smoking, poor diet, and lack of physical exercise that may
affect the expression of those genes in negative ways.
What is the chemical composition of our genetic instructions?
Explain how the human genome encodes more proteins than it
MODES OF INHERITANCE
Genetics has the power of prediction. Knowing how genes
are distributed in meiosis and the combinations in which
they can join at fertilization makes it possible to calculate
the probability that a certain trait will appear in the offspring
of two particular individuals. The patterns in which genes
are transmitted in families are termed
modes of inheritance.
Chromosomes and Genes Come in Pairs
The basis of being able to make genetic predictions is the way
in which genes are carried as parts of chromosomes. From the
moment of conception, a human cell is diploid. Chromosome
display the 23 chromosome pairs
in size order
. Pairs 1 through 22 are
¯mz), chromosomes that do not carry genes that
Until recently, the ± eld of medical genetics dealt mostly
with rare disorders that can be traced to the malfunction
or absence of single genes. However, information from the
human genome sequence is providing a new view of physi-
ology as a complex interplay of gene functions.
Looking at the human body in terms of multiple, interacting genes
A related field, proteomics, focuses on the
proteins that speciF
c cell types produce. A proteomics approach to
studying the function of the breast as a gland, for example, compares
the proteins in a healthy epithelial cell in a milk duct of a woman who
is not pregnant or breastfeeding to the proteins in the same type of
cell from a new mother who is breastfeeding.
As important as genes are, they do not determine who
we are, nor do they act alone. The environment inﬂ uences
how genes are expressed—under which circumstances they
are transcribed into mRNA and those mRNA molecules trans-
lated into protein. The environment includes the chemical,
physical, social and biological factors surrounding an indi-
vidual. A common medical condition may result from several
inherited risk factors and exposure to certain environmental
uences. For example, development of cardiovascular dis-
ease may reﬂ ect inheritance of speci± c variants of speci± c
genes that control blood pressure, blood clotting, and choles-
terol metabolism, but also lifestyle inﬂ
uences such as stress,
±rom DNA to gene to chromosome. Chromosomes consist of a continuous DNA double helix and associated proteins.
Chromosomes condense to become visible under a microscope just prior to cell division.