Non-Mendelian inheritance review

Key terms

Variations involving single genes

• Incomplete dominance. Two alleles may produce an intermediate phenotype when both are present, rather than one fully determining the phenotype.

  Diagram of a cross between $C^WC^W$C, start superscript, W, end superscript, C, start superscript, W, end superscript (red) and $C^RC^R$C, start superscript, R, end superscript, C, start superscript, R, end superscript (white) snapdragon plants. The F1 plants are pink and of genotype $C^RC^W$C, start superscript, R, end superscript, C, start superscript, W, end superscript.

  An example of this is the snapdragon plant. A cross between a homozygous white-flowered plant ($C^WC^W$C, start superscript, W, end superscript, C, start superscript, W, end superscript) and a homozygous red-flowered plant ($C^RC^R$C, start superscript, R, end superscript, C, start superscript, R, end superscript) will produce offspring with pink flowers ($C^RC^W$C, start superscript, R, end superscript, C, start superscript, W, end superscript).
• Codominance. Two alleles may be simultaneously expressed when both are present, rather than one fully determining the phenotype.

• Multiple alleles. Mendel studied just two alleles of his pea genes, but real populations often have multiple alleles of a given gene.

  Allelic series of the color gene C in rabbits.

  • A $CC$C, C rabbit has black fur.
  • A $c^{ch}$c, start superscript, c, h, end superscript$c^{ch}$c, start superscript, c, h, end superscript rabbit has chinchilla coloration (grayish fur).
  • A $c^hc^h$c, start superscript, h, end superscript, c, start superscript, h, end superscript rabbit has Himalayan (color-point) patterning, with a white body and dark extremities.
  • A $cc$c, c rabbit is albino, with a pure white coat.

  An example of this is the gene for coat color in rabbits (the $C$C gene) which comes in four common alleles: $C$C, $c^{ch}$c, start superscript, c, h, end superscript, $c^h$c, start superscript, h, end superscript, and $c$c.
• Pleiotropy. Some genes affect many different characteristics, not just a single characteristic.

  Simple schematic illustrating pleiotropy.

  In pleiotropy, one gene affects multiple features (feature 1, feature 2, feature 3.

  Caption: One gene affects multiple characteristics.

  An example of this is Marfan syndrome, which results in several symptoms (unusually tall height, thin fingers and toes, lens dislocation, and heart problems). These symptoms don’t seem directly related, but as it turns out, they can all be traced back to the mutation of a single gene.
• Lethal alleles. Some genes have alleles that prevent survival when homozygous or heterozygous.

  Two yellow mice ($A^YA$A, start superscript, Y, end superscript, A genotype) are crossed to one another. The Punnett square for the cross is:

  		$A^Y$A, start superscript, Y, end superscript	$A$A
  $A^Y$A, start superscript, Y, end superscript		$A^Y$A, start superscript, Y, end superscript$A^Y$A, start superscript, Y, end superscript (dies as embryo)	$A^Y$A, start superscript, Y, end superscript$A$A (yellow)
  $A$A		$A^Y$A, start superscript, Y, end superscript$A$A (yellow)	$A$A$A$A (agouti/brown)

  There is a phenotypic ratio of 2:1 yellow:brown among the mice that survive to birth.

  A classic example of an allele that affects survival is the lethal yellow allele, a spontaneous mutation in mice that makes their coats yellow. Mice that are homozygous ($A^YA^Y$A, start superscript, Y, end superscript, A, start superscript, Y, end superscript) genotype die early in development. Although this particular allele is dominant, lethal alleles can be dominant or recessive, and can be expressed in homozygous or heterozygous conditions.

Polygenic inheritance and environmental effects

• Polygenic inheritance. Some characteristics are polygenic, meaning that they’re controlled by a number of different genes. In polygenic inheritance, traits often form a phenotypic spectrum rather than falling into clear-cut categories.

  Human skin color chart showing spectrum of skin pigmentation

  An example of this is skin pigmentation in humans, which is controlled by several different genes.
• Environmental effects. Most real-world characteristics are determined not just by genotype, but also by environmental factors that influence how genotype is translated into phenotype.

  Blue and pink hydrangea flowers

  An example of this is the hydrangea flower. Hydrangea of the same genetic variety may vary in color from blue to pink depending on the pH of the soil they are in.

Common mistakes and misconceptions

• Some people confuse pleiotropy and polygenic inheritance. The major difference between the two is that pleiotropy is when one gene affects multiple characteristics (e.g. Marfan syndrome) and polygenic inheritance is when one trait is controlled by multiple genes (e.g. skin pigmentation).
• Codominance and incomplete dominance are not the same. In codominance, neither allele is dominant over the other, so both will be expressed equally in the heterozygote. In incomplete dominance, there is an intermediate heterozygote (such as a pink flower when the parents' phenotypes are red and white).