Chapter 15 Biology

The Chromosomal Basis of Inheritance

Introduction

The chromosomal theory of inheritance connects Mendel’s principles of genetics to the physical behavior of chromosomes during meiosis. This chapter explores how genes are carried on chromosomes, how their behavior explains inheritance patterns, and how chromosomal alterations can lead to genetic disorders.

Concept 15.1: Mendelian Inheritance and Chromosomes

Morgan’s Choice of Experimental Organism

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

• Wild type: Most common phenotype in a population.

• Mutant phenotype: Traits differing from the wild type.

• Chromosome theory of inheritance: Genes are located on chromosomes, and their behavior during meiosis explains inheritance patterns.

• Example: Morgan’s discovery of white-eyed mutants in fruit flies supported the chromosomal theory.

Relationship Between Genes and Chromosomes

Genes are located on chromosomes, which duplicate before cell division. During meiosis, homologous chromosomes and their alleles segregate, leading to genetic variation in offspring.

• Alleles: Different versions of a gene.

• Homologous chromosomes: Chromosome pairs, one from each parent, carrying the same genes at the same loci.

• Meiosis: Process that separates homologous chromosomes and sister chromatids.

• Fertilization: Combines maternal and paternal chromosomes in offspring.

Concept 15.2: Sex-Linked Genes and Patterns of Inheritance

Sex Chromosomes and Sex Determination

Humans and other mammals have two types of sex chromosomes: X and Y. Females have two X chromosomes (XX), while males have one X and one Y (XY). The SRY gene on the Y chromosome determines male development.

• X chromosome: Contains about 1,100 genes.

• Y chromosome: Contains about 78 genes, including SRY.

• SRY gene: Triggers development of testes in males.

• Sex-linked gene: Located on either sex chromosome.

Systems of Sex Determination

Different organisms use various systems for sex determination, including X-Y (humans), X-0 (insects), Z-W (birds), and haplo-diploid (bees).

• X-Y system: Males are XY, females are XX.

• X-0 system: Males have one X, females have two Xs.

• Z-W system: Males are ZZ, females are ZW.

• Haplo-diploid system: Males are haploid, females are diploid.

Inheritance of X-Linked Genes

X-linked genes exhibit unique inheritance patterns. Disorders caused by recessive alleles on the X chromosome are more common in males, as they have only one X chromosome.

• Examples: Color blindness, Duchenne muscular dystrophy, hemophilia.

• Carrier female: Heterozygous for a recessive X-linked allele.

• Affected male: Hemizygous for a recessive X-linked allele.

X Inactivation in Female Mammals

In female mammals, one X chromosome in each cell is randomly inactivated during embryonic development, forming a Barr body. Heterozygous females are mosaics for X-linked traits.

• Barr body: Inactive X chromosome in female cells.

• Mosaic: Presence of two cell populations with different active X chromosomes.

• Example: Calico cats display mosaic fur coloration due to X inactivation.

Concept 15.3: Linked Genes and Genetic Recombination

Linked Genes

Genes located near each other on the same chromosome tend to be inherited together and are called linked genes. Morgan’s experiments with fruit flies showed that linked genes do not assort independently.

• Linked genes: Genes on the same chromosome inherited together.

• Genetic recombination: Production of offspring with combinations of traits different from either parent.

• Crossing over: Exchange of genetic material between homologous chromosomes during meiosis, creating recombinant types.

Concept 15.4: Chromosomal Alterations and Genetic Disorders

Alterations of Chromosome Number

Abnormal chromosome number can result from nondisjunction, where chromosomes fail to separate properly during meiosis. This leads to aneuploidy, such as trisomy (three copies) or monosomy (one copy).

• Nondisjunction: Failure of chromosome separation during meiosis.

• Aneuploidy: Abnormal number of chromosomes.

• Trisomy: Three copies of a chromosome (e.g., Down syndrome).

• Monosomy: One copy of a chromosome (e.g., Turner syndrome).

Down Syndrome (Trisomy 21)

Down syndrome is caused by trisomy 21, resulting in developmental and physical abnormalities. The risk increases with maternal age.

• Symptoms: Intellectual disability, characteristic facial features.

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

Aneuploidy of Sex Chromosomes

Nondisjunction of sex chromosomes leads to syndromes such as Klinefelter (XXY), Turner (X0), and others. Some conditions are viable and may have mild or no symptoms.

• Klinefelter syndrome: XXY males, often with mild symptoms.

• Turner syndrome: X0 females, sterile.

• XXX females: Usually healthy, may have learning disabilities.

• XYY males: Typically normal.

Polyploidy

Polyploidy is the presence of more than two complete sets of chromosomes, common in plants but rare in animals. Polyploids are often more normal in appearance than aneuploids.

• Triploidy (3n): Three sets of chromosomes.

• Tetraploidy (4n): Four sets of chromosomes.

Alterations of Chromosome Structure

Chromosome breakage can lead to structural changes: deletion, duplication, inversion, and translocation. These alterations can cause genetic disorders.

• Deletion: Loss of a chromosome segment.

• Duplication: Repetition of a segment.

• Inversion: Reversal of a segment within a chromosome.

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

• Example: Cri du chat syndrome (deletion on chromosome 5), chronic myelogenous leukemia (translocation).

Summary Table: Types of Chromosomal Alterations

Type	Description	Example Disorder
Deletion	Removes a chromosomal segment	Cri du chat syndrome
Duplication	Repeats a chromosomal segment	Some cancers
Inversion	Reverses a segment within a chromosome	May cause infertility
Translocation	Moves a segment to a nonhomologous chromosome	Chronic myelogenous leukemia

Key Equations

• Recombination frequency: $\frac{\text{Number of recombinant offspring}}{\text{Total number of offspring}} \times 100$