Introduction to Genetics

 

Terms you must know:

gene:  section of DNA that codes for one protein

trait:  characteristic that can vary from one organism to another  (ex:  eye color)

diploid:  term used to describes cells with two copies of every gene  (this means one copy of the gene comes from you mother, the other from your father)

allele:  one of many different forms of a gene (you could get a blue allele for the eye color gene from your mother and a brown allele for eye color gene from your father)

 

homozygous:  an individual who has two of the same alleles for a gene (a pea plant could have two tall alleles (TT))

heterozygous:  an individual who has two different alleles for a gene (a pea plant could have one tall allele and one short allele (Tt))

body cells:  non sex cells (skin cells, muscles cells, nerve cells, fat cells)

gametes:  sex cells;  these cells contain ½ of the genetic material of body cells (sperm or egg) 

independent assortment:  individual separation of genes during the formation of gametes  (An individual plant with one tall allele and one short allele for height, and one round allele and one wrinkled allele could form four different types of sperm:  1)  tall. round carrying sperm  2)  tall wrinkled carrying sperm  3)  short, round carrying sperm  4)  short, wrinkled carrying sperm.  Each of these types would make up ¼ of the plants sperm.)

dominant:  an allele that would cover up a recessive allele

recessive:  an allele that is covered up by a dominant allele (an individual would has to have two recessive alleles for a recessive trait to be expressed)

genotype:  the genetic makeup of an individual (the plant was heterozygous tall (Tt))

phenotype:  the physical appearance of a characteristic (the plant was homozygous green (GG)

 

 

  The body cells of all the organisms we will study during the genetics unit, including humans, are diploid.  One copy of every gene came from you mother, the other copy came from your father.  Your gametes, sperm or egg, only have one copy of every gene.

 

You must learn how to represent alleles with letters so that you can work genetics problems.

 

For example, lets look at pea plant height.  The allele for tall plants is dominant over the allele for short plants. 

Since the dominant allele is tall, this trait will be represented by the letter “t”.  The first letter of the name of the dominant allele is usually used to represent a trait.

The dominant allele is represented by a capital letter.

In our example, the allele for tall plants would be represented by a capital t- “T”.

The recessive allele is represented by a lower case letter.

In our example, the allele for short plants would be represented by a lower case t-  “t”.

 

An individual who has two tall alleles would have the genotype  “TT”.   This individual would be called homozygous tall.  Its phenotype, or physical appearance, would be tall.

 

An individual who has one tall allele and one short allele would have the genotype  “Tt”.  This individual would be called heterozygous tall.  Its phenotype, or physical appearance, would be tall.

 

An individual who has two short alleles would have the genotype “tt”.  This individual would be called short (or homozygous short).  Its phenotype would be short.