Chapter 15: The Chromosomal Basis of Inheritance

Introduction to Chromosomal Inheritance

The Chromosomal Theory of Inheritance connects Mendel’s laws of inheritance with the physical behavior of chromosomes during meiosis. This chapter explores how genes are carried on chromosomes and how their behavior explains inheritance patterns observed by Mendel.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Genes for different traits assort independently if they are on different chromosomes.

Section 15.1: Chromosomes and Mendelian Inheritance

Morgan’s Experiments and Drosophila melanogaster

Thomas Hunt Morgan used the fruit fly, Drosophila melanogaster, as a model organism to demonstrate that genes are located on chromosomes. This provided the physical basis for Mendelian inheritance.

• Advantages of Drosophila:

  • Short life cycle (~3 weeks)

  • Produces hundreds of offspring

  • Four pairs of chromosomes (three autosomal, one pair of sex chromosomes: XX or XY)

  • Easy to culture in the laboratory

  • Large sample sizes allow for accurate statistical analysis

Mutations in Drosophila

Many mutations have been studied in fruit flies, including changes in eye color, wing shape, body color, and size. These mutations have been crucial for understanding gene function and inheritance patterns.

• Original mutation studied: Eye color (white vs. red)

• Other mutations: Wing size/shape, body color, and more

Fruit Fly Eye Color Experiment

Morgan’s experiments with eye color in fruit flies demonstrated X-linked inheritance, where the pattern of inheritance differs between males and females due to the presence of sex chromosomes.

Section 15.2: Sex-Linked Genes and Inheritance Patterns

Autosomal vs. Sex Chromosomes

• Autosomal Chromosomes: Chromosomes not involved in determining sex.

• Sex Chromosomes: Chromosomes (X and Y) that determine the sex of an organism.

Punnett Squares and Sex-Linked Traits

Sex-linked traits, especially those on the X chromosome, show unique inheritance patterns. Males (XY) are more likely to express X-linked recessive traits because they have only one X chromosome.

• Homozygous Dominant Female (XCXC): Cannot pass on the trait.

• Homozygous Recessive Male (XcY): Shows the trait.

• Carrier Female (XCXc): Can pass on the gene but does not show the trait.

Examples of X-Linked Disorders

• Hemophilia: Recessive disorder where blood does not clot properly due to missing proteins.

• Red/Green Colorblindness: X-linked trait; males need only one defective gene, females need two.

Normal Color Vision and Color Blindness

Humans have three types of cone cells for color vision (S, M, L cones). Color blindness results from defects in one or more types of cones.

• S-cones: Blue sensitivity (420–440 nm)

• M-cones: Green sensitivity (530–540 nm)

• L-cones: Red sensitivity (560–580 nm)

• Rods: Active in low light, no color vision

Types of Color Blindness

• Dichromacy: Only two types of cones function.

• Protanopia: No L-cones (red); X-linked recessive.

• Deuteranomaly: No M-cones (green); X-linked recessive.

• Tritanopia: No S-cones (blue); rare, not X-linked.

• Monochromacy: Total color blindness.

Sex-Linked Gene Inheritance

• Females:

  • XCXC – unaffected

  • XCXc – carrier

  • XcXc – affected

• Males:

  • XYC – unaffected

  • XYc – affected

X-Inactivation

In females, one X chromosome is randomly inactivated in each cell early in development. This leads to mosaic expression of X-linked genes, as seen in calico cats, where coat color patches result from different X chromosomes being active in different cells.

Section 15.3: Linked Genes and Genetic Mapping

Linked Genes and Recombinant Chromosomes

Linked genes are located close together on the same chromosome and tend to be inherited together. Crossing over during meiosis can produce recombinant chromosomes with new combinations of alleles.

• Parental types: Chromosomes with the same gene combinations as the parents.

• Recombinant types: Chromosomes with new combinations due to crossing over.

Recombination Frequency and Genetic Maps

Recombination frequency is used to create genetic linkage maps. The frequency of recombination between two genes is proportional to their distance apart on a chromosome.

• 1 map unit (MU): Equivalent to 1% recombination frequency.

• Genetic map: Shows relative positions of genes based on recombination frequencies.

• Cytogenetic map: Based on physical banding patterns on stained chromosomes.

• Physical map: Based on actual DNA sequence distances (in base pairs).

Section 15.4: Alterations of Chromosome Number or Structure

Nondisjunction and Aneuploidy

Nondisjunction occurs when homologous chromosomes fail to separate properly during meiosis, resulting in gametes with abnormal chromosome numbers (aneuploidy).

• Monosomic: Missing one chromosome (2n-1)

• Trisomic: Extra chromosome (2n+1)

Polyploidy

Polyploidy is the presence of extra complete sets of chromosomes. It is more common in plants and can sometimes provide advantages, such as increased resistance to certain diseases.

• Example: Wild strawberries (2N), cultivated strawberries (8N)

Human Disorders Due to Nondisjunction

• Down Syndrome (Trisomy 21): Physical growth delays, intellectual disability, characteristic facial features.

• Edward’s Syndrome (Trisomy 18): Small size, heart defects, severe intellectual disability.

• Patau Syndrome (Trisomy 13): Intellectual disability, motor disorder, polydactyly, kidney defects.

• Klinefelter Syndrome (XXY): Males with hypogonadism, infertility.

• Jacob’s Syndrome (XYY): Males, usually undiagnosed, taller, more acne.

• Triple-X Syndrome (XXX): Females, sometimes learning difficulties.

• Turner Syndrome (XO): Females, short stature, webbed neck, low fertility.

Changes in Chromosome Structure

• Cri du chat Syndrome: Deletion in the short arm of chromosome 5; severe intellectual disability, small head, cat-like cry.

Section 15.5: Exceptions to Standard Mendelian Inheritance

Genomic Imprinting

Genomic imprinting is an epigenetic phenomenon where certain genes are expressed or silenced depending on whether they are inherited from the mother or father. This can lead to disorders if the active copy is deleted or mutated.

• Prader-Willi Syndrome: Deletion of paternal chromosome 15; maternal gene inactive.

• Angelman Syndrome: Deletion of maternal chromosome 15; paternal gene inactive.

Mitochondrial Inheritance

Mitochondria are inherited exclusively from the mother because the egg provides the cytoplasm and organelles. Mitochondrial disorders affect cellular respiration and energy production.

• Three-Person Babies: A reproductive therapy to prevent mitochondrial disease by using mitochondrial DNA from a donor egg.

Y-Chromosome Inheritance

The Y chromosome is passed from father to son with little recombination, making it useful for tracing paternal lineage over many generations. Mutations accumulate and persist along a single paternal line.

Tracing Human Migration and Ancestry

Both mitochondrial DNA (maternal line) and Y-chromosome DNA (paternal line) are used to trace human ancestry and migration patterns. Different haplotypes and haplogroups reveal the history of human populations.

• Mitochondrial Eve: The most recent common matrilineal ancestor of all living humans (~150,000 years ago).

• Y Chromosome Adam: The most recent common patrilineal ancestor (160,000–300,000 years ago).

Case Study: The Lemba People

The Lemba, a Bantu ethnic group in southern Africa, have oral traditions of descent from ancient Israelites. Y-DNA analysis supports a paternal lineage connection to populations in Israel.