Physical development is comprised of various stages that begin at conception and end at death. Conception occurs when the sperm meets the egg and creates a zygote. The genetics of a child are determined by the hereditary material in the DNA of the parents. The baby receives sets of chromosomes from each parent that determines the traits the child receives. There are two combinations of genes known as the genotype and the phenotype, which vary because of alleles. There are dominant and recessive alleles, dominant being the stronger ones and recessive the weaker, and the combination of the two determine which traits the child will receive. The stage of prenatal development that begins at conception and lasts through the second week of pregnancy is known as the germinal stage. During this stage, a process known as fertilization, in which two gametes join together, occurs. Shortly thereafter, the blastocyst is formed and implanted in the uterus. The blastocyst is made up of three layers known as the ectoderm, mesoderm, and endoderm. Ectoderm is the future nervous system and skin, the mesoderm is the future skeleton and muscles, and the endoderm is the future respiratory and digestive system. The next stage of prenatal development is known as the embryonic stage and it occurs from the third to the ninth week of pregnancy. This stage is known as the embryonic stage because the blastocyst becomes an embryo. Two hundred and thirty different cell types are formed in a process known as differentiation. This period in the womb is associated with the start of the heart beat and the formation of arms and legs. It is also the most vulnerable stage of prenatal development. The final stage of prenatal development is known as the fetal stage and it occurs from the ninth week of pregnancy up until birth. It is known as the fetal stage because the embryo becomes a fetus. The most amount of growth occurs during this stage from the development of bones, body parts, hair, nails, sex organs, and brain and nervous system. Once the child is born he or she is known as a neonate throughout their first month of life. It is important for the parents to bond with their child upon his or her birth in order to create a sense of security and attachment. There are two types of infant development known as cephalocaudal, which starts at the head and moves down the body, and proximodistal, which starts at the center of the body and moves outward. Infants experience development of the senses as well as the motor skills. The development of the gross and fine motor skills allow the child to reach various milestones throughout infancy. Childhood development consists of neural growth, somatic growth, and reproductive growth. Neural growth deals with the development of the brain, somatic growth with developments in height and weight, and reproductive growth with the development of the reproductive organs and the beginning of puberty. Adolescence is known as the stage in which puberty occurs. Puberty begins when a part of the brain known as the hypothalamus releases growth hormones, which causes a growth spurt as well as sexual development and the whole process lasts between three and four years. These growth spurts can cause body image and self esteem issues, which can result in eating disorders, drug use, and mental issues. Once a child is fully grown, the process of aging begins, in which the body begins to decline in aspects such as the brain, bone, skin, senses, and lungs.