Chromosomal Basis of Inheritance

Introduction

The chromosomal theory of inheritance connects the behavior of chromosomes during meiosis and fertilization to Mendel’s laws of inheritance. This theory explains how genes are carried on chromosomes and how their segregation and independent assortment during meiosis account for the patterns of inheritance observed by Mendel.

Mendel’s Laws and Chromosome Behavior

The Law of Segregation

Definition: The law of segregation states that each individual has two alleles for each gene, which segregate (separate) during gamete formation, so that each gamete receives only one allele.

• During meiosis, homologous chromosomes (and thus alleles) separate into different gametes.

• This explains why offspring inherit one allele from each parent.

• Example: In pea plants, the allele for round seeds (R) and the allele for wrinkled seeds (r) segregate during gamete formation.

The Law of Independent Assortment

Definition: The law of independent assortment states that alleles of different genes assort independently of one another during gamete formation, provided the genes are on different chromosomes.

• During metaphase I of meiosis, homologous chromosome pairs align independently at the metaphase plate.

• This leads to genetic variation in gametes.

• Example: The inheritance of seed shape (R/r) and seed color (Y/y) in peas occurs independently if the genes are on different chromosomes.

Chromosome Behavior During Meiosis

Meiosis is the process by which diploid cells produce haploid gametes. The behavior of chromosomes during meiosis explains Mendel’s laws:

• Homologous chromosomes separate during anaphase I (segregation).

• Different chromosome pairs align independently during metaphase I (independent assortment).

Equation:

where n is the number of chromosome pairs.

Genetic Crosses and Phenotypic Ratios

Monohybrid and Dihybrid Crosses

• Monohybrid cross: Involves one gene (e.g., Rr x Rr). The F2 generation shows a 3:1 phenotypic ratio.

• Dihybrid cross: Involves two genes (e.g., RrYy x RrYy). The F2 generation shows a 9:3:3:1 phenotypic ratio if genes assort independently.

Gamete Formation and Chromosome Segregation

• Each gamete receives one chromosome from each homologous pair.

• Random orientation of chromosome pairs during metaphase I leads to genetic diversity.

Linked Genes and Exceptions to Independent Assortment

Linked Genes

Genes located close together on the same chromosome tend to be inherited together and do not assort independently. These are called linked genes.

• Linked genes violate the law of independent assortment.

• Crossing over during meiosis can separate linked genes, creating new combinations.

• Linkage: The tendency of genes on the same chromosome to be inherited together.

Genetic Recombination

• Crossing over during prophase I of meiosis exchanges genetic material between homologous chromosomes.

• This process increases genetic variation by producing recombinant chromosomes.

Sex Determination in Humans

Sex Chromosomes

In humans and many animals, sex is determined by a pair of sex chromosomes: X and Y.

• Females: XX

• Males: XY

• Eggs always carry an X chromosome; sperm carry either an X or a Y chromosome.

• The sex of the offspring is determined by whether the sperm contributes an X or a Y chromosome.

Autosomes vs. Sex Chromosomes

• Humans have 22 pairs of autosomes (non-sex chromosomes) and 1 pair of sex chromosomes.

• Autosomes are inherited independently of sex chromosomes.

Table: Comparison of Autosomes and Sex Chromosomes

Summary of Key Terms

• Allele: Different forms of a gene.

• Homologous chromosomes: Chromosome pairs, one from each parent, that are similar in shape, size, and genetic content.

• Gamete: A reproductive cell (sperm or egg) with half the number of chromosomes of a somatic cell.

• Phenotype: Observable traits of an organism.

• Genotype: Genetic makeup of an organism.

• Crossing over: Exchange of genetic material between homologous chromosomes during meiosis.

Checkpoint Questions

• Which of Mendel’s laws have their physical basis in the orientation of pairs of homologous chromosomes in metaphase I of meiosis? Answer: The law of segregation.

• Why do genes located on the same chromosome often not assort independently? Answer: Because they are linked and tend to be inherited together unless separated by crossing over.

Additional info: Some explanations and definitions have been expanded for clarity and completeness based on standard biology textbooks.