PCAT Exam: Biological Processes - Diversity of Life Forms

Genetics
Genetics is the study of heredity, which is the transmission of traits from one generation to the next, and hereditary variation. The chromosomes passed from parent to child contain hereditary information in the form of genes. Each gene has specific sequences of DNA that encode proteins, start pathways, and result in inherited traits. In the human life cycle, one haploid sperm cell joins one haploid egg cell to form a diploid cell. The diploid cell is the zygote, the first cell of the new organism, and from then on, mitosis takes over and nine months later, there is a fully developed human that has billions of identical cells.
The monk Gregor Mendel is referred to as the father of genetics. In the 1860s, Mendel came up with one of the first models of inheritance, using peapods with different traits in the garden at his abbey to test his theory and develop his model.

His model included three laws to determine which traits are inherited; his theories still apply today, even after genetics has been studied more in depth.
1. The Law of Segregation: When two parent cells form daughter cells, the alleles segregate and each daughter cell only inherits one of the alleles from each parent.
2. The Law of Independent Assortment: Different traits are inherited independent of one another because in metaphase, the set of chromosomes line up in random fashion – mom’s set of chromosomes do not line up all on the left or right, there is a random mix.
3. The Law of Dominance: Each characteristic has two versions that can be inherited. The gene that encodes for the characteristic has two variations, or alleles, and one is dominant over the other.

Dominant and Recessive Traits
Each gene has two alleles, one inherited from each parent. As mentioned, dominant alleles are noted in capital letters (A) and recessive alleles are noted in lower case letters (a). There are three possible combinations of alleles among dominant and recessive alleles: AA, Aa (known as a heterozygote), and aa. Dominant alleles, when mixed with recessive alleles, will mask the recessive trait. The recessive trait would only appear as the phenotype when the allele combination is aa because a dominant allele is not present to mask it.
Although most genes follow the standard dominant/recessive rules, there are some genes that defy them. Examples include cases of co-dominance, multiple alleles, incomplete dominance, sex-linked traits, and polygenic inheritance.
In cases of co-dominance, both alleles are expressed equally. For example, blood type has three alleles: IA, IB, and i. IA and IB are both dominant to i, but co-dominant with each other. An IAIB has AB blood. With incomplete dominance, the allele combination Aa actually makes a third phenotype. An example: certain flowers can be red (AA), white (aa), or pink (Aa).

Mendel’s Laws of Genetics and Punnett Squares
Mendel’s first law of genetics is the principle of segregation and states that alleles will segregate into different cells during the formation of gametes in meiosis. Mendel’s second law of genetics is the principle of independent assortment and states that genes for different traits will be assigned to different gametes independent of the others. Together, these two laws state the assumptions on which genetic probabilities are based.
Punnett squares are simple graphic representations of all the possible genotypes of offspring, given the genotypes of the parent organisms. For instance, in the above example with the species of bird with black or white feathers, A represents a dominant allele and determines white colored feathers on a bird. The recessive allele a determines black colored feathers on a bird. If both parents are heterozygous (Aa, the x- and y-axis of the square), the offspring will have the possible genotypes AA, Aa, Aa, and aa.

Phenotypically, three offspring would have white feathers and one would have black feathers, as shown in the Punnett square below:
 

Monohybrid and Dihybrid Genetic Crosses
Genetic crosses represent all possible permutations of gene combinations, or alleles. A monohybrid cross investigates the inheritance pattern of a single gene such as in the above example of the birds with black or white feathers. Both parents must have heterozygous gene pairs in a monohybrid cross.
The phenotypic ratio for a monohybrid cross is 3:1 (AA, Aa, Aa, aa) in favor of the dominant gene. A dihybrid cross investigates the inheritance patterns of two genes that are related, for example A and B. A dihybrid cross has a phenotypic ratio of 9:3:3:1 with nine offspring inheriting both dominant genes, six offspring inheriting a single dominant and a single recessive gene, and one offspring inheriting both recessive genes.

Mutations
Mutations are permanent alterations to an organism’s genetic DNA sequence. Mutations can result from DNA failing to replicate accurately. They can also result from environmental influences such as radiation or chemicals. Mutations occur randomly and spontaneously at low rates.
Mutations can occur in reproductive and non-reproductive cells. Those occurring in non-reproductive cells are termed somatic mutations. Those occurring in reproductive cells (eggs or sperm) are termed germ line mutations. Although somatic mutations cannot be transmitted to offspring, germ line mutations are transmitted to offspring and can be advantageous, neutral, or disadvantageous.
In general, single-nucleotide alterations to DNA, or point mutations, can be either silent or same-sense (so that an identical amino acid sequence is encoded), missense (so that a different amino acid sequence is encoded), or nonsense (so that a new stop codon is encoded, and the resultant protein is truncated). They can also either be transitions (a purine base to another purine base or vice versa) or transversions (a purine base to a pyrimidine base or vice versa).

The four common multi-nucleotide mutations are:
1. Insertions: One or more nitrogen bases are added or inserted into the typical DNA sequence.
2. Deletions: One or more nitrogen bases are removed or deleted from the typical DNA sequence.
3. Inversions: A length of DNA is removed and reattached in reverse order from the typical DNA sequence.
4. Translocations: A length of DNA is removed and reattached in an alternate place or chromosome than where found in the typical DNA sequence.