Designer Genes

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Revision as of 04:38, 3 February 2012 by Luo (talk | contribs) (Eh, this will need a lot of work next year, but here's a start.)
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Designer Genes (Division C) and Heredity (Division B) are based on genetics. This year Mendelian genetics and most things dealing with dominant/recessive alleles are out. Another major element to the event is molecular biology (introns/exons, mitosis/meiosis, leading/lagging strand). Make sure you know transcription. translation, replication, Hardy Weinburg equilibrium (p^2 + 2pq+ q^2 = 1, p+q=1), and the other topics in your rule book. For the 2006-2007 school year, the event will focus more on the molecular biology and biotechnology elements of genetics.


DNA is made up of three things --> Phosphate group, a five carbon sugar (deoxyribose) and nitrogenous bases. Nitrogenous bases are Adene and Guanine -- which are purines, and Thymine and Cytosine -- which are pyrimidines. The sugar -- deoxyribose and the phosphate groups form the backbone or the sides of the double helix "ladder" and the nitrogenous bases stick out from the chain like "rungs" of the ladder.

Watson, Crick, and Maurice Wilkins are credited with finding the structure of DNA. Read The Double Helix by James D. Watson. It is a really nice book.

Nitrogenous bases Adenine only bonds with thymine and guanine bonds only with cytosine. This is called base pairing.

DNA Replication When a cell divides, it makes a duplicate copy of its DNA so the produced cell will have a set of DNA molecules. This is DNA replication -- it is also called DNA synthesis. Enzymes (DNA polymerase) "unzip" the two strands of the double helix structure and insert appropriate nitrogenous bases.

A Karyotype is a chart that shows sets of chromosomes. It can detect chromosome disorders with too little or too less chromosomes in a set. It can detect Trisomy 21 - Down syndrome, XXY Syndrome - Klinefelter's syndrome, X syndrome - Turner's syndrome, etc.

Punnett squares

Those are called Punnett squares. These Punnett squares show the cross between tall and short plants. The first Punnett square shows a cross between two heterozygous tall plants. The second Punnett sqare shows a cross between a homozygous tall plant and a homozygous short plant. 'T' means tall and 't' means short. Tall dominates over short, in this case. Therefore, Tt is tall. TT is tall too. tt is short.


Ways In Which Traits Are Passed On

Single factor

crosses Single factor crosses show only one trait -- one pair of alleles with definite dominant and recessive traits. The traits are like tall and short. T is tall and t is short. T is definitely dominant. The Punnett square above is used to show single factor crosses.

Two factor cross

Two factor crosses show two different traits, each of which is on a different chromosome, it is like a single factor cross, but just two traits instead of one. Like in the single factor cross where T was tall and t was short. In two factor crosses, suppose that wrinkled skin is a trait. W is smooth skin and w is wrinkled skin. The Punnett Square used for this is as follows:


This cross is TtWw? x TtWw?...cross between two heterogeneous tall, wrinkled plants.

different combination of alleles phenotype - physical appearance of the combination

Three factor crosses

Three factor crosses show three different traits. Just like one or two factor crosses.

Incomplete dominance

In some unusual cases such as pink flowers, and medium sized plants, gene pairs for a given trait fail to establish dominance and the heterozygous condition is expressed as an intermediate between the two alleles. Often, to draw attention to this situation, the letter 'I' is used to designate the gene allele. In snapdragon plants, the genotype IrIr? appears red, IwIw? appears white, and the heterozygous IrIw? appears pink. In all cases of incomplete dominance, the number of genotypes equals the number of phenotypes.


Epistatis is where one set of genes stops or inhibits the action of another genes. Epistasis genes can either be recessive or dominant. The gene for no pigment (p) in the skin(albinism) is recessive to normal pigmentation(P). For any pigment to appear at all, at least one gene for enzyme S must also be present. That's like even if there is a pigment, but enzyme S is not present, the person is albino. PpSs? is normal, PPss? is albino, ppSS is albino, and so on. To not be albino, there needs to be at least one P and one S.

Sex determination

In humans, the male and female share 22 of the 23 pairs of chromosomes in each body cell. The 23rd pair is known as the sex chromosomes because it determines the sex of the individual. In the male, the sex chromosome consists of an X and a Y chromosome(XY) while the pair in females consists of two X chromosomes(XX). The male is the one who determines the sex of the child and teh female gives an X to all eggs while the male randomly produces about 50% X sperm and 50% Y sperm.

Sex-linked traits

Sex-linked traits are features that are associated with the genes on the sex chromosomes, usually X. Examples of those are recessive genes for color-blindness and hemophilia.

Sex-influenced traits

Sex influenced traits are traits that show up more in one sex than they do in the other as a definite phenotype. Usually influenced more by hormones in the male or female.

Multiple genes

Most phenotypic features are controlled by more than one set of non-allelic genes acting on height, skin color, intelligence, and hair and eye color. Usually this type of problem is seen as a typical two or three, etc factor cross with the more dominants, the more expression of the trait in question.

Multiple alleles

There may be more than the usual two alleles for any given gene. Especially, this appears in fur or pelt conditions of domestic animals. The problem usually uses 'I' (for incomplete dominance) and some prearranged superscript.

Practice test

This is how the test could look like. I have never been in this event but this is what my Biology genetics/DNA test looked like. It might help you. It might not. Still, it is better than nothing. The answers may be posted later.

Part I - DNA

1. Which of the following nucleotide pair bonds would be found in a DNA molecule? a. adenine-guanine b. guanine-cytosine c. adenine-cytosine d. cytosine-uracil

2. The backbone of a DNA molecule is made of which two components? a. phosphate molecules and ribose sugars b. deoxyphosphate molecules and ribose sugars c. phosphate molecules and deoxyribose sugars d. deoxyphosphate molecules and deoxyribose sugars

3. Ribosomes are made of _____. a. rRNA and protein b. tRNA and mRNA c. rRNA and mRNA d. protein and mRNA

4. Watson and Crick were the first to suggest that DNA is ______. a. a short molecule b. the shape of a double helix c. a protein molecule d. protein and tRNA

5. The chromosome abnormality that occurs when part of one chromosome breaks off and is added to a different chromosome is ______. a. deletion b. nondisjunction c. translocation d. inversion

6. Which of the following would be least likely to happen as a result of a mutation in a person's skin cells? a. skin cancer b. reduced functioning of the skin cell c. no change in the functioning of the skin cell d. the person's offspring have mutated skin

7. THe?? process by which a DNA molecule is copied is called ___________. a. binary fission b. mitosis c. replication d. translation

8. A DNA nucleotide may be made up of a phosphate group along with ________. a. a deoxyribose sugar and uracil b. ribose sugar and adenine c. deoxyribose sugar and thymine d. ribose sugar and cytosine

9. Which series is arranged in order from largest to smallest in size? a. chromosome, nucleus, cell, DNA, nucleotide b. cell, nucleus, chromosome, DNA, nucleotide c. nucleotide, chromosome, cell, DNA, nucleus d. cell, nucleotide, nucleus, DNA, chromosome

10. Messenger RNA is formed in the process of ______. a. transcription b. translation c. replication d. mutation

11. X rays, ultraviolet light, and radioactive substances that can change the chemical nature of DNA are classified as ________. a. growth regulators b. metamorphic molecules c. hydrolytic enzymes d. mutagens

12. After DNA replication, the two DNA molecules that are made ___________ a. are complimentary b. are identical c. must replicate again d. cannot replicate again

13. Bacteriophages are __________. a. tiny bacteria b. bacteria of the sametype c. lipids and ribonucleic acid d. viruses

14. The order in which specific amino acids bond to mRNA is initially determined by a. rRNA b. tRNA c. base pairing d. DNA

15. In RNA, the code word AUG that specifies methionine can also serve as a(n) a. anticodon b. stop codon c. initiator codon d. all are correct

16. The two strands of a DNA double helix are ______- a. identical b. purines c. pyrimidines d. complementary

17. Both DNA and RNA _______ a. contain ribose b. are single stranded c. contain nucleotides d. contain uracil