Genetics: Inheritance, Traits, and DNA Profiling
Genetics Fundamentals
Chromosomes and Genes
Homologous Chromosomes: Pairs of chromosomes with the same size, shape, and gene locations (one from each parent).
Gene: Basic hereditary unit, made of DNA sequences.
Alleles: Different forms of the same gene (e.g., yellow (Y) vs. green (y) seed color in peas).
Genetic Terminology
Homozygous: Two identical alleles for a gene (e.g., YY or yy).
Heterozygous: Two different alleles for a gene (e.g., Yy).
Dominant Allele: Expressed even if only one copy is present (e.g., Y for yellow seeds).
Recessive Allele: Expressed only when two copies are present (e.g., y for green seeds).
Phenotype: Physical traits.
Genotype: Genetic makeup (e.g., Yy or YY).
Genetic Crosses and Traits
Monohybrid Cross: Genetic cross focusing on one trait.
P Generation: Parental generation.
F1 Generation: First offspring generation from P cross.
F2 Generation: Offspring from F1 crosses.
Autosomal Traits: Genes on non-sex chromosomes (seed color in peas).
Sex-Linked Traits: Genes on the X chromosome.
Wild-Type: Most common phenotype.
Mutant: Phenotype from gene mutation (dominant or recessive).
Purebred: Individual from a line where one phenotype has been the only form present for many generations.
Pedigrees
Visualize inheritance across generations:
- Circles: Females
- Squares: Males
- Shaded: Affected individuals
Autosomal Dominant: Males and females equally affected. Does not skip generations.
Autosomal Recessive: Males and females equally affected. Traits may skip generations.
Sex-Linked Recessive: Affects males more often. No male-to-male transmission.
Variable Number of Tandem Repeats (VNTRs)
Definition and Variability
Definition: Short sequences of DNA repeated multiple times in a head-to-tail fashion.
Variability: The number of repeats can differ widely between individuals.
Example: A 17-base pair (bp) VNTR can repeat 70 to 450 times, with loci sizes ranging from 1190 to 7650 bp. Detected using RFLP (Restriction Fragment Length Polymorphism) analysis after cutting DNA with restriction enzymes.
VNTR Detection and Applications
VNTRs are visualized as DNA fragments of varying lengths in an autoradiograph. Each individual has two VNTR alleles: one inherited from each parent.
- Two-band patterns: Common in profiles, representing alleles from both parents.
- Single-band patterns: Occur when both alleles are identical.
Forensic DNA Profiling: DNA profiles are created by analyzing VNTR loci. Probability of matching profiles at multiple loci for unrelated individuals is extremely low. In forensic analysis, typically 13 VNTR loci are analyzed to ensure precision.
Paternity Testing
Example: The child inherits DNA bands from both mother and father. If no match is observed, paternity is excluded.
Uncertainty Reduction: Testing more VNTR loci minimizes the likelihood of false results.
Disease Diagnosis (Cystic Fibrosis)
- Homozygous wt (wild-type): Both alleles are normal; no disease.
- Heterozygous (carrier): One normal allele and one mutated allele; no symptoms but can pass the mutation.
- Homozygous disease allele: Both alleles are mutated; individual has CF.