The Chromosomal Basis of Inheritance

The Chromosomal Basis of Inheritance

Introduction

The chromosomal theory of inheritance connects Mendelian genetics with the behavior of chromosomes during meiosis. This chapter explores how genes are carried on chromosomes, how their behavior explains inheritance patterns, and how alterations in chromosome number or structure can lead to genetic disorders.

Concept 15.1: Mendelian Inheritance and Chromosome Behavior

Chromosomes and Genes

Genes are located on chromosomes, and their behavior during meiosis explains Mendel's laws of segregation and independent assortment.

• Chromosomes are structures within cells that contain DNA and carry genetic information.

• Each chromosome contains many genes, which are specific sequences of DNA that code for proteins.

• During meiosis, homologous chromosomes separate, ensuring that offspring inherit one allele from each parent.

• Mendel's Law of Segregation states that the two alleles for each gene separate during gamete formation.

• The Law of Independent Assortment states that alleles of genes on nonhomologous chromosomes assort independently.

Morgan’s Experiments with Drosophila

Thomas Hunt Morgan used the fruit fly Drosophila melanogaster to demonstrate that genes are located on chromosomes.

• Fruit flies are ideal for genetic studies due to their short generation time and few chromosomes.

• Wild type refers to the normal phenotype, while mutant phenotypes are alternatives caused by mutations.

• Morgan discovered a white-eyed mutant, linking the gene for eye color to the X chromosome.

Chromosome Theory of Inheritance

Morgan’s findings supported the chromosome theory of inheritance, which states that genes are carried on chromosomes and that their behavior during meiosis accounts for inheritance patterns.

Concept 15.2: Sex-Linked Genes and Inheritance Patterns

Sex Chromosomes and Sex Determination

Sex chromosomes determine the biological sex of an organism and carry genes that exhibit unique inheritance patterns.

• Humans have two types of sex chromosomes: X and Y.

• Females are XX; males are XY.

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

• Different organisms use different systems for sex determination (e.g., X-Y, X-0, Z-W, haplo-diploid).

Sex-Linked Genes

Genes located on sex chromosomes are called sex-linked genes. X-linked genes are found on the X chromosome, while Y-linked genes are found on the Y chromosome.

• The human X chromosome contains about 1,100 genes; the Y chromosome contains about 78 genes.

• Inheritance of X-linked genes shows unique patterns, as males have only one X chromosome.

Inheritance Patterns of X-Linked Genes

X-linked recessive disorders are more common in males because they have only one X chromosome.

• Examples include color blindness, Duchenne muscular dystrophy, and hemophilia.

• Females can be carriers if they are heterozygous for the gene.

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.

• If a female is heterozygous for an X-linked gene, she will express both alleles in different cells.

• This phenomenon is visible in the coat color of calico cats.

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.

• Linked genes do not assort independently.

• Morgan’s experiments with Drosophila showed that some gene pairs are inherited together more often than expected by chance.

Genetic Recombination and Crossing Over

Genetic recombination produces offspring with combinations of traits different from either parent. Crossing over during meiosis can separate linked genes, creating recombinant chromosomes.

• Parental types have phenotypes matching the parents; recombinant types have new combinations.

• The recombination frequency can be used to map the distance between genes on a chromosome.

Concept 15.4: Alterations of Chromosome Number or Structure

Nondisjunction and Aneuploidy

Nondisjunction occurs when chromosomes fail to separate properly during meiosis, leading to gametes with abnormal chromosome numbers.

• Aneuploidy is the presence of an abnormal number of chromosomes in a cell.

• A monosomic zygote has one chromosome missing; a trisomic zygote has an extra chromosome.

• Down syndrome (trisomy 21) is a common aneuploid condition in humans.

Aneuploidy of Sex Chromosomes

Nondisjunction of sex chromosomes can result in syndromes such as Klinefelter (XXY), Turner (X0), and others. These conditions often have less severe effects than autosomal aneuploidies.

Polyploidy

Polyploidy is the condition of having more than two complete sets of chromosomes. It is common in plants and can result in triploidy (3n) or tetraploidy (4n).

Alterations of Chromosome Structure

Chromosome breakage can lead to four types of structural changes:

• 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.

Disorders Caused by Chromosomal Alterations

Some genetic disorders are caused by structural changes in chromosomes. For example, cri du chat syndrome results from a deletion in chromosome 5, and chronic myelogenous leukemia (CML) is associated with a translocation between chromosomes 9 and 22 (the Philadelphia chromosome).

Summary Table: Types of Chromosomal Alterations

Type	Description	Example Disorder
Deletion	Removes a chromosomal segment	Cri du chat syndrome
Duplication	Repeats a segment	Charcot-Marie-Tooth disease
Inversion	Reverses a segment within a chromosome	Some forms of hemophilia A
Translocation	Moves a segment to a nonhomologous chromosome	Chronic myelogenous leukemia (CML)

Key Equations

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

Conclusion

The chromosomal basis of inheritance provides a framework for understanding how genes are transmitted from one generation to the next, how genetic variation arises, and how chromosomal abnormalities can lead to disease. Mastery of these concepts is essential for further study in genetics and molecular biology.