Genetics - Beyond Mendel - Part 1

Key Ideas

Circumstances that influence ratios between phenotypes

I. Dominance Relationships : The various ways in which 2 alleles at one gene locus can effect the phenotype.

When the information is in conflict, the recessive allele is usually ignored, and the dominant allele is expressed.

How one gene can dominate another--

Recessivity may mean that a gene is simply not functioning. An example of this is albinism, in which a recessive cannot fulfill its role in producing the pigment melanin. Two alleles failing to produce pigment results in an albino individual.

recessive alleles may be functioning, but to a lesser degree than dominant genes.

In some instances, dominance is apparent visually, but on closer examination or measurement there is a difference between having one or two genes functioning normally. In fruit flies, for example, there may be less pigment produced by one gene than by two.

Some dominant alleles are also known as inhibitor alleles, since they prevent recessive alleles from expressing themselves. The dominant white allele of white leghorn chicken is an example. This allele prevents the expression of colored feathers.

Partial dominance (also known as incomplete dominance) in snapdragons is seen when red and white are crossed. The offspring are pink -- a blending of the two traits. Phenotypic and genotypic ratios are the same ( : : ) This is a diagnostic feature of partial dominance.

Codominance occurs when one homozygote expresses a trait differently from the other homozygote, but the heterozygote shows both traits. It is common among genetic traits that can only be measured by biolchemical tests - See blood groups

There are three ways lethality can be expressed in the genome
- Partial dominants with a lethal allele survive but are different than the homozygous normal individual. A good example is seen in achondroplasia or achondroplastic dwarfism. Homozygous offspring die as embryos, while heterozygotes survive. For lethality to be expressed, two abnormal alleles must be present.
- In the recessive lethal condition homozygous individuals can not survive. A good example is the white seedling allele of maize (corn) -- no chlorophyll -- no survivability.
- Dominant lethals can exist but only when the condition is expressed after the organism has reached sexual maturity. The example in man is Huntington's chorea.

The blood groups M and N are an example of codominance. When tested with antisera, agglutinations show that each genotype can be biochemically identified. Both M and N are clearly expressed.