Notes on The Chromosomal Basis of Inheritance

Mendel's "hereditary factors" are now identified as genes.
Genes are located on chromosomes, observable by tagging chromosomes with fluorescent dye.

Mitosis and meiosis were first described in the late 1800s.
The chromosome theory of inheritance states that Mendelian genes are found at specific loci on chromosomes.
Chromosomes undergo segregation and independent assortment during reproduction.
The behavior of chromosomes during meiosis explains Mendel's laws of segregation and independent assortment.

Law of Segregation: The two alleles for each gene separate during gamete formation.
Law of Independent Assortment: Genes on nonhomologous chromosomes assort independently of one another.
Gametes recombine during fertilization to produce a phenotypic ratio of 9:3:3:1 in the F2 generation.

Thomas Hunt Morgan provided the first solid evidence linking genes to specific chromosomes using the fruit fly, Drosophila melanogaster.
Characteristics of fruit flies:
- High offspring production.
- Rapid generation time (about every two weeks).
- Only four pairs of chromosomes.
Noted the presence of wild-type and mutant phenotypes (e.g., eye color).

Morgan's experiment involved crossing white-eyed male flies with red-eyed female flies:
- F1 generation: All had red eyes.
- F2 generation: Displayed a 3:1 ratio (red:white), with only males showing white eyes, indicating a sex-linked inheritance.
The white-eyed mutant allele is located on the X chromosome.

In humans, sex is determined by X (larger) and Y (smaller) chromosomes.
X-linked genes show specific inheritance patterns:
- Females: XX; Males: XY.
- A female must be homozygous (two copies) for a recessive trait to express it, whereas a male needs only one copy (hemizygous).
Recessive X-linked disorders (e.g., color blindness, Duchenne muscular dystrophy, hemophilia) are more common in males due to their hemizygous condition.

In female mammals, one X chromosome is randomly inactivated during early development, forming a Barr body.
Heterozygous females become mosaics for expression of X-linked traits, as some cells express one allele and others express the alternate allele.

Linked genes are located on the same chromosome and tend to be inherited together.
Morgan's experiments with fruit flies showed linked genes affect inheritance.
Nonparental phenotypes arise from genetic recombination via crossing over during meiosis.

Alfred Sturtevant constructed genetic maps based on recombination frequencies.
Genes far apart on a chromosome show higher recombination frequencies, while closely linked genes show less crossing over.
1 map unit corresponds to a 1% recombination frequency.

Large-scale chromosomal alterations can lead to developmental disorders or miscarriages.
Nondisjunction causes gametes to have abnormal numbers of chromosomes:
- Aneuploidy: Abnormal chromosome count, such as monosomy (1 copy) or trisomy (3 copies).
- Polyploidy: More than two sets of chromosomes, common in plants.
Types of chromosomal structural changes:
- Deletion: Segment is removed.
- Duplication: Segment is repeated.
- Inversion: Segment is reversed.
- Translocation: Segment is moved from one chromosome to another.

Down syndrome (Trisomy 21) is characterized by three copies of chromosome 21, with increased incidence in older mothers.
Syndromes like Klinefelter (XXY) and Turner Syndrome (X0) are results of sex chromosome nondisjunction.
Structural alterations can lead to syndromes like cri du chat, linked to specific deletions.

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