Patterns of Inheritance
So far, we've looked at traits that are inherited in a strictly dominant/recessive mode. Aside from the magic six and laws of probabilities, the most important thing to remember about dominant/recessive mode of inheritance is
1. It is sufficient to possess one dominant allele to show the dominant trait.
2. It is necessary to possess two copies of the recessive allele to show the recessive trait.
These two principles will allow you to move forward and backward through a pedigree or problem and not get hung up on numbers.
Test-cross
Since one dominant allele is sufficient to display a dominant phenotype, how can we determine the genotype of a specimen showing a dominant trait? Another principle comes into play here.
A recessive phenotype is a known genotype.
A recessive phenotype must have two copies of the recessive allele, so we automatically know its genotype. This is used in the determination of the genotype of dominant characters in plants, animals, and microbes.
Example: Cross a pea plant producing purple flowers (P_) with a plant producing white flowers (pp).
Case 1. The purple parent is homozygous (PP). When crossed with the white parent, we would not expect to see any white flowers. If ten F1 plants mature and produce purple plants only, this may not be enough to confirm the genotype of the purple parent. With plants, it's better to self-fertilize to determine the genotype. With animals, it takes a great deal of work to establish that the animal in question is indeed true-breeding.
Case 2: The purple parent is heterozygous (Pp). If any white flowers are produced in the F1 generation, we must assume that the purple parent is heterozygous. Numbers are not important to establish the genotype of the parent as long as the recessive trait appears in the F1 generation.
Incomplete dominance
In this mode of inheritance, we see a phenotype in the heterozygote that does not correspond to either of the two parental phenotypes. This does not mean that the alleles are blended, only that the traits appear to be blended. The F1 hybrids will show a phenotype that is somewhere in between the two parental varieties.
When red snapdragons are crossed with white snapdragons, the F1 hybrids produce pink flowers. When the F1 hybrids are crossed, the F2 generation will exhibit a 1:2:1 ratio for both genotype and phenotype. 25% will be homozygous red, 25% will be homozygous white, and 50% will be heterozygous pink.
CAUTION: Different authors use different legends when establishing incomplete dominance,
some will use RR for red and R'R' (read as "r-prime,r-prime') for white. Others, such as Campbell will use a capital letter to denote the character (C for colour) and superscripts for the traits: CwCw for white and CRCR for red (imagine the W and R are superscripted!). Still others will use different letters (but this should really be reserved for codominance)
Try these (Hint the genotypic ratio of a dihybrid cross - you are NOT responsible for this - is 1:2:1:2:4:2:1:2:1)
Codominance
Codominance is similar to incomplete dominance in that there are three phenotypes. The difference lies in the appearance of the heterozygote, and this is where many students confuse the two. Incomplete dominance means that neither homozygote phenotype is able to mask the other. Codominance means that the two homozygote phenotypes appear together in the heterozygote. The prefix "co" means together like co-operate, cohabit, coexist...
Roan cattle are the offspring of red (RR) and white (WW) parents. They're not pink! Their fur shows red and white together, never blended, never intermediate. We'll see codominance again when we look at the inheritance of blood types in humans.
Sex-linked inheritance
In mammals, sex is determined by two chromosomes. The mother passes on one X chromosome to each of her children; the father passes on an X chromosome to his daughters and a Y chromosome to his sons.