Why are cells small?
Surface to volume ratio
Diffusion of wastes and nutrients.
Chromosomes
The DNA molecule exists for most of the cell cycle as chromatin , a long, uncoiled single strand.
Chromatin will coil around proteins called histones to form chromosomes .
Chromosomes are packages of DNA which can be transferred to new daughter cells when the cell reproduces through mitosis .

Chromosome Structure
Homologous chromosomes - contrasting chromosomes of same type, one from each parent.
Chromatid - one half of the "X" chromosome shape. The two halves are sister chromatids and exact copies of each other. They will go to separate daughter cells during meiosis.
Centromere - protein band that joins the two sister chromatids.

Human body cells contain 46 chromosomes
23 pairs
22 pairs of autosomes
1 pair of sex chromosomes
X chromosome from mom’s egg
X or Y chromosome from dad’s sperm
= XY, = XX
Sex-linked traits have a loci (singular locus) on the X chromosome, so that only one recessive allele is required for expression in males, since males have only one X chromosome.

Somatic (body) cells are diploid (2n) - two sets of homologous chromosomes, one from mom, one from dad.
Sex cells ( gametes ) are haploid (n) - one set, no homologies
Sex cells are formed through the process of meiosis .
Multiple allele traits are those traits which are governed by more than one single allele
(or gene)

A karyotype is a print of the chromosomes of a cell. It is a diagnostic tool used to evaluate the total # of chromosomes in the cell.
Nondisjunction - the failure of two homologies to separate during meiosis, resulting in a condition known as trisomy
(three bodies) =
2n +1 = 47 chromosomes in humans.
Down’s syndrome (mongolism) is trisomy
21, of the 21st chromosome pair.
Aneuploidy - the condition of missing a chromosome due to nondisjunction:
2n - 1 = 45 chromosomes.
Turner's syndrome X__