Title says it all. I feel like this is something I should know. I've googled it, but seems like all the results assume the reader has a basic understanding of the topic. Which I lack in. Lay-mans terms if you please
Very good question. As I'm sure you know, the heritable traits of an individual are passed in their genes. The DNA from the mother is a randomized mix from the maternal grandparents; the father passes a randomized mix of his parent's genes. By this process a trait not seen in generations, such as blue eyes, may appear in family that recalls no blue eyed relatives. In this case the gene for blue eyes is said to be recessive, since it passed for generations without expressing itself. Brown eyes is the dominant gene for humans, and the other shades are recessive.
The central idea to dominance and recessivity is this: in this system, alleles do not act independently. Thus, in a system in which the A allele is dominant at a gene (which has, in a diploid genome, two alleles), any individual has the phenotype. So let's say allele 'A' confers brown eyes and that it is dominant. Any individual inheriting an 'A' allele then will then have brown eyes. Let's say further that 'a' confers blue eyes - when not overridden by an 'A' allele. Any individual who then inherits two 'a' alleles at this same gene will then have blue eyes. It is possible to carry alleles for blue eyes yet have or express brown eyes. Example Let's say that you have two parents (most people do) and then that each of them have brown (brown being dominant in this case) eyes, but have one blue -eyed child! How can this be? Simple: They have brown eyes, which are dominant, so you know that each of them has at least one A allele. You can express their genotype (the alleles they have at a gene) as A_. The _ indicates that you don't actually know the second allele. But you can tell from their progeny using a simple Punett's Square: Dad............. A _ Mom A ............. AA _A _ .............. A_ aa The father is along the top, the mother along the left-hand side. Both are scored A_. The four genotypes in the square (AA, A_, _A, aa) are ordered according to the alleles they can pass on. BUT they have a blue-eyed son, so his genotype must be aa. (He's at lower right.) Thus, each of the brown-eyed parents is carrying an 'a' allele also. Thus, their genotype must be Aa. Both are heterozygotes. If all their children were brown-eyed, then all of them must carry an A allele and you could never be sure about the genotype of the parents. Both might be AA (only able to give a dominant brown phenotype), or one might be AA (only able to give a dominant brown phenotype) and the other could be whatever, since brown is dominant. Hope that helps.
Genes come in pairs (one supplied by mom and one supplied by dad). Variants of a gene are called alleles. If at least one of the alleles is dominant it is expressed in the phenotype (it is dominant). Phenotype is how alleles are expressed. So recessive alleles are only expressed when both are recessive. Allele A (dominant) -> brown eyes Alelle a (recessive) -> blue eyes (or really just any other color than brown) combination AA -> phenotype brown eyes combination Aa -> phenotype brown eyes combination aA -> phenotype brown eyes combination aa -> phenotype blue eyes Example 1. parent 1, genotype AA (homozygous genotype), brown eyes (phenotype) parent 2, genotype Aa (heterozygous genotype), brown eyes (phenotype) Posible combinations: AA, Aa, AA, Aa So in this case there 100% chance that the offspring will have brown eyes. Example 2. parent 3, genotype Aa, brown eyes parent 4, genotype aa, blue eyes Possible combinations: Aa, Aa, aa, aa In this case the is 50% chance of brown eyed offspring and 50% chance of blue eyed offspring.
Genes come in pairs. If a pair contains two different genes, typically one of them is dominant and the other is recessive. This means that the characteristic or trait supplied by the dominant gene will be manifested. The only way the characteristic or trait supplied by a recessive gene can be manifested is for both of the genes to be the recessive one. As has been noted, in humans the gene for brown eyes is dominant whereas the gene for blue eyes is recessive. This means that only people with two blue-eye genes will have blue eyes, and the rest will have brown, regardless of whether they have one or two brown eye genes. (I'm simplifying; obviously there are other genes for green eyes, etc.) This means that most people will have brown eyes, even if quite a few of us might have one blue eye gene. The same is true of hair. Brown hair is dominant, blond is recessive. So people with only one blond hair gene will have brown hair, even if there are a whole bunch of them. Again, blond hair is unusual in most human populations, regardless of how many members have one copy of the gene. If the dominant gene is a useful one, the prevalence of the trait it programs will be beneficial to the species. If it is not useful, it might endanger the species. At the very least, all individuals with that gene--either one or two--are likely to die off, and the species will be left with only the individuals with a paired-up recessive gene. This is an unstable equilibrium, of course. Over the generations, a mutation may occur, resulting in the emergence of a new gene, which happens to be dominant. As the organisms breed with each other, this new gene will spread throughout the population. If it happens to manifest a dangerous trait (for example, bright red fur on an arctic rodent so all the predators can spot it easily), it could result in the species dying off.
What would happen is that individuals with that dominant gene would not live to reproduce, and not be able to transmit it to future generations.
