Genetics Study Guide: Sep 19

Genetic Variation

Types of Genetic Variation

Genetic variation refers to differences in DNA sequences among individuals within a population. These variations are the basis for diversity in traits and can influence susceptibility to diseases, physical characteristics, and more.

• SNPs (Single Nucleotide Polymorphisms): SNPs are the most common type of genetic variation, involving a change in a single nucleotide (A, T, C, or G) in the DNA sequence. For example, a SNP may change a cytosine (C) to a thymine (T) at a specific position.

• Copy Number Variation (CNV): CNVs refer to differences in the number of copies of a particular gene or DNA segment among individuals. Approximately 10% of the 30,000 human genes can vary in copy number, which can have significant effects on phenotype and disease susceptibility.

Example: Variation in the BMP3 gene affects muzzle length in dogs, resulting in medium, shortened, or elongated muzzles depending on the breed.

Application: CNV in the CCL3L1 gene is associated with susceptibility to HIV/AIDS.

Genetic Testing: Applications and Considerations

Genetic testing can identify traits, disease risks, and breed ancestry in animals and humans. Commercial tests (e.g., Embark for dogs) analyze DNA for dozens of traits and compare samples to reference populations.

• Pros: Enables lifestyle changes, helps breeders avoid pairing carriers of recessive disorders, and provides insight into ancestry.

• Cons: May reveal unexpected or stressful results, similar to human genetic testing.

Example: Dog size prediction based on genetic markers; breed identification using long segments of chromosomal DNA compared to reference breeds.

PCR and Detection of Repeat Expansions

Polymerase Chain Reaction (PCR) for CAG Repeat Analysis

PCR is a technique used to amplify specific DNA regions. It can be used to detect the number of trinucleotide repeats (e.g., CAG) in genes such as HTT (Huntington's disease gene).

• Control PCR Product: 420 bp from a patient with 40 CAG repeats. Each CAG repeat is 3 nucleotides, so the variable region is $40 \times 3 = 120$ bp, and the conserved region is $420 - 120 = 300$ bp.

• Experimental PCR Products: 354 bp and 462 bp from a new patient. The number of CAG repeats is calculated by subtracting the conserved region and dividing by 3.

Example Calculation:

• 354 bp product: $354 - 300 = 54$ bp variable region; $54 / 3 = 18$ CAG repeats.

• 462 bp product: $462 - 300 = 162$ bp variable region; $162 / 3 = 54$ CAG repeats.

Mitosis

Stages and Key Events

Mitosis is the process by which a eukaryotic cell divides to produce two genetically identical daughter cells. It consists of several stages, each with distinct events.

• Prophase: Chromosomes condense, spindle fibers form.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids are pulled to opposite poles of the cell.

• Telophase: Nuclear membranes reform, spindle fibers disappear.

• Cytokinesis: Cytoplasm divides, cell plate forms in plant cells.

Quiz Example: During anaphase in plant cells, daughter chromosomes are pulled to opposite ends of the cell (correct answer: E).

Inheritance

Pedigree Analysis and Modes of Inheritance

Pedigree charts are used to track inheritance patterns of traits across generations. Deducing genotypes from pedigrees helps identify carriers and affected individuals, especially for X-linked recessive traits.

• X-linked Recessive: Trait is more common in males; females can be carriers or affected if homozygous recessive.

• Genotype Deduction: For some individuals, possible genotypes can be narrowed down to two options based on family history.

Example: Determining the genotype of female #5 in generation III in an X-linked recessive pedigree.

Meiosis

Process and Introduction of Genetic Variation

Meiosis is the process by which gametes (sperm and egg cells) are produced, reducing the chromosome number by half and introducing genetic variation.

• Meiosis I: Homologous chromosomes separate; crossing over occurs during prophase I, introducing genetic variation.

• Meiosis II: Sister chromatids separate, resulting in four haploid cells.

• Oogenesis: In females, meiosis produces one ovum and polar bodies; DNA content changes at each stage.

Quiz Example: Calculating DNA content in polar bodies based on initial DNA amount in primary spermatocytes.

Chi-Square Analysis

Testing Genetic Ratios

The chi-square test is used to determine whether observed genetic data fit expected Mendelian ratios. It compares observed and expected counts for each phenotype.

• Formula:

$\chi^2 = \sum \frac{(O - E)^2}{E}$

• Application: Testing whether pea plant pod color and shape data fit a 9:3:3:1 ratio.

• Steps:

  1. Determine expected counts for each phenotype based on the ratio.

  2. Calculate $(O - E)^2 / E$ for each category.

  3. Sum the values to obtain the chi-square statistic.

Expected Ratio	Observed #	Expected #	O-E	(O-E)2/E
9	9	Expected	O-E	Calculated
3	33	Expected	O-E	Calculated
3	126	Expected	O-E	Calculated
1	29	Expected	O-E	Calculated

Example: Use the observed counts for each phenotype and compare to expected counts based on the 9:3:3:1 ratio.

Additional info: Some context and explanations were expanded for clarity and completeness, including definitions, examples, and stepwise calculations for PCR and chi-square analysis.