Prenatal Development PDF

. ··--- ·--·--- - ---, - zygote separated into two clusters of cells, and two individuals with the...

. ··--- ·--·--- - ---, - zygote separated into two clusters of cells, and two individuals with the same genetic makeup developed. © Laura Dwight Photography Patterns of Genetic Inheritance (T~o forms of each gene occur at the same place on the chromosomes, one inherited from the finother and one from the father. Each form of a gene is called an allele. If the alleles fi:om both parents are alike, the child is homozygous-and will display the inherited trait. If-the alle_l~ differ, then the child is heterozygous, and relationships between the alleles determine Uhe trait that will appear. Dominant-Rec_es~ive Relationships In many heterozygous parrmgs, dominant-. r,ecessive inheritance occurs: Only one allele affects the child's characteristics. It is called dominant; the second allele, which has n effect, is called recessive. Hair color is an example. The allele for dark hair is dominant (we can represent it with a capital D), whereas the one for blond hair is recessive (symbolized by a lowercase b). A child who inherits a homozygous pair of dominant alleles (DD) and a child who inherits a heterozygous pair (Db) will both be dark-haired, even though their genotypes differ. Blond hair can result only from having two recess·ive alleles (bb). Still, heterozygous individuals with just one recessive allele (Db) can pass that trait to their children. Therefore, they are called carriers of the trait..... , ,,. factors Linked to Fraternal Maternal Twinning , ~ Ethnicity ~.. , 9ccyrs in 4 per 1,000 births among Asians, 8 per.1,000 births among whites, 12 to 16 per l ,000 births among blacks '. ~ Family :history of twlnnJng D