Your Federal Quarterly Tax Payments are due April 15th Get Help Now >>

Genetics by lifemate



The scientific study of heredity is called genetics. A trait is a characteristic that can be passed from
parent to offspring. The first significant work in genetics was done by Gregor Mendel, an Austrian
monk. He used garden peas to study how traits were passed from one generation to another. Garden
peas have some traits that are easy to see which made it easy to study the results. Several of these
traits have only two ways they are expressed, as in pea color. The color is either yellow or green, no
intermediate or mixture of the two different colors.

Mendel hypothesized that each trait was controlled by a distinct factor. Since there were two different
forms of the trait there must be at least two forms of each factor.

We now know that these factors are called genes. Genes are sections of chromosomes that code for
a trait. Most organisms have two copies of every gene and chromosome, one from each parent.

We refer to the different forms of these factors (GENES), as alleles. An allele is the distinct form of
the gene, as an example dominant or recessive. If an organism has two alleles for a trait only one of
them is usually expressed, or visible.

A dominant allele is the form of a gene that is fully expressed when two different alleles are present.
A recessive allele is the form of a gene that is not expressed when paired with a dominant allele.

 An organism that receives the same genetic traits from both parents is called a purebred. An
organism that receives that receives different forms of a genetic trait are called hybrids.

We use the uppercase and lowercase forms of the same letter to represent the alleles because they
are two versions of the same gene. As a rule, an uppercase letter represents a dominant allele, while
a lowercase letter represents a recessive gene.

The genetic makeup of an organism is called the GENOTYPE. This not necessarily what genes are
expressed but what the organism actually contains. Such as YY for two dominant alleles an Yy for a
dominant and recessive although both would look the same.

The PHENOTYPE is the actually outward appearance or how the organism behaves based on it’s
genetic structure. As in the example above both YY and Yy would exhibit the same phenotype.

An organism that has the same forms of a specific allele is called HOMOZYGOUS for that trait.
An organism that has the different forms of a specific allele is called HETEROZYGOUS for that trait.

     This law states that each pair of genes segregates, or separates during meiosis.

      This law states that gene pairs segregate into gametes randomly and independently of
each other. In other words the way one pair of genes separates has no effect on the
separation of another pair of genes (non linked genes only).

       This law states that a recessive gene is only expressed when the organism has no copy
of the corresponding dominant allele.
Since the LAW OF INDEPENDENT ASSORTMENT basically says that what gene you will receive is
the result of random chance. Probability is very useful in the study of genetic traits. It can even be
used to predict the possible outcome of the genotypes and phenotypes of specific breeding

Scientist use a tool called a PUNNETT SQUARE. It is a grid used for organizing genetic information.
A Punnett square shows probabilities and NOT ACTUAL RESULTS!! Below is an example of a
punnett’s square possible results based on the garden pea example where Y is dminant and y is
recessive. If two heterozygous yellow pea plants (the dominant trait) are crossed what is the possible
out come?

Yellow heterozygous genotype equals Yy so that :

                  Y             y            YY phenotype would be Yellow

           Y     YY            Yy             Yy phenotype would be yellow

           y      Yy           yy
                                               yy phenotype would be green

This is the normal ratio for the phenotype that is found when every a heterozygous is crossed with a
heterozygous A RATIO OF 3 (DOMINANT) : 1 (RECESSIVE)

This is what is called a monohybrid cross. Mono because it deals with just one allele, hybrid because
the alleles are not the same.

Predicting two traits (a dihybrid cross) using a punnett’s square is like tossing two coins at the same
time. Because the coins are not attached in any way the results of one toss does not affect the results
of the other (Law of independent assortment).

To predict the results of a dihybrid cross, you use a 4 X 4 Punnett square. You list the allele in the
gametes of one parent along the top of the square and the alleles in the gametes of the other parent
along the side. Then combine the alleles in the boxes.

In this example we will use the heterozygous yellow peas (Yy) and heterozygous round peas (Rr)

The genotype of this plant would be : RrYy

In order to make sure you include all of the possible gametes use the foil method as taught in
F = first           make your 4 letters into two sets of two letters RrYy = (Rr) (Yy)
O = outer
I = inner           then use these to find you gametes
L = last                   (Rr)             X                     (Yy)

                        RY           Ry          rY           ry

                       First        outer       inner         last
Now use these gametes to predict your results using a Punnett square

                                     RY        Ry      rY                 ry

                            RY       RRYY       RRYy        RrYY          RrYy

                           Ry        RRYy      RRyy         RrYy          Rryy

                           rY        RrYY      RrYy     rrYY             rrYy

                            ry       RrYy      Rryy     rrYy             rryy

The phenotype ratio of this cross is 9:3:3:1

   This means 9 round yellow peas
              3 wrinkled yellow peas
              3 round green peas
              1 wrinkled green pea


      Scientists can determine if an organism is homozygous or heterozygous dominant by doing test
crosses. In a TEST CROSS the organism that is of an unknown genotype (heterozygous or
homozygous dominant) will be crossed with a homozygous recessive organism. Based on the
predictions using a Punnett square if the unknown genotype is heterozygous about half the offspring
should be recessive. If the unknown genotype is homozygous none of the off sring could be

               If unknown is Homozygous                            If unknown is Heterozygous

                  Y         Y                                                    Y        y

           y      Yy       Yy                                                   Yy   yy

           y      Yy        Yy                                       y          Yy    yy

         Note: No recessive                                        Note: half recessive

To top