If you have a litter of kittens that includes 2 long haired, 2 tailless and 4 short haired, how can this occur?
The presence of certain traits in offspring depends on the genotype of the parents and whether the trait is dominant or recessive. A dominant trait is one which is expressed when only one copy of the gene is present. Thus, if B represents a dominant gene, the corresponding trait will be expressed in individuals which have two copies (BB) and those who have only one copy (Bb). When the individual has zero copies of the dominant gene (bb), the recessive trait will be observed.
In cats, both short hair (A) and tailless are dominant traits. Long hair and tailed are the corresponding recessive traits, respectively. However, the tailless gene is lethal when in the homozygous form. Homozygous means both copies are present (MM).
In this example, at least one of the parents must have been tailless (Mm), because two tailed (mm) parents cannot produce tailless offspring. See the possible three cases below.
1. Two tailed parents: mm x mm
Possible offspring: mm mm mm mm (100% tailed)
2. One tailed parent x one tailless: mm x Mm
Possible offspring: mM mm mM mm (50% tailed, 50% tailless)
3. Two tailless parents: Mm x Mm
Possible offspring: MM Mm mM mm (25% lethal, 50% tailless, 25% tailed)
With respect to coat length, these are the possible cases:
1. Both parents long-haired (aa x aa)
Possible offspring: aa aa aa aa (100% long haired)
2. Both parents short haired homozygous (AA x AA)
Possible offspring: AA AA AA AA (100% short haired)
3. Both parents short haired heterozygous (Aa x Aa)
Possible offspring: AA Aa Aa aa (75% short haired, 25% long haired)
4. One long haired, one short haired heterozygous (aa x Aa)
Possible offspring: aA aa aA aa (50% short haired, 50% long haired)
5. One long haired, one short haired homozygous (aa x AA)
Possible offspring: aA aA aA aA (100% short haired)