Chapter 13: Modern Understandings of Inheritance Flashcards
What is the chromosomal theory of inheritance?
Genes are located on chromosomes, and chromosomes undergo segregation and independent assortment during meiosis.
Proposed by Sutton and Boveri (1902), explaining Mendel’s laws. Supported by Morgan’s fruit fly experiments (e.g., X-linked eye color in Drosophila).
What is genetic linkage, and how does it violate Mendel’s laws?
Linkage: Genes located close together on the same chromosome tend to be inherited together.
Violates independent assortment (Mendel’s 2nd law) because linked genes do not segregate independently. Example: Pea plant genes for flower color and pollen shape.
How is recombination frequency used in gene mapping?
Recombination frequency = Number of recombinant offspring / Total offspring × 100.
Expressed in map units (centimorgans). Key Insight: Higher recombination frequency = genes farther apart on a chromosome.
Why are males more likely to express X-linked recessive disorders?
Males are hemizygous (XY): They have only one X chromosome.
X-linked recessive alleles (e.g., hemophilia, color blindness) always express in males. Females require two recessive alleles (XcXc) to show the trait.
What is X-inactivation, and what is its significance?
In female mammals, one X chromosome is randomly inactivated in somatic cells, forming a Barr body.
Ensures dosage compensation (equal gene expression between XX females and XY males). Example: Calico cats (coat color patches reflect X inactivation).
How are linkage maps constructed?
Based on recombination frequencies between genes.
1 map unit = 1% recombination frequency. Example: If genes A and B recombine 15% of the time, they are 15 map units apart.
What is nondisjunction, and what are its consequences?
Failure of chromosomes to separate during meiosis I/II.
Results in aneuploidy (abnormal chromosome number). Examples: Trisomy 21 (Down syndrome), Monosomy X (Turner syndrome).
Name and define four types of chromosomal mutations.
Deletion: Loss of a chromosome segment.
Duplication: Extra copy of a segment.
Inversion: Reversed segment orientation.
Translocation: Segment moves to a nonhomologous chromosome.
Example: Philadelphia chromosome in leukemia.
What is genomic imprinting? Provide an example.
Parent-of-origin effect: Gene expression depends on whether the allele is inherited from the mother or father.
Example: Prader-Willi syndrome (deletion on paternal chromosome 15) vs. Angelman syndrome (maternal deletion).
What is extranuclear inheritance?
Genes located in mitochondria or chloroplasts (not nuclear DNA).
Inherited maternally (egg contributes cytoplasm). Example: Mitochondrial diseases (e.g., Leber’s hereditary optic neuropathy).
Interpret these pedigree symbols: Filled square, Half-filled circle, Diamond.
Filled square: Affected male.
Half-filled circle: Carrier female.
Diamond: Unknown gender.
Used to track autosomal dominant/recessive or sex-linked traits (e.g., hemophilia in European royal families).
What is epistasis? Provide an example.
One gene masks or modifies the expression of another gene.
Example: Coat color in Labrador retrievers: Gene B/b (black/brown) and E/e (pigment deposition). ee genotype results in yellow coat, regardless of B/b.
What are multifactorial traits?
Influenced by multiple genes + environment (e.g., height, heart disease).
Show continuous variation (bell curve distribution). Example: Skin color (polygenic + sun exposure).
Name three methods of genetic testing.
Karyotyping: Visualizes chromosome number/structure.
FISH (Fluorescent in situ hybridization): Locates specific DNA sequences.
DNA Sequencing: Identifies mutations.
Example: BRCA1 for breast cancer risk.
What are ethical concerns in modern genetics?
Privacy: Misuse of genetic data (e.g., discrimination by employers).
Eugenics: Selecting ‘desirable’ traits.
Gene Editing: CRISPR’s unintended consequences.
Example: Off-target effects.
How can environment influence gene expression?
Phenocopy: Environmental factor mimics genetic trait.
Example: Vitamin D deficiency causing rickets, a condition also caused by genetic mutations. Smoking increasing lung cancer risk in individuals with certain alleles.
What is mosaicism?
Presence of two or more genetically distinct cell lines in one individual.
Caused by somatic mutations during development. Example: Mosaic Down syndrome (some cells have trisomy 21).
How does CRISPR-Cas9 work?
A gene-editing tool using a guide RNA to direct Cas9 enzyme to cut specific DNA sequences.
Applications: Correct genetic disorders (e.g., sickle cell anemia), modify crops. Controversies: Ethical concerns in human germline editing.
What role do telomeres play in aging?
Telomeres: Protective caps on chromosome ends that shorten with each cell division.
Shortened telomeres trigger cell senescence/aging. Telomerase enzyme (active in stem cells/cancer) extends telomeres.
What was the goal of the Human Genome Project (HGP)?
Map all human genes (~20,500) and sequence the 3 billion DNA base pairs.
Impact: Advanced understanding of genetic diseases, personalized medicine, and evolutionary biology.
