Chromosomal and Molecular Basis of Inheritance

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Chromosomal and Molecular Basis of Inheritance

Introduction

This study guide covers the chromosomal and molecular foundations of inheritance, focusing on how genes are transmitted, the structure and function of chromosomes, and the molecular mechanisms underlying genetic information transfer. Key experiments and concepts in classical and molecular genetics are included.

Relationship Between Genes and Chromosomes

Genes and Chromosomes

Genes are segments of DNA that encode functional products, usually proteins.
Genes are located on chromosomes, which are long DNA molecules associated with proteins.
Each chromosome carries many genes; homologous chromosomes carry the same genes but may have different alleles.
During meiosis, homologous chromosomes separate, ensuring each gamete receives one version of each gene.

Example: Inheritance of eye color in Drosophila (fruit flies) demonstrates gene location on chromosomes.

Sex Determination and Sex Chromosomes

Chromosomal Basis of Sex

Humans and many animals have two types of sex chromosomes: X and Y.
Females: XX; Males: XY.
The presence of a Y chromosome typically determines male development (SRY gene).

Other Sex Determination Systems

X-0 system: Some insects (e.g., grasshoppers); females XX, males X0.
Z-W system: Birds, some fish; females ZW, males ZZ.
Haplo-diploid system: Bees and ants; females diploid, males haploid.

Sex Linkage and Inheritance Patterns

Sex-Linked Genes

Genes located on sex chromosomes are called sex-linked genes.
X-linked genes: Found on the X chromosome; human X chromosome has ~1,100 genes.
Many X-linked genes are unrelated to sex determination.

Inheritance of X-Linked Genes

X-linked recessive traits are more common in males (only one X chromosome).
Females need two copies of the allele to express the trait; males need only one (hemizygous).

X Inactivation 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.

Gene Linkage and Recombination

Linked Genes

Genes located close together on the same chromosome tend to be inherited together.
Genes within 50 map units are considered linked genes.

Genetic Recombination

Genetic recombination produces offspring with combinations of traits differing from either parent.
Unlinked genes assort independently (Mendel's law of independent assortment).
Linked genes can be separated by crossing over during meiosis.

Linkage Maps

Linkage maps show the relative positions of genes on a chromosome based on recombination frequencies.
1 map unit = 1% recombination frequency.

Chromosomal Behavior and Abnormalities

Abnormal Chromosome Number

Nondisjunction: Failure of homologous chromosomes or sister chromatids to separate properly during meiosis.
Aneuploidy: Abnormal number of chromosomes (e.g., monosomy, trisomy).
Polyploidy: More than two complete sets of chromosomes; common in plants.

Alterations of Chromosome Structure

Deletion: Removes a chromosomal fragment.
Duplication: Repeats a segment.
Inversion: Reverses orientation of a segment within a chromosome.
Translocation: Moves a segment from one chromosome to another.

Examples of Chromosomal Disorders

Down syndrome (Trisomy 21): Three copies of chromosome 21; frequency increases with maternal age.
Klinefelter syndrome (XXY): Extra X chromosome in males.
Turner syndrome (X0): Monosomy X in females; only known viable human monosomy.
Cri du chat syndrome: Deletion on chromosome 5; intellectual disability and catlike cry.

Exceptions to Standard Inheritance Patterns

Genomic Imprinting

Phenotype depends on which parent contributed the allele.
Involves silencing of certain genes depending on parental origin.
Example: Igf2 gene in mice.

Inheritance of Organelle Genes

Extranuclear genes are found in organelles (mitochondria, chloroplasts).
Inherited maternally because the zygote's cytoplasm comes from the egg.
Mutations in mitochondrial DNA can cause diseases affecting muscular and nervous systems (e.g., mitochondrial myopathy).

Molecular Basis of Inheritance

DNA as the Genetic Material

Experiments by Griffith, Avery, Hershey, and Chase demonstrated that DNA is the genetic material, not protein.
Chargaff's rules: In DNA, the amount of adenine (A) equals thymine (T), and guanine (G) equals cytosine (C).

Structure of DNA

Watson and Crick, using Franklin's X-ray crystallography data, deduced the double helix structure of DNA.
DNA consists of two antiparallel strands with complementary base pairing (A-T, G-C).

Table: Types of Chromosomal Alterations

Type	Description	Example/Effect
Deletion	Removes a chromosomal fragment	Cri du chat syndrome
Duplication	Repeats a segment	May cause developmental abnormalities
Inversion	Reverses orientation of a segment	Usually less harmful, but can affect gene expression
Translocation	Moves a segment to another chromosome	Can cause cancers (e.g., CML)

Key Equations and Concepts

Map unit (centimorgan): 1% recombination frequency = 1 map unit
Chargaff's rules: and

Example Poll Question (from notes)

Question: The agouti gene in mice plays a role in determining coat color. At this locus, the genotype AA produces an “agouti coat,” and the heterozygote Aa produces a yellow coat. The aa homozygotes, however, die very early in development. What is the expected phenotypic ratio of live mice resulting from a cross of two Aa mice?

Genotypic ratio: 1 AA : 2 Aa : 1 aa (but aa die)
Phenotypic ratio (live mice): 1 agouti : 2 yellow

Summary

Genes are located on chromosomes, which segregate and assort independently during meiosis.
Sex determination and sex-linked inheritance involve specialized chromosomes and unique inheritance patterns.
Gene linkage, recombination, and chromosomal abnormalities contribute to genetic diversity and disease.
DNA is the molecular basis of inheritance, with a double helix structure and specific base pairing.