How to Ace AP Biology (2018): Traits

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Frameshift Mutation
A frameshift mutation is a genetic mutation caused by indels (insertions or deletions) of a number of nucleotides in a DNA sequence that is not divisible by three. Due to the triplet nature of gene expression by codons, the insertion or deletion can change the reading frame (the grouping of the codons), resulting in a completely different translation from the original. The earlier in the sequence the deletion or insertion occurs, the more altered the protein. A frameshift mutation is not the same as a single-nucleotide polymorphism in which a nucleotide is replaced, rather than inserted or deleted. A frameshift mutation will in general cause the reading of the codons after the mutation to code for different amino acids. The frameshift mutation will also alter the first stop codon ("UAA", "UGA" or "UAG") encountered in the sequence. The polypeptide being created could be abnormally short or abnormally long and will most likely not be functional.


Base Deletion
The smallest single base deletion mutations occur by a single base flipping in the template DNA, followed by template DNA strand slippage within the DNA polymerase active site. Deletions can be caused by errors in chromosomal crossover during meiosis, which causes several serious genetic diseases.

A base deletion near the start of a gene would be most likely to have a catastrophic effect on the functioning of a protein because it can introduce a stop codon near the beginning of the gene.


RFLP Analysis
RFLP analysis is a technique that exploits variations in homologous DNA sequences. Through this analysis, individuals can be identified based on unique patterns of restriction enzyme cutting in specific regions of DNA.

The basic technique for the detection of RFLPs involves fragmenting a sample of DNA with the application of a restriction enzyme, which can selectively cleave a DNA molecule wherever a short, specific sequence is recognized in a process known as a restriction digest. The DNA fragments produced by the digest are then separated by length through a process known as agarose gel electrophoresis and transferred to a membrane via the Southern blot procedure.


Sickle Cell Trait
Sickle cell trait describes a condition in which a person has one abnormal allele of the hemoglobin beta gene (is heterozygous), but does not display the severe symptoms of sickle cell disease that occur in a person who has two copies of that allele (is homozygous). Those who are heterozygous for the sickle cell allele produce both normal and abnormal hemoglobin (the two alleles are co-dominant with respect to the actual concentration of hemoglobin in the circulating cells).


People with sickle-cell trait are heterozygous for the sickle-cell allele, have increased resistance to malaria, and produce normal and abnormal hemoglobin.


The frequency of heterozygosity for the sickle cell anemia allele is unusually high, presumably because this reduces the frequency of malaria. This relationship is related to natural selection because the traits that are the most advantageous (reduction of disease) will continue to be highly expressed over the given population.


Sex Linked Traits
In humans, as well as in many other animals and some plants, the sex of the individual is determined by sex chromosomes—one pair of non-homologous chromosomes. Until now, we have only considered inheritance patterns among non-sex chromosomes, or autosomes. In addition to 22 homologous pairs of autosomes, human females have a homologous pair of X chromosomes, whereas human males have an XY chromosome pair. Although the Y chromosome contains a small region of similarity to the X chromosome so that they can pair during meiosis, the Y chromosome is much shorter and contains fewer genes. When a gene being examined is present on the X, but not the Y, chromosome, it is X-linked.

Males are more often affected by sex-linked traits than females because male hormones such as testosterone often alter the effects of mutations on the X chromosome.


Triple X syndrome is a genetic disorder that affects about 1 in 1,000 females. Females normally have two X chromosomes in all cells — one X chromosome from each parent. In triple X syndrome, a female has three X chromosomes. Many girls and women with triple X syndrome don't experience symptoms or have only mild symptoms. In others, symptoms may be more apparent — possibly including developmental delays and learning disabilities.


The triple X syndrome is usually caused by malformation of an egg or sperm cell, or by an error early in embryo development. There are few to no symptoms of this condition.

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