Mendelian Inheritance

In 1866, Gregor Mendel performed a number of simple but ingenious breeding experiments with garden pea plants and observed consistent, predictable patterns of inheritance. Mendel understood that some sort of hereditary factor was involved. We know today that he was studying phenotypic traits that were the result of single genes with several allelic forms (such as smooth and wrinkled seed coat). From his observations, Mendel developed a number of principles, today referred to as Mendel's Laws of Inheritance :

The Law of Segregation : Each allele possessed by a parent will be passed into separate gametes during meiosis.

The Law of Independent Assortment : In each gamete, alleles of one gene separate independently of all other genes, allowing for new combinations of alleles through recombination.

The Law of Dominance : Each gene has two alleles, one inherited from each parent. Alleles are either dominant , co-dominant , or recessive in their expression; dominant alleles mask the expression of recessive alleles.

Based on the patterns of inheritance that Mendel observed, it is possible to make predictions about the probability of a particular allele being passed on to an offspring, and to make predictions about the phenotypic expression of an allele in the next generation.

For example, consider coat color for Rattus norvegicus , an animal with a variable reputation as a pest, an essential laboratory animal, and a household pet. Rats come in a flurry of colors from black to mottled brown to white. The coat color is controlled by a number of genes, demonstrating a range of dominant/recessive and incomplete dominance inheritance.

For example:
Gene 1: The base coat color of the rat is defined by the agouti gene (A) where:

• A (agouti) is dominant, resulting in a band of yellow and a band of dark brown, or "ticking", on each hair
• a (non-agouti) is recessive, and removes all yellow, resulting in a dark brown hair

Gene 2: The Brown gene affects the penetration of the dark brown pigment in hair, where:

• B (Brown) is dominant, resulting in deep penetration of the pigment
• b (diluted) is recessive, and dilutes the dark brown pigment

Thus one can predict the genotype and phenotypes given crosses between parents of known genotypes, illustrated in the following punnett square

	AB	Ab	aB	ab
AB	AABB	AAbB	aABb	aAbB
Ab	AABb	AAbb	aABb	aAbb
aB	AaBB	AabB	aaBB	aabB
ab	AaBb	Aabb	aaBb	aabb

Where:

	Agouti/Brown has the standard yellow/dark brown bands, and results in agouti (grey) color.
	Agouti/diluted has a diluted dark brown band in the hair, and results in a cinnamon color.
	Non-agouti/Brown has no yellow and brown, and results in a chocolate color.
	Non-agouti/brown has no yellow and diluted dark brown, and results in a light to white color.

The expected ratios for random breeding in a population with the given genotypes are:
56% agouti
19% cinnamon
19% chocolate
6% white

The dominant agouti phenotype can be affected by other genes, resulting in dilutions or variations such as Amber, Blue Agouti, Chinchilla, Cinnamon, Cinnamon Pearl, and Fawn. Each of these are produced by an allele of a different gene. The recessive non-agouti phenotype can also be affected by a second gene, resulting in variations such as Beige, Black, Blue, Champagne, Coffee, Lilac, Mink, Silver, Siamese, and Himalayan.

Deviations from Mendelian Expression:

Since Mendel's discoveries, a number of other patterns of gene expression have been identified which deviate from the "classic" Mendelian dominant/recessive patterns; these include:

• Co-dominance - Neither allele is dominant over the other, thus both are expressed. Example: In human blood types, the A and B alleles are co-dominant and the O allele is recessive. Individuals with the AB genotype are phenotypically distinct (type AB blood) from individuals with the AA or AO (type A), BB or BO (type B), and OO (type O) genotype.
  
  
• Incomplete dominance - The heterozygote is an intermediate between the homozygous phenotypes. Example: In snap dragon plants, a flower homozygous for the red allele (RR) is red in color; homozygous for the white allele (WW) results in a white flower, but a heterozygous RW results in a pink flower.
  
  
• Pleotropy - The seemingly unrelated expression of multiple phenotypic traits caused by a single gene. Example: Coat color in cats is due in part to the gene that produces melanin in melanocytes (cells). Melanocytes are also responsible for transmitting chemical signals from the ear to the brain. Thus, defects in the melanin gene produce white cats with a tendency to deafness.
  
  
• Epistasis - When one gene influences the phenotypic expression of another gene; a form of multigenic trait where several genes influence a single phenotype. Example: The black and brown coat color in Labrador retrievers is controlled by two alleles of the same gene - black is dominant. However, there is a second gene that facilitates deposition of pigment into the hairs; the recessive allele for this second gene is deficient in depositing pigment. Thus, when a dog is homozygous recessive for the second gene, no pigment is deposited into the hair, and the resulting coat is blonde. Epistatic genes can be dominant or recessive.
  
  
• Somatic Mosaicism - The effect of mutation in one cell at the two-cell stage in embryonic development in an individual. This can give rise to two different cell lines in a single individual, each of which has a different phenotype. Example: Two different color eyes in the same individual.
  
  
• Sex Linkage - The phenotypic expression of an allele is dependent on the sex of the individual and is directly tied to the sex chromosomes. Dominant sex-linked genes are expressed in all males (with a single X chromosome) and all females, even if heterozygous for the allele. Recessive sex-linked genes are expressed in all males, but only those females that are homozygous for the recessive allele. Example (sex-linked recessive): horns in sheep that appear only in males.