Final answer:
Inheritance of jaw shapes like a cleft or smooth chin depends on the genotypes of the parents. The evolution of australopithecines reveals different jaw shapes adapted to their diets. Human jaws develop and grow during childhood, and jaw shapes may also play a role in sexual selection through the good genes hypothesis.
Step-by-step explanation:
Jaw shapes and their characteristics can be explained through genetic factors and evolutionary adaptations. For instance, if one parent has a cleft chin, which is a dominant trait, and the other has a smooth chin, a recessive trait, the inheritance of these features by their offspring depends on the alleles carried by the parents. If the cleft chin parent is homozygous dominant (BB), the offspring will inherit a cleft chin. If they are heterozygous (Bb), there is a 50% chance for each offspring to have either a cleft or smooth chin.
Furthermore, the evolution of gracile and robust australopithecines showcases varied jaw shapes and teeth, associated with dietary habits. Gracile forms had smaller teeth and less protruding jaws, while robust australopithecines had larger teeth and jaws with a sagittal crest, indicating stronger jaw muscles for chewing tougher plant materials. The development of jaw shapes in hominids can also reflect diet and environmental adaptations.
The human skull at birth shows disproportionally large orbits and brain case compared to the underdeveloped jaws. During childhood, the maxilla and mandible grow in size as teeth appear, contributing to the growth of the face. The mandible forms the lower jaw and becomes the single movable bone in the adult skull after fusing during early childhood.
In terms of sexual selection, features such as jaw shapes may be influenced by the good genes hypothesis, where males develop traits that signal genetic superiority to attract mates, leading to healthier offspring. This selection process impacts the evolution of physical characteristics including those of the jaw.