wk4 Mendel CATGEN Student

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Genetics is the study of the heredity and variation in specific populations of organisms where you know
the genetic makeup of every organism in the population, and can carefully control breeding experiments.
Gregor Mendel was the first person to formulate the basic “laws” of genetics that are still in effect today.
For this reason, the study of genetics is usually referred to as Mendelian genetics. The following are some
of the terms associated with the study of genes and heredity that are necessary for you to learn to better
understand the genetic process.

A GENE (sometimes called a cistron) is a section of DNA that codes for a single polypeptide.

An ALLELE is one version of that DNA section. There is often more than one version of the same DNA
section. Alleles can be dominant (those that are expressed) or recessive (those that can be masked).

GENOTYPE is the gene makeup of an individual. Remember, every individual has two copies of every
chromosome (one from each parent), so they have two copies of each gene.

PHENOTYPE    is the expressed characteristic of a gene that can be measured in some manner by sight,
sound, touch, taste, or chemical testing, etc.

HOMOZYGOUS refers to a genotype where both alleles are identical for a single trait. For example, AA
(homozygous dominant) or aa (homozygous recessive).

HETEROZYGOUS      refers to a genotype in which two alleles are different for a single trait (Aa).

EPISTASIS refers to the interaction of two different genes, where one gene will interfere or modify the
phenotypic expression of the other gene. For instance, if a cat has the genotype to produce black fur,
another “epistatic” gene can interfere with that expression and cause the cat to be white.

CATGEN—A Genetic Model

Understanding how genes are passed from generation to generation in a controlled population greatly
enhances our ability to understand how gene inheritance works in natural populations.

The genetic model used in CATGEN is based on a current understanding of the action and interaction of
genes that control coat pattern and color in domestic cats. The genes are those that are typical for Siamese
cats. All of these genes are on autosomes, except the O gene, which is located on the X chromosome.

CATGEN allows you to mate cats of known genotype and observe the genotypes and phenotypes of the
kittens that result from those matings. The program is very easy to use; menu screens guide you through
the exercises. You will need to keep careful records of the genotypes and phenotypes of the parents and
their kittens.

In most genetic crosses, letters are used to indicate a particular gene. A capital letter indicates a dominant
allele, while a lower case letter indicates a recessive allele. The following is a list of the genes that you
will be using for the CATGEN model.


Alleles for Coat Color in CATGEN Exercise:
Six genes influence the color and pattern of a cat’s fur (W, C, X, A, B, D). Use this key to determine the
coat color of your cat. Start with the “W gene” and work your way down to a bold box. The description of
coat color in the bold box is your cat’s color scheme.


                                    W gene

                       W__                           ww

                     All white                  not all white

                          C__                                                             cc

                       Full color                                                      Siamese

                                                                           A__                       aa
   XoX+            XoXo or XoY           X+X+ or X+Y                 Tabby/lynx points            No points
  Calico            Orange/Red          Not orange/red                      B__                      bb
                                                                        Black points             Brown points

                                                                           D__                        dd
                                                                        Blue points              Lilac points
                    A__                    aa
                Tabby stripes           No stripes

                     B__                    bb
                    Black                 Brown

                     D__                   dd
                    Dense                 Dilute


Monohybrid Cross

SET UP the following cross for a typical cat (not Siamese):

Parent cats:               Female Cat           x        Male Cat

genotype:                      Bb                             Bb


What kinds of gametes (eggs) can the mother produce? (List all possibilities.)

What kinds of gametes (sperm) can the father produce? (List all possibilities.)

PRODUCE several litters of kittens from these parent cats until you have at least 40 kittens and RECORD
the phenotypes for each litter in the following table. NOTE what possible genotypes result in each
       Litter Size           No. of Black Kittens      No. of Brown Kittens       Ratio of Black:Brown

Genotypes possible in black kittens:

Genotypes possible in brown kittens:

This type of cross is a called MONOHYBRID CROSS. If mono means “one,” what does “hybrid” mean?

What would happen if one of the parents in the above cross had the genotype BB?


Test Cross

A test cross is done when you want to find out the genotype of an organism (a cat, in this case) with a
dominant phenotype. You know that the cat will be either BB or Bb, but to determine which genotype the
cat has you can cross the cat with one whose genotype is known—one with the recessive phenotype can
only have a homozygous recessive genotype (bb).

Cross the following parent cats:
                             Female             x           Male

genotype:                      Bb                            bb


How is a test cross different from a monohybrid cross?

PRODUCE several litters of kittens with these parents until you have at least 40 offspring and RECORD
the phenotypes in the table below. NOTE the sexes of the kittens.

       Litter Size           No. of Black Kittens        No. of Brown Kittens    Ratio of Black:Brown

Are there black kittens of both sexes?

Are there brown kittens of both sexes?

How does the ratio between the different colors of kittens differ from those that resulted from the mono-
hybrid cross?


Dihybrid Cross

If a monohybrid cross is a cross between two organisms that are heterozygous for one gene, what does a
“dihybrid” cross mean?

SET UP the following dihybrid cross:

Parents:                  Female Cat           x         Male Cat

genotype:                    AaBb                         AaBb


What are the different types of gametes the female could produce?

What are the different gametes the male could produce?

PRODUCE at least 64 kittens by crossing these two parents repeatedly and RECORD the number of
kittens of each possible phenotype.

 Size of Litter    Black Tabby      Brown Tabby           Black           Brown             Ratio

What is the most common phenotype among the kittens?

What genotypes produce that phenotype?

What is the least common phenotype among the kittens?

What genotypes produce that phenotype?

What genotypes produce a brown tabby kitten?

What genotypes produce a black kitten without stripes?

Now TRY a dihybrid cross between the following parents:

Parents:                     Female             x          Male

genotype:                    BbDd                          BbDd

What phenotypes do you get from this cross? In what ratio? (This is called a phenotypic ratio).

Sex Linkage

So far, you have looked at genes located on autosomes. Now look at a gene for coat color that is attached
to the X sex chromosome. Cats, like humans, are female if they have two X chromosomes, and male if
they have one X and one Y chromosome.

Cross the parent cats:    Female (XX)           x        Male (XY)

genotype:                    O+ O+                         Oo Y


The female cat is homozygous for what coat color?

The male cat is HEMIZYGOUS for what coat color?

What does “hemizygous” mean?

Cats with an O+ allele can be any color except red; their coat color is determined by the genes with the
dominant B, D and W alleles. Female cats that are heterozygous show patches of red color and patches of
some other colors, and are called tortoise-shells (shortened to tortie here).

Any time you see a three-color cat you know it must be female. Why?

CROSS the parents listed above until you have produced at least 40 offspring and FILL IN the following


   Kitten Phenotype         No. of Female Kittens       No. of Male Kittens        Possible Genotypes

     Black or Brown



Is it possible for these parents to produce a red, male kitten? Why or why not?

CROSS a tortie female to a black male. LIST their genotypes, and the genotypes and phenotypes of their

If you feel pretty confident that you understand how genes (DNA) control coat color in cats, TRY the
following crosses:

                             Female             x          Male

                            AaBbDd                       AaBbDd
                            AaBbWw                       AaBbWw

Human Genetics

DNA genes work together to provide incredible variety in every trait that makes up an organism. No
wonder no two of us is exactly alike!

Human genetics is of more interest to most people than cat genetics, but in human populations, as in any
natural, uncontrolled population, it is not easy to study gene variation, yet. The Human Genome Project
has recently succeeded in mapping every DNA sequence present in humans. This does not mean that
every allele has been mapped for each sequence or that scientists have decoded what each DNA sequence
does, but work will continue and will probably affect your life in many ways in the near future.

There are a few human phenotypic traits that are regulated by a single pair of genes. Some of these are
listed in the table below. FILL IN the table on the next page using your own personal information, as well
as information from the entire class (the population).

Based on your data, what does “dominant” mean?

What does “recessive” mean?

Why might some recessive genes be more common in a natural population than their dominant alleles?


     Characteristic        Possible       Personal Genotype/        Number in            Percent of
      Phenotype            Genotype           Phenotype            Population for      Population for
                                                                  Each Phenotype      Each Phenotype
 Widow’s peak                W__
 Continuous                  ww

 Free earlobes               U __
 Attached earlobes           uu

 Freckles                    F __
 No Freckles                 ff

 Knuckle hair                H __
 No knuckle hair             hh

 Bent little finger          B __
 Straight little finger      bb

 Tongue roller               T __
 Non-roller                  tt

 Normal color vision
                             N __
 Red-green color

If Sue and her immediate family all have attached earlobes, except for her maternal grandfather who has
free lobes, what is the genotype of her maternal grandmother?

Her maternal grandfather’s genotype?

Everyone in Ted’s family can roll their tongue except Ted. What is his mother’s genotype?

His father’s genotype?

His sister’s genotype?

The allele for a type of red-green color blindness is found on the X chromosome in humans. If George is
color blind, but the other members of his immediate family all have normal color vision, what is his
father’s genotype?

His mother’s genotype?

His brother’s genotype?


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