Chapter 15 study guide

Chapter 15: The Chromosomal Basis of Inheritance

Associating Genes with Specific Chromosomes

Early geneticists sought to connect Mendel's principles with physical structures in cells. The work of Thomas Hunt Morgan using Drosophila melanogaster (fruit flies) was pivotal in linking genes to chromosomes.

• Morgan's Experiments: Used fruit flies due to their simple genetics (only 4 pairs of chromosomes) and prolific breeding.

• Key Discovery: Eye color in fruit flies is linked to the sex of the fly, indicating that some genes are located on sex chromosomes.

• Sex Chromosomes: Males are XY, females are XX. Genes on sex chromosomes show unique inheritance patterns.

Sex-Linked Traits

Traits controlled by genes located on the sex chromosomes (primarily the X chromosome) are called sex-linked traits.

• X-Linked Genes: Most sex-linked genes are found on the X chromosome.

• Inheritance Patterns:

  • Males (XY) have only one X chromosome, so a single recessive allele on the X will be expressed.

  • Fathers pass X-linked genes to all daughters but never to sons.

  • Mothers can pass X-linked genes to both sons and daughters.

• Examples: Red-green color blindness, Duchenne muscular dystrophy, hemophilia, and fragile X syndrome.

Genotypes for Red-Green Color Blindness

• XNY: Normal male

• XNXN: Normal female

• XnY: Color-blind male

• XnXn: Color-blind female

• XNXn: Carrier female (normal phenotype)

Punnett Square Example: Carrier Female × Color-Blind Male

	Xn	Y
XN	XNXn (carrier female)	XNY (normal male)
Xn	XnXn (color-blind female)	XnY (color-blind male)

• 50% XNXn: Normal female (carrier)

• 50% XnXn: Color-blind female

• 50% XNY: Normal male

• 50% XnY: Color-blind male

X-Linked Recessive Disorders

• More Common in Males: Males are more frequently affected due to having only one X chromosome.

• Examples:

  • Red-green color blindness: 8% of Caucasian males affected.

  • Duchenne muscular dystrophy: Progressive muscle degeneration, early death.

  • Hemophilia: Deficiency of clotting factors VIII or IX, leading to excessive bleeding.

  • Fragile X syndrome: Most common inherited cause of intellectual disability.

Linked Genes and Genetic Recombination

Genes located close together on the same chromosome are called linked genes and tend to be inherited together. However, recombination can occur, producing new allele combinations.

• Linked Genes: Hundreds or thousands per chromosome (except Y chromosome).

• Genetic Recombination: Nonparental allele combinations are produced due to crossing over during meiosis.

• Crossing Over: Homologous chromosomes exchange segments, breaking linkage between genes.

Linkage Maps

• Definition: A genetic map based on recombination frequencies.

• Map Units: One map unit = 1% recombination frequency; indicates relative distance and order, not precise location.

Genetic Variation and Natural Selection

• Recombinant Chromosomes: Crossing over creates new allele combinations in gametes.

• Random Fertilization: Further increases genetic variation.

• Importance: Genetic variation is essential for evolution by natural selection.

Alterations of Chromosome Number or Structure

Large-scale chromosomal changes can cause genetic disorders or developmental abnormalities.

• Nondisjunction: Failure of homologous chromosomes or sister chromatids to separate properly during meiosis.

• Result: Gametes with abnormal chromosome numbers (e.g., 22 or 24 instead of 23 in humans).

Aneuploidy

• Definition: Abnormal number of a particular chromosome.

• Monosomic: Only one copy of a chromosome (2n-1).

• Trisomic: Three copies of a chromosome (2n+1).

Polyploidy

• Definition: More than two complete sets of chromosomes.

• Types:

  • Triploidy (3n): Three sets.

  • Tetraploidy (4n): Four sets.

• Common in Plants: Rare in animals.

Alterations in Chromosome Structure

• Causes: Radiation, viruses, chemicals.

• Types:

  • Deletion: Loss of a chromosome segment (e.g., Williams syndrome, cri du chat syndrome).

  • Duplication: Repetition of a segment, resulting in extra genetic material.

  • Inversion: Segment reverses orientation (e.g., inv dup 15 syndrome).

  • Translocation: Segment moves to a nonhomologous chromosome (e.g., Alagille syndrome).

Down Syndrome (Trisomy 21)

• Most Common Autosomal Trisomy: Three copies of chromosome 21.

• Symptoms: Short stature, flat face, eyelid folds, stubby fingers, intellectual disability, and other physical features.

• Incidence: About 1 in 830 births in the U.S.

• Maternal Age Effect: Frequency increases with maternal age.

Human Syndromes: Abnormal Sex Chromosome Number

Syndrome	Sex	Chromosomes	Chromosome #
Down	M/F	3 #21	47
Poly X	F	XXX(X)	47/48
Klinefelter	M	XX(X)Y	47/48
Jacobs	M	XYY	47
Turner	F	X	45

Descriptions of Syndromes

• Turner Syndrome (XO): Female, short stature, malformed features, possible heart/kidney defects, sterile. Only known human monosomy.

• Klinefelter Syndrome (XXY): Male, underdeveloped genitalia, long limbs, learning difficulties, sterile.

• Poly-X Females (XXX or XXXX): Tall, thin, generally healthy, possible learning disabilities.

• Jacobs Syndrome (XYY): Male, tall, acne, speech/reading problems.

Inheritance of Organelle Genes (Extranuclear Genes)

Some genes are located outside the nucleus, in organelles such as mitochondria and chloroplasts.

• Extranuclear Genes: Found in mitochondria, chloroplasts, and other plastids; typically small, circular DNA molecules.

• Maternal Inheritance: These genes are inherited from the mother because the egg provides most of the cytoplasm to the zygote.

• Evidence: Studies of variegated (yellow/white patches) leaves in plants demonstrated non-Mendelian inheritance patterns.