flowered pea plant with a white-flowered pea plant—known as the P generation—he obtained an F1 generation of all purple-flowered pea plants. After crossbreeding two F1 purple-flowered pea plants, he obtained an F2 generation with a ratio of 3 purple-flowered pea plants to 1 white-flowered pea plant.

Non-Mendelian Genetics/ Non-Mendelian Genetics is the branch of inheritance patterns that do not follow the basic recessive and dominance laws that is characteristic of Mendelian genetics. One example is incomplete dominance. In this type of inheritance, both types of alleles contribute to the phenotype, resulting in an appearance that is in between the phenotypes of the two parents.

In co-dominance, both alleles are expressed fully. For example, human blood groups ABO are from the three alleles, IA, IB, and IC. If an individual has the alleles IA and IB, they will have AB type blood.


Sometimes, one gene is responsible for more than one phenotype and this is referred to as pleiotropy. Pleitropy is observed in PKU (phenylketonuria)disease in which one gene causes mental retardation as well as reduced pigmentation in skin, among other complications.

Another type of inheritance is sex-linked inheritance. In human females, two X chromosomes are present, and in human males, one X and one Y chromosomes are present. Because genes on the X chromosome is not present in the Y chromosome, the inheritance pattern is significantly different from any other patterns of heredity. For recessive traits, males only need one copy of the allele on the X chromosome to display the recessive phenotype, whereas females still need two recessive alleles on both their X chromosomes to have the recessive phenotype. A standard example of such recessive sex-linked inheritance is hemophilia.

Autosomal Dominant[edit]

  1. Affected individuals have at least one affected offspring
  2. The phenotype appears every generation
  3. Unaffected progeny do not transmit the trait to their offspring
  4. Two unaffected parents have only unaffected offspring

Autosomal Recessive[edit]

  1. Unaffected parents can have affected offspring
  2. Affected progeny are both male and female
  3. May skip generations

X-linked Dominant[edit]

  1. Trait is never passed from father to son
  2. All daughters of an affected father are affected

X-linked Recessive[edit]

  1. Trait is never passed from father to son
  2. Males are much more likely than females to be affected
  3. Males cannot be carriers (only one X chromosome)

Maternal Inheritance[edit]

  1. Due to inheritance of alleles on mitochondria genes
  2. Trait is passed exclusively from mother to all progeny (eggs provide the bulk of cytoplasm for the zygote, and mitochondria are located in the cytoplasm)
  3. Trait is not passed from father to progeny (sperm provide essentially no cytoplasm to the fertilized zygote)



References/ 1.http://faculty.evansville.edu/de3/b10004/PDFs/9_Inheritance.pdf 2. http://www.nature.com/scitable/topicpage/pleiotropy-one-gene-can-affect-multiple-traits-569 3. http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/mendel1.htm