Chapter 15: The Chromosomal Basis of Inheritance

Big Idea: Chromosomes are made of genes that determine heredity

Chromosomes are structures within cells that contain DNA, the hereditary material. Genes, which are segments of DNA, are located on chromosomes and are responsible for the inheritance of traits from one generation to the next.

Concept 15.1: Mendelian Inheritance and Chromosome Behavior

The Chromosome Theory of Inheritance

The chromosome theory of inheritance states that genes are located on chromosomes, and the behavior of chromosomes during meiosis accounts for inheritance patterns. This theory was developed after biologists noticed parallels between Mendel's 'hereditary units' and the behavior of chromosomes during cell division.

• Mendel’s hereditary units were theoretical until the early 1900s.

• Sutton and Boveri independently proposed the chromosome theory of inheritance around 1902.

• Thomas Hunt Morgan provided the first solid evidence linking a specific gene to a specific chromosome using fruit flies (Drosophila melanogaster).

Genes and Chromosomes: Physical Evidence

Fluorescent tagging techniques allow scientists to visualize the location of specific genes on chromosomes, providing direct evidence for the physical basis of inheritance.

Relationship Between Genes and Chromosomes

Genes are located on chromosomes, which duplicate before cell division. During meiosis, homologous chromosomes separate, ensuring that each gamete receives one chromosome from each pair. This process explains Mendel’s laws of segregation and independent assortment.

• Each chromosome carries many genes.

• Homologous chromosomes have the same genes at the same loci, but may have different alleles.

• Meiosis ensures genetic diversity through the separation and recombination of chromosomes.

Morgan’s Experiments with Fruit Flies

Choice of Experimental Organism

Drosophila melanogaster (fruit fly) was chosen for genetic studies due to its short generation time, high offspring number, and only four pairs of chromosomes. Morgan distinguished between wild type (normal) and mutant phenotypes, such as red eyes (wild type) and white eyes (mutant).

Scientific Inquiry: Eye Color Inheritance in Fruit Flies

Morgan crossed a white-eyed male with a red-eyed female. All F1 offspring had red eyes, but in the F2 generation, only males had white eyes, suggesting the gene for eye color is located on the X chromosome.

• Females have two X chromosomes (XX), males have one X and one Y (XY).

• The white-eyed trait is recessive and X-linked.

The Chromosomal Basis of Mendel’s Laws

Mendel’s law of segregation and law of independent assortment can be explained by the behavior of chromosomes during meiosis. The separation of homologous chromosomes and the random assortment of chromosome pairs lead to genetic variation.

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

• Law of Independent Assortment: Alleles of different genes assort independently if they are on different chromosomes.

Concept 15.2: Sex-Linked Genes and Inheritance Patterns

Sex Chromosomes and Sex Determination

Sex chromosomes (X and Y) determine the biological sex of an individual. The X-Y system is found in humans and many other mammals, but other systems exist in different organisms.

• XX = female, XY = male in humans.

• The SRY gene on the Y chromosome triggers male development.

• Other systems: X-0 (insects), Z-W (birds), haplo-diploid (bees).

Inheritance of X-Linked Genes

X-linked genes exhibit unique inheritance patterns. Males (XY) are hemizygous for X-linked genes, so recessive traits are more commonly expressed in males. Females (XX) must inherit two copies of a recessive allele to express the trait.

• Hemizygous: Having only one allele for a gene (as in males for X-linked genes).

• X-linked recessive disorders are more common in males (e.g., color blindness, hemophilia).

X Inactivation in Female Mammals

In female mammals, one X chromosome in each cell is randomly inactivated during embryonic development, forming a Barr body. This leads to mosaicism for X-linked traits in heterozygous females.

• Barr body: The condensed, inactive X chromosome in female cells.

• Example: Calico cats have patches of different fur colors due to X inactivation.

Concept 15.3: Linked Genes and Genetic Recombination

Linked Genes

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

• Parental types: Offspring with phenotypes matching the parents.

• Recombinant types: Offspring with new combinations of traits.

• A 50% recombination frequency indicates genes are on different chromosomes (unlinked).

Genetic Recombination: Crossing Over

Crossing over during meiosis can break the physical connection between genes on the same chromosome, resulting in genetic recombination. This process increases genetic diversity in offspring.

• Crossing over occurs between homologous chromosomes during prophase I of meiosis.

• Linked genes can be separated if crossing over occurs between them.

Abnormalities in Chromosome Number and Structure

Nondisjunction and Aneuploidy

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis, leading to gametes with abnormal chromosome numbers. Fertilization of such gametes results in aneuploidy.

• Aneuploidy: Abnormal number of chromosomes (e.g., trisomy, monosomy).

• Monosomic: Only one copy of a chromosome.

• Trisomic: Three copies of a chromosome.

Alterations of Chromosome Structure

Chromosome breakage can lead to structural changes:

• Deletion: Loss of a chromosomal segment.

• Duplication: Repetition of a segment.

• Inversion: Reversal of a segment within a chromosome.

• Translocation: Movement of a segment to a nonhomologous chromosome.

Human Conditions Due to Chromosomal Alterations

Alterations in chromosome number and structure can cause various syndromes. Some aneuploidies are survivable and result in characteristic symptoms.

• Down syndrome (Trisomy 21): Caused by three copies of chromosome 21; risk increases with maternal age.

• Klinefelter syndrome (XXY): Males with an extra X chromosome.

• Turner syndrome (XO): Females with only one X chromosome; the only viable monosomy in humans.

Additional info: Other chromosomal disorders include XYY males (usually normal) and XXX females (may have learning disabilities).