Special Genetics

Incomplete Dominance: When one homozygous dominant parent crosses with a homozygous recessive parent, the offspring would be only one type—heterozygous. However, sometimes heterozygous offspring has different trait with the dominant parent. For example, some heterozygous flowers have pink color when their parents have red and yellow flowers.

Multiple Alleles: In different blood types, there are three types of alleles--I^a, I^b, and i. The first two alleles are both dominant, and the last one is recessive. Blood type A can be produced by two I^a or one I^a and i. Similarly, blood type B can be formed by two I^b or one I^b and i. Blood type O can only formed by two recessive alleles. And Blood type AB can be formed by one I^a and one I^b.

Sex-linked Genes: Genes that locate on a sex chromosome are sex-linked genes. X chromosome carries more genes than Y chromosome. Males only have one allele for most X-linked genes. Females have two alleles for X-linked genes. In a Punnett square, when a male is crossed with a female, 50% of the offspring would be a boy because there are two alleles that have Y chromosome. Diseases such as hemophilia and color blindness can be passed on through sex-linked crosses.

Dihybrid Cross

The difference between a dihybrid cross and a monohybrid cross is that a dihybrid cross involves two types of alleles at the same time.

Usually, we can figure out the offspring of a dihybrid cross in two ways. The first way is to take one allele from each trait at a time from a single parent and combine every two to form a gamete. List the male gametes on one row of the Punnett square and female gametes on the column of the Punnett square. We can have 16 genotypes from the cross and then conclude the genotypic and phenotypic ratios.

The second way is to create two Punnett squares by separating the two traits. Put one allele at a time from each parent on the sides of the square. If there were two traits such as black/brown hair and brown/blue eyes, the first Punnett square can be allele combinations for hair color and the second square for eye color. Then, we can determine the genotypic and phenotypic ratios of offspring by combining the traits of two squares.

Mendelian Genetics (Monohybrid Cross)

In order to do Mendelian genetics problems, we must understand these two laws.

• The principle of segregation (First Law): The two members of a gene pair (alleles) segregate (separate) from each other in the formation of gametes. Half the gametes carry one allele, and the other half carry the other allele.
• The principle of independent assortment (Second Law): Genes for different traits assort independently of one another in the formation of gametes.

In a monohybrid cross, we analyze organisms with one trait. Let's take the color of fur on a type of bear for example. The black fur is the dominant trait, and the brown fur is the recessive trait. B and b are alleles respectively for each trait. We can have combinations of different types of bears (homozygous dominant, heterozygous dominant, and homozygous recessive)--(BB, Bb, bb). When two bears cross, they form gametes by assorting the alleles independently according to the Mendelian laws.

Operon System

An operon system controls the rate of protein synthesis. There are two types of operons. The first type is called repressible operon, in which protein is being synthesized. In order to stop the process, an amino acid called tryptophan has to be created in order to transform the inactive repressor to active repressor. In this process, RNA polymerase reads DNA to create mRNA. So polypeptides are created for tryptophan. Then tryptophan can attach to protein that blocks the RNA polymerase.

Another type of operon is inducible operon. This type of system is not producing protein at first. Allolactose is served as an inducer to inactivate the repressor. In the Arabinose operon system, which is an inducible one, Arabinose is added to start the protein synthesis. RNA polymerase is able to come in and arabinase is produced to digest sugar. When there is too much enzyme, the system will shut off and the process would repeat over and over.

Protein Synthesis

DNA is able to create protein through protein synthesis. Even though DNA contains information that encodes protein, DNA molecules cannot go out the nuclear membrane because their shape is too large. Therefore, DNA is transcribed into mRNA. First, the DNA strands are replicated. RNA polymerase reads DNA from 3’ to 5’ and synthesizes a strand of messenger RNA in the 5’ to 3’ direction. In mRNA, every 3 RNA bases form a codon. In the processing of RNA, introns, which are noncoding parts of DNA, are cut out via splicesomes. Only exons are left. A G-cap (Guanine), is put on the mRNA to protect the message. And a poly A tail with many A nucleotides is added as well. In translation, mRNA is first decoded to produce polypeptides that make up the amino acids. The transfer RNA carries amino acids to the ribosome where proteins are processed. mRNA and tRNA make codons and anticodons match with each other in the ribosome. There are three sites in the ribosome. A site accepts codons and anticodons; P site binds the peptides; and E site makes the protein exit. Start codon is known as Met (AUG).

DNA Replication

DNA replication is a process in which two identical copies are created from one original DNA molecule. This process occurs in all living organisms and provides the basis for inheritance. It is a semiconservative process, meaning that each original serves as a template for the production of a complementary strand. 

The origin of replication is called promoter. The replication fork is created by helicase, which breaks the strands into two parts by breaking hydrogen bonds between nucleobases. There are two parts of each strand, the leading strand and the lagging strand. Then, RNA Primase comes in to create RNA that has a polar 3’ end. Its job is to help DNA polymerase read since DNA polymerase is polar. DNA polymerase III creates new nucleotides for the new strand of DNA. The bond in between the bases is a phosphodiester bond. It is important to distinguish that DNA is read from 3’ to 5’, but its synthesized from 5’ to 3’. Afterwards, DNA Polymerase I comes to replace RNA with DNA. However, the bond between these replaced DNA nucleotides is still incomplete. These nucleotides are known as Okazaki fragments. Finally, ligase glues DNA nucleotides together by phosphodiester bonds. This is the process of DNA Replication.