The Moro reflex occurs in infants up to 6 months old. This reflex occurs when infants feel like they are falling, and specifically involves the infant spreading and unspreading his arms while crying out for help. 

Out of these three units, genes are the building blocks and the smallest unit. Each gene codes for one or more proteins, though only one protein will be produced from each gene transcript. Genes are comprised of DNA sequences. A single strand of DNA will carry the code for numerous genes. When DNA is condensed and wrapped around histone proteins, it creates the structure of a chromosome. Chromosomes are formed from tightly coiled DNA strands, as well as protein components.

Chromosomes are composed of DNA, which is composed of genes. As such, genes are the least complex element, followed by DNA and chromosomes.

Genes frequently come in different varieties, known as alleles. Certain alleles can be dominant to others, such that the presence of a dominant allele masks the presence of a recessive allele. Every time a dominant allele is present in an individual's genome, that individual will display the trait coded by the dominant form of the gene.

Note that there is no such thing as a "dominant gene" or "recessive gene." Take, for example, the gene that codes for eye color. One does not say that the gene for eye color is dominant or recessive; this suggests that presence of a "recessive eye color gene" would result in no eye color whatsoever. In contrast, one can say that the allele for blue eyes is recessive to the allele for brown eyes, such that an individual with both forms of the allele will always display brown eyes. Note that eye color is, in reality, far more complex that this explanation would suggest.

The Moro reflex is an involuntary type of reflex (response to stimulation) during which a baby will respond to loss of support by spreading out his/her arms (abduction), unspreading the arms (adduction), then usually crying. Another common name for this reaction is the startle reflex.

Babies tend to develop in a pattern called cephalocaudal development. This means that the first areas to develop are the head and neck, and the last are the legs and feet. We also develop smell and taste in the womb, then hearing, then finally sight.

Teratogens are chemicals or viruses that can reach the embryo or fetus during prenatal development and cause great harm. Moderate to high amounts of alcohol consumed by the mother may cause problems for the developing fetus; thus it is an example of a teratogen. 

Fetal alcohol syndrome (FAS) is a collection of cognitive abnormalities in young children caused by a mother's heavy drinking during pregnancy. In severe cases of FAS, there may be severe physical deficiencies as well.

The rooting reflex is an infant's tendency to turn his/her head toward something that touches his/her cheek. This allows the baby to find the nipple during breastfeeding.

Menarche, a girl's first menstrual period, is often viewed as the biological marker indicating the beginning of adolescence. 

After the conception of a child, the following two weeks are known as the Germinal Period. This is followed by the Embryonic Period, which lasts another 2-8 weeks. After the Embryonic Period is complete, the Fetal Period occurs, and the child is born at its conclusion.