BackChromosomal Basis of Inheritance and Genetic Disorders
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Chapter 15: Chromosomes
Chromosome Theory of Inheritance
The chromosome theory of inheritance connects the physical movement of chromosomes in meiosis to Mendel’s laws of inheritance. This theory explains how genes are carried on chromosomes and how their segregation and independent assortment during meiosis account for inheritance patterns.
Genes are located on chromosomes, which are duplicated and passed to offspring during cell division.
Chromosomes undergo segregation and independent assortment, mirroring Mendel’s laws.
Fruit flies (Drosophila melanogaster) are a model organism for studying inheritance due to their short generation time and easily distinguishable traits.
Sex-linked traits are associated with genes located on sex chromosomes (X and Y).
Example: Morgan’s experiments with fruit flies demonstrated that eye color is linked to the X chromosome, supporting the chromosome theory of inheritance.
Sex-Linked Genes and Inheritance Patterns
Sex-linked genes are found on sex chromosomes and exhibit unique inheritance patterns. In humans and other mammals, the X and Y chromosomes determine sex.
Females: XX; Males: XY
X-linked genes: Genes located on the X chromosome; males are more likely to express recessive X-linked traits due to having only one X chromosome.
Y-linked genes: Genes located on the Y chromosome; fewer and mostly related to male sex determination.
Examples of X-linked disorders: Color blindness, hemophilia
Example: A male with a recessive allele on the X chromosome will express the trait, while a female must have two copies to express it.
Patterns of Inheritance: Linked Genes and Genetic Recombination
Linked genes are genes located close together on the same chromosome and tend to be inherited together. Genetic recombination occurs when crossing over during meiosis separates linked genes, producing new combinations of traits.
Genetic recombination: Production of offspring with combinations of traits differing from either parent.
Crossing over increases genetic variation by exchanging segments between homologous chromosomes.
Recombination frequency is used to map the relative positions of genes on chromosomes.
Type of Cross | Expected Phenotype Ratio |
|---|---|
Unlinked genes | 9:3:3:1 (dihybrid cross) |
Linked genes (no crossing over) | Parental types only |
Linked genes (with crossing over) | Parental and recombinant types |
Additional info: The greater the distance between two genes, the higher the probability that a crossover will occur between them.
Alterations of Chromosome Number and Structure
Changes in chromosome number or structure can lead to genetic disorders. These alterations may result from errors during meiosis or fertilization.
Nondisjunction: Failure of homologous chromosomes or sister chromatids to separate properly during meiosis, resulting in abnormal chromosome numbers.
Aneuploidy: Presence of an abnormal number of chromosomes (e.g., trisomy 21 causes Down syndrome).
Polyploidy: Organisms have more than two complete sets of chromosomes; common in plants.
Structural changes: Deletions, duplications, inversions, and translocations can disrupt gene function.
Type of Chromosomal Alteration | Description |
|---|---|
Deletion | Loss of a chromosome segment |
Duplication | Repeat of a chromosome segment |
Inversion | Reversal of a chromosome segment |
Translocation | Segment joins a nonhomologous chromosome |
Example: Cri du chat syndrome is caused by a deletion on chromosome 5.
Exceptions to Standard Mendelian Inheritance: Mitochondrial DNA
Some traits are inherited through extranuclear genes, such as mitochondrial DNA, which is passed only from the mother.
Mitochondrial disorders are inherited maternally.
Chloroplast DNA in plants is also inherited from the maternal parent.
These genes affect cellular respiration and energy production.
Additional info: All humans descended from a common maternal ancestor based on mitochondrial DNA.
Concept Check: Genetic Crosses and Calculations
Genetic crosses can be used to predict the probability of offspring inheriting certain traits.
Example: Crossing a green-wrinkled homozygous parent (yyrr) with a yellow-round heterozygous parent (YyRr) yields a 50% parental phenotype and 50% recombinant phenotype.
Equations:
Additional info: Punnett squares and test crosses are essential tools for predicting genetic outcomes.
