Biology 30 Unit III- Genetics
The cells in your body can be divided up into two groups:
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Somatic- these cells make up most of your body tissues and organs.
Gametes- these are the sex cells; they are the cells in the reproductive organs. In females the
germ cells are the ova, or eggs. In males the gametes are the spermatozoa, or sperm cells.
Each cell will carry genetic information via DNA (Deoxyribonucleic acid). Genetic information is found in
cells in the form of chromosomes. Chromosomes are thread-like structures made up of DNA and
proteins.
Genetics is the study of inheritance patterns and variation in organisms.
Traits are distinguishing characteristics that are inherited, i.e. eye colour,
leaf shape, and tail shape. Scientists knew traits were inherited long before
they knew how traits were passed on.
Each of the body cells in the human body contains 46 chromosomes and 23 pairs. Each pair is referred to
as a homologous pair. Homologous chromosomes are two chromosomes in which one is inherited from
the mother and the other is from the father. Homologous chromosomes will have the same appearance
and length, as well as the same genes.
Chromosome pairs 1 to 22 are the autosomal chromosomes, while pair 23 is the sex chromosomes. The
sex of an organism is determined by the XY system. If a human has two X chromosomes then they are a
female. Human males have an X and Y chromosome.
Sexual reproduction involves the fusion of two gametes that results in offspring that are a genetic
mixture of both parents.
Fertilization is the fusion of an egg and sperm. The nuclei of the egg and sperm fuse together to form
one nucleus. The egg and sperm contain HALF the number of chromosomes. Therefore when the egg
and sperm fuse, they will eventually form an organism with the full 23 pairs of chromosomes.
Body cells are diploid: They contain two copies of each chromosome; one from the mother and one
from the father. In humans, the diploid number is 46.
Gametes are haploid: They contain only one copy of each chromosome. Each egg or sperm cell contains
22 autosomal chromosomes and one sex chromosome.
Mendel
Mendel’s units of heredity are now called genes. A gene provides the cell with a
set of instructions to make specific proteins. Each gene will have a specific
locus; a specific location on a chromosome. Think of a locus as the gene’s
address; it tells where a gene is located on a chromosome.
An allele is any of the alternative forms of a gene that may occur at a specific
locus. Each cell has two alleles for each gene. Homozygous alleles are two
alleles that are the same at one locus.; i.e. they both code for blue eyes.
Heterozygous alleles are two alleles that are different at a given locus; i.e. one codes for white flowers
while the other codes for pink flowers.
Genotype: is the genetic makeup of a specific set of genes.
Phenotype: is the physical makeup, or characteristics of an organism.
A dominant allele is expressed when two different alleles or two dominant alleles are present. A
recessive allele is expressed only when two are present.
A Punnett Square is a grid system that is used to predict the
genotypes of an organism.
One allele from each parent is written in one grid box. Each possible genetic combination is equally likely
to occur.
Monohybrid Crosses
Monohybrid crosses examine only one trait.