Designer Genes

Designer Genes (Division C) and Heredity (Division B) are based on genetics and molecular biology (introns/exons, mitosis/meiosis, leading/lagging strand, etc).

DNA
DNA is made up of three things --> Phosphate group, a five carbon sugar (deoxyribose) and nitrogenous bases. Nitrogenous bases are Adenine 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.

Karyotypes
A karyotype is a chart that shows each chromosome. Each karyotype displays 23 pairs of chromosomes, including the X/Y chromosomes. Every pair is assigned a number (except for the sex chromosomes; they are always referred to as the X and Y chromosomes). Some genetic disorders can be detected by analyzing the number of chromosomes and/or the sex chromosomes. The gender of the individual can also be deduced from looking at the sex chromosomes. If there is an X and a Y, the individual is a male. A female has two X chromosomes and no Y chromosome.

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.



Single factor (Monohybrid)
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 (Dihybrid)
Two factor crosses, or dihybrid crosses, are similar to single-factor crosses except that in a two-factor cross, two traits are crossed rather than one trait in a single-factor cross. An example of a two-factor cross is pictured to the left.



Here, two heterozygotes are crossed (RrYy x RrYy). The "R" allele represents the shape of the seed and the "Y" allele represents the color. It is important to note the genotypic and phenotypic ratios for a heterozygous dihybrid cross. Regardless of the alleles, if two dihybrid heterzygotes are crossed, then the resulting phenotypic ratio will be:

9 D/D: 3 D/R: 3 R/D: 1 R/R (D = homozygous dominant, R= homozygous recessive).

and the genotypic ratio will be:

1 D/D: 2 D/H: 1 D/R: 4 H/H: 4 H/D: 1 R/D: 2 R/H: 1 R/R (H = heterozygous).

So, the phenotypic ratio for the pictured dihybrid cross is:

9 round/yellow:3 round/green: 3 wrinkled/yellow: 1 wrinkled/green.

Some terms are defined below. On tests, be extra careful when you spot these terms as they are easily confused with each other.


 * Genotype: The different combinations of the alleles.
 * Phenotype: The physical appearance of the offspring.


 * Genotypic ratio: The ratio of the combination of alleles.
 * Phenotypic ratio: The ratio of the physical appearance.

Three factor crosses (Trihybrid)
Three factor crosses show three different traits. Just like one or two factor crosses. Example: AaBBcc x AAbbCc

Incomplete dominance
In some unusual cases such as 4 o'clock flowers, 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.

Example: In 4 o'clock flowers, the genotype RR (homozygous dominant) appears red, rr (homozygous recessive)appears white, and Rr (heterozygous)appears pink. In all cases of incomplete dominance, the number of genotypes equals the number of phenotypes.

Epistasis
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 them...like 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 are: a. are complimentary b. are identical c. must replicate again d. cannot replicate again

13. Bacteriophages are: a. tiny bacteria b. bacteria of the same type c. lipids and ribonucleic acids 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

 1. B 2. C 3. A 4. B 5. C 6. D 7. C 8. C 9. B 10. A 11. C (?) 12. A (?) 13. D 14. D (?) 15. D 16. D 17. A