Study Notes: The Chromosomal Basis of Inheritance (Chapter 15)

The Chromosomal Basis of Inheritance

Introduction

The chromosomal basis of inheritance explains how genes are carried on chromosomes and how their behavior during meiosis accounts for inheritance patterns. This chapter integrates Mendelian genetics with cytological observations of chromosomes.

Relationship Between Genes and Chromosomes

Genes on Chromosomes

• Genes are located on chromosomes, which are duplicated before cell division.

• Each chromosome carries one version of a gene (allele).

• During meiosis I, homologous chromosomes separate and alleles segregate; in meiosis II, sister chromatids separate.

• Offspring inherit one allele from each parent.

Homologous Chromosomes

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

• Fertilization restores the diploid number, combining maternal and paternal chromosomes.

Chromosome Theory of Inheritance

Historical Development

• In 1902, Sutton and Boveri developed the chromosome theory of inheritance, linking Mendel’s laws to chromosome behavior.

Mendelian Laws and Chromosome Behavior

Law of Segregation

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

• Fertilization recombines alleles at random.

Law of Independent Assortment

• Alleles of genes on nonhomologous chromosomes assort independently during meiosis.

• Results in a 9:3:3:1 phenotypic ratio in the F2 generation for dihybrid crosses.

Experimental Evidence: Thomas Hunt Morgan and Drosophila

Association of Genes with Chromosomes

• Thomas Hunt Morgan provided the first solid evidence associating a specific gene with a specific chromosome using Drosophila melanogaster (fruit fly).

• He discovered a mutant fly with white eyes instead of the wild-type red eyes.

Sex-Linked Inheritance

• The gene for eye color is located on the X chromosome.

• Red eye allele (w+) is dominant; white eye allele (w) is recessive.

• Crosses showed that only males could have white eyes, indicating X-linked inheritance.

Punnett Square Example

	Xw+ (Egg)	Xw (Egg)
Xw+Y (Sperm)	Red-eyed female	Red-eyed female
XwY (Sperm)	Red-eyed male	White-eyed male

Sex-Linked Genes

X-Linked and Y-Linked Traits

• X-linked recessive disorders are much more common in males than females because males have only one X chromosome (hemizygous).

• Y-linked traits are passed from father to all sons.

• Short segments at the ends of the Y chromosome are homologous with the X, allowing pairing during meiosis.

• The SRY gene (sex-determining region on the Y) is responsible for development of testes in embryos.

Inheritance of X-Linked Genes

• X chromosomes carry many genes unrelated to sex.

• Most Y-linked genes are related to sex determination.

Homozygous vs Hemizygous

Genotype	Sex	Phenotype
XaXa	Female	Affected
XAXa	Female	Carrier
XaY	Male	Affected

Linked Genes

Definition and Behavior

• Linked genes are located on the same chromosome and tend to be inherited together.

• They do not assort independently, violating Mendel’s law of independent assortment.

Experimental Evidence

• Testcrosses with Drosophila showed that most offspring had parental phenotypes, but some had recombinant phenotypes due to crossing over.

Testcross Results Table

Phenotype	Number of Offspring
Wild type (gray-normal)	965
Black-vestigial	944
Gray-vestigial	206
Black-normal	185

Genetic Recombination

Recombinant Chromosomes

• Genetic recombination produces offspring with combinations of traits differing from either parent.

• Recombinant chromosomes bring alleles together in new combinations in gametes.

• Random fertilization further increases genetic variation.

Recombination Frequency Formula

Recombination frequency is calculated as:

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

Abnormal Chromosome Number

Nondisjunction

• Nondisjunction occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis.

• This results in gametes with extra or missing chromosomes.

Aneuploidy

• Aneuploidy is the result of fertilization involving gametes with abnormal chromosome numbers.

• A monosomic zygote has only one copy of a chromosome.

• A trisomic zygote has three copies of a chromosome.

• Imbalance of gene expression can lead to developmental disorders.

Down Syndrome (Trisomy 21)

Characteristics

• Down syndrome is an aneuploid condition resulting from three copies of chromosome 21.

• It affects about one out of every 830 children born in the United States.

• The frequency of Down syndrome increases with maternal age, possibly due to longer wait times for eggs to complete meiosis.

Summary Table: Chromosomal Abnormalities

Term	Definition	Example
Nondisjunction	Failure of chromosomes to separate during meiosis	Leads to aneuploidy
Aneuploidy	Abnormal number of chromosomes	Down syndrome (Trisomy 21)
Monosomy	One copy of a chromosome	Turner syndrome (XO)
Trisomy	Three copies of a chromosome	Down syndrome (Trisomy 21)

Key Terms

• Chromosome theory of inheritance

• Homologous chromosomes

• Allele

• Sex-linked gene

• Linked genes

• Genetic recombination

• Nondisjunction

• Aneuploidy

• Down syndrome

Additional info:

• These notes expand on the provided slides and images, adding definitions, examples, and tables for clarity and completeness.