INTRODUCTION - Dragon genetics by liwenting

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									University of Pittsburgh at Bradford           Science in Motion                                   Biology Lab


                                       Activity 1: I’m all Keyed Up

Introduction
        One of the first steps used to study how traits are inherited is to design a key. Using the letters of
the alphabet, simply select a letter to symbolize the trait you are investigating. For example, you might
want to use the letter “n” to symbolize the length of the dragon’s neck.

         The upper case version of the letter you choose represents the dominant allele for the trait; the
lower case version of the letter represents the recessive allele for the trait. An allele is one of the two or
more alternative forms of the same gene or segment of DNA on a chromosome. Each allele carries the
genetic information that codes for specific traits (for instance, the neck length gene for our dragon). Each
allele from one parent will pair with an allele from the other parent, to create a complete code for a
specific genetic trait. An allele is dominant when it completely masks the presence of the allele it is
paired with. The trait that is hidden or masked is the recessive trait.

        Suppose that in our hypothetical dragon, long necks are dominant to short necks. If a baby dragon
inherited even one long neck allele (N) from either parent, the baby would have a long neck. Since the
most simple inheritance patters involve transmission of one trait from the mom and one trait from the dad,
all “NN” and “Nn” dragons would have a long neck. The only letter combination that would represent a
short-necked dragon would be “nn”. This would mean both the mom and the dad contributed one
recessive, short neck allele (n) during the fertilization process.

                          Activity 2: Would Someone Please Take Charge?
Introduction
        The simplest form of genetic inheritance for a single trait involves receiving one piece of genetic
information (one allele) from the mother and one piece of genetic information (one allele) from the father.
Sometimes the information from the mom and dad is identical, and sometimes it is not. The resulting
combination of alleles is referred to as a genotype; the physical expression of the trait that is coded for is
referred to as a phenotype. For example, the genotype for our long necked dragon could be either NN or
Nn; the phenotype for this trait would be a long neck.

                                   Activity 3: Do Your Ears Hang Low?
Introduction
         The combination of alleles inherited from your parents is called a genotype. When a person
exhibits a dominant trait, he/she probably does not know his/her genotype for that trait. Take, for
example, the genetic trait of tongue rolling. A tongue rolling allele (R) is dominant; a non-tongue rolling
allele (r) is recessive. The alleles you have for that trait could be identical. For example, you may have
received a tongue rolling allele (R) from both you mom and dad; in that case, you would also be a tongue
roller, with a genotype of “RR”. Or you may have received a non-tongue rolling allele (r) from both your
mom and dad; in that case you would not be able to roll you tongue and your genotype would be “rr”. In
either case, when both alleles for a trait are identical, they are referred to as homozygous, from the Greek
words “homo” (meaning “same”) and “zygote” (from “zygoun”, meaning “to join”). A zygote is a cell
that is formed at conception, by the union of two gametes (egg and sperm).

       Sometimes the combination of alleles inherited from your parents is different. For example, you
may have received a tongue rolling allele from you mom, but not your dad. In this case, your genotype
would be “Rr”. This combination of alleles is referred to as heterozygous, from the Greek words “hetero”
(meaning “different”) and “zygote”.
Introduction adapted from Science Kit® & Boreal® Laboratories “Dragon Genetics”                        1
University of Pittsburgh at Bradford          Science in Motion                                 Biology Lab
         The important thing to remember is that both “RR” and “Rr” will result in a tongue rolling kid. In
a simple inheritance pattern involving two alleles, it is only necessary to receive one copy of the dominant
allele to exhibit the dominant trait. A person who exhibits a dominant trait will probably not know his/her
genotype for that trait. On the other hand, a person who exhibits a recessive trait always knows what
his/her genotype is because no dominant allele is present.

                                       Activity 4: Enter the dragon

Introduction
       Thomas Morgan and others performed experiments with Drosophila melanogaster (the fruit fly),
and provided proof that genes are arranged linearly on chromosomes. Activities #4 and #5 combine
Gregor Mendel’s postulates:
          o each trait is determined by two genes, one from the mother and one from the father;

                 and

             o genes exhibit dominance or recessiveness.

In this activity, as well as in Activity 5, you will pair up the mom and dad genes and learn how they align
with each other on the chromosomes. When doing this, remember that a gene is a segment of DNA
located at a particular place on a chromosome; each gene or DNA segment codes for a specific trait.
Alleles are the various versions of each gene. For instance, there is a gene that codes for blood type. The
various versions or alleles of that gene code for different blood types. One allele codes for A type blood;
another version codes for B type blood; and a third version codes for O type blood.

                                       Activity 5: Dragon Genetics

Introduction
        It takes more than a single pair of chromosomes to make up a living organism. In humans, each
parent contributes 23 chromosomes, for a total of 46 (or 23 pairs). A fruit fly has eight chromosomes
(four from each parent). In our hypothetical dragon, each parent dragon contributes four chromosomes,
for a total of eight. All eight chromosomes (four from the mother and four from the father) are needed to
complete the genotype of the baby dragons.

         The sex of the baby dragon is determined by one set of chromosomes. The mother always donates
an X chromosome to her offspring because, as a female, her genotype is always XX. The father may
donate either an X chromosome or a Y chromosome because, as a male, his genotype is always XY. An
XX combination results in a female offspring; and XY combination results in a male offspring. In this
case, the X and Y chromosomes not only code for gender, but also for the secondary sex characteristic of
ear frills. A dragon with an XX genotype will be a female; a female will never have ear frills. An XY
genotype will be a male; males will always have ear frills.

       For is activity, one student in each pair will receive an “egg” filled with four dragon
chromosomes; the other student gets a “sperm” filled with four dragon chromosomes. You must
determine the genotypes and phenotypes of your dragon “offspring”. You will then construct and color
your dragon according to the resulting phenotype you determine the dragon to have.




Introduction adapted from Science Kit® & Boreal® Laboratories “Dragon Genetics”                               2

								
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