Chapter 15: The Chromosomal Basis of Inheritance

Relationship Between Genes and Chromosomes

The discovery of chromosomes provided the physical basis for Mendel's abstract laws of inheritance. Genes are located on chromosomes, which are structures that carry genetic information and vary in length and morphology. Chromosomes duplicate before cell division, resulting in sister chromatids that are partitioned into daughter cells during meiosis.

• Gene Location: Genes are specific sequences found at particular loci on chromosomes.

• Homologous Chromosomes: Chromosomes that carry the same types of genes but may have different alleles.

• Sister Chromatids: Duplicated chromosomes joined at the centromere, each containing identical genetic information.

• Alleles: Different versions of a gene found at the same locus on homologous chromosomes.

Chromosomal Basis of Mendel's Laws

Mendel's laws are explained by the behavior of chromosomes during meiosis. The law of segregation and the law of independent assortment are supported by the separation and alignment of chromosomes.

• Law of Segregation: The two alleles for each gene separate during meiosis I, specifically during anaphase I.

• Law of Independent Assortment: Alleles of genes on non-homologous chromosomes assort independently due to random alignment during metaphase I.

• Gamete Formation: Each gamete receives one allele for each gene, resulting in genetic diversity.

Dihybrid Crosses and Phenotypic Ratios

Crosses involving two traits (dihybrid crosses) demonstrate the law of independent assortment. The classic phenotypic ratio for a dihybrid cross is 9:3:3:1, representing the combinations of parental and recombinant phenotypes.

• Fertilization: Random recombination of alleles from gametes restores diploidy and produces offspring with varied phenotypes.

• Phenotypic Ratio: The 9:3:3:1 ratio is observed when both parents are heterozygous for two traits.

Chromosomal Theory of Inheritance: Model Organisms

Drosophila melanogaster as a Model Organism

The fruit fly Drosophila melanogaster was used by T.H. Morgan to demonstrate the chromosomal basis of inheritance. Drosophila is advantageous due to its short generation time, large number of offspring, and simple chromosome structure (four pairs).

• Wild Type vs. Mutant: Wild type refers to the common phenotype; mutant refers to deviations from the wild type.

• Sex Chromosomes: Drosophila has distinct X and Y chromosomes, allowing for studies of sex-linked inheritance.

Experimental Evidence for Chromosomal Inheritance

Morgan's experiments with Drosophila showed that traits such as eye color are linked to sex chromosomes. Crosses between wild type females and mutant males revealed unique patterns of inheritance in the F1 and F2 generations.

• P Generation: Wild type female (red eyes) × mutant male (white eyes).

• F1 Generation: All offspring have red eyes (wild type).

• F2 Generation: Female offspring all have red eyes; male offspring show a 50:50 ratio of red to white eyes.

Sex-Linked Genes and Patterns of Inheritance

Sex Chromosomes and Sex-Linked Genes

Sex chromosomes (X and Y) carry genes that determine sex and other traits. Genes located on sex chromosomes are called sex-linked genes. The X chromosome contains over 1000 genes, while the Y chromosome contains fewer than 100, most related to male development and fertility.

• X-Linked Genes: Genes found on the X chromosome; can affect traits unrelated to sex.

• Y-Linked Genes: Genes found on the Y chromosome; mostly related to male development.

• Hemizygosity: Males have only one X chromosome, so they express any allele present, whether dominant or recessive.

Sex Determination Systems

Different organisms use various systems for sex determination, including the XY system (humans, Drosophila), XO system (grasshoppers), ZW system (birds), and haplodiploid system (bees).

Inheritance of X-Linked Genes

X-linked genes exhibit unique inheritance patterns due to the hemizygosity of males. X-linked recessive traits are more commonly expressed in males, as they have only one X chromosome. Females can be carriers if they are heterozygous for the trait.

• Carrier Female: Heterozygous for an X-linked recessive trait; can pass the trait to offspring.

• Affected Male: Hemizygous for the recessive allele; expresses the trait.

• Inheritance Patterns: Punnett squares illustrate the possible outcomes for offspring based on parental genotypes.

Examples of X-Linked Traits

Red-green color blindness is a common X-linked recessive trait. Males are more frequently affected due to their hemizygosity. Females can be carriers or, rarely, affected if they inherit two recessive alleles.

• Carrier Mother: Can produce sons with a 50% chance of being affected.

• Affected Father: Can produce daughters who are carriers or affected, depending on the mother's genotype.

X Inactivation in Female Mammals

Dosage Compensation and Barr Bodies

In female mammals, one X chromosome in each cell is randomly inactivated during embryonic development, forming a Barr body. This ensures dosage compensation between males and females. Females heterozygous for X-linked genes can be mosaics, displaying two different phenotypes in different regions of their body.

• Barr Body: Inactivated X chromosome, condensed and transcriptionally silent.

• Mosaicism: Presence of two cell populations expressing different alleles due to random X inactivation.

• Example: Tortoiseshell cats display patches of different fur colors due to X inactivation.

Summary Table: Key Concepts

Key Equations

• Probability of offspring genotype (Punnett square):

• Phenotypic ratio for dihybrid cross:

Additional info:

• Hemizygosity in males means that X-linked recessive traits are more frequently expressed in males.

• X inactivation is random but permanent for all descendants of a cell.

• Sex determination systems vary among organisms and are not always based on sex chromosomes.