Chapter 15 Lecture Notes

The Chromosomal Basis of Inheritance

Introduction

The chromosomal basis of inheritance explains how genes are transmitted from one generation to the next through chromosomes. This concept integrates Mendelian genetics with cytology, providing a framework for understanding genetic inheritance, sex determination, and chromosomal mutations.

Chromosome Theory of Inheritance

Key Principles

• Genes are located on chromosomes at specific positions called loci.

• Each chromosome contains dozens to thousands of genes, depending on its size.

• Chromosomes are the vehicles for genetic information, passing traits from parents to offspring.

Sex Chromosomes and Autosomes

Types and Structure

• Autosomes: Non-sex chromosomes; humans have 44 autosomes (22 pairs).

• Sex chromosomes: X and Y chromosomes determine biological sex in humans.

• The X chromosome is much larger than the Y chromosome and contains more genes.

• X and Y chromosomes are mostly non-homologous but share small homologous regions for pairing during meiosis.

Sex Determination in Humans

Mechanism

• Females: 44 autosomes + XX

• Males: 44 autosomes + XY

• The presence of the SRY gene (Testis Determining Factor, TDF) on the Y chromosome initiates male development.

• The sperm's sex chromosome determines the sex of the offspring.

Sex-Linked Genes and Inheritance Patterns

Location and Expression

• Genes located on the X or Y chromosome are called sex-linked genes.

• The X chromosome contains about 1000 genes, most unrelated to sexual characteristics.

• The Y chromosome contains about 30 genes, with ~15 important for male fertility.

• Males are hemizygous for most X and Y genes, having only one copy.

• This leads to unique inheritance patterns, especially for X-linked traits.

X-Linked Inheritance

• Alleles for X-linked genes are shown as superscripts on the X chromosome (e.g., XA, Xa).

• Y chromosome lacks most alleles present on X, so males express recessive X-linked traits if present.

• Females with one recessive allele are carriers (heterozygous), while males with the allele always express the trait.

Example: Red/Green Color Blindness

• An X-linked recessive trait affecting about 8% of males in the US.

• Females are less likely to express the trait unless they inherit two copies of the recessive allele.

Pedigree Analysis

• Pedigrees are used to trace inheritance patterns of traits through families.

• Example: Hemophilia B in European royal families, an X-linked recessive disorder.

Y-Linked Inheritance

• Traits determined by genes on the Y chromosome appear only in males.

• Example: Some forms of male infertility are Y-linked.

X Inactivation in Female Mammals

Mechanism and Consequences

• In females, one X chromosome in each cell is randomly inactivated early in embryonic development.

• This leads to mosaicism for X-linked traits (e.g., tortoiseshell cats).

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

Chromosomal Mutations

Types and Effects

• Alterations in chromosome number or structure can cause developmental disorders or miscarriage.

• Common types include aneuploidy (abnormal number) and structural mutations (changes in chromosome structure).

Aneuploidy

• Results from missing or extra chromosomes.

• Monosomy: Only one copy of a chromosome (e.g., Turner syndrome, XO).

• Trisomy: Three copies of a chromosome (e.g., Down syndrome, trisomy 21).

• Detected by karyotype analysis, which arranges chromosomes by size and number.

Cause: Meiotic Non-disjunction

• Non-disjunction occurs when chromosomes fail to separate properly during meiosis I or II.

• This leads to gametes with abnormal chromosome numbers, resulting in aneuploid offspring.

Structural Chromosomal Mutations

• Involve changes to large sections of DNA, affecting many genes.

• Types include:

  • Translocation: A segment moves to a different chromosome.

  • Inversion: A segment is reversed within the chromosome.

  • Duplication: A segment is repeated.

  • Deletion: A segment is lost.

• Causes include errors during crossing over, radiation, and carcinogens.

Transposable Elements

Definition and Impact

• Transposable elements ("jumping genes") are DNA segments that can move within the genome.

• They can promote unequal crossover events during meiosis, leading to duplications or deletions.

Summary Table: Chromosomal Disorders

Disorder	Cause	Chromosomes
Down Syndrome	Extra copy of chromosome 21	Trisomy 21
Patau Syndrome	Extra copy of chromosome 13	Trisomy 13
Edward's Syndrome	Extra copy of chromosome 18	Trisomy 18
Triple X Syndrome	Extra X chromosome in females	XXX
Turner Syndrome	Missing X in females	XO
Klinefelter Syndrome	Extra X chromosome in males	XXY
XYY Syndrome	Extra Y chromosome in males	XYY
Cri-du-Chat Syndrome	Deletion on chromosome 5	5p-
Fragile X Syndrome	Duplications on X	X
Acute Myelogenous Leukemia	Translocation between chromosomes 8 and 21	t(8;21)

Key Equations and Concepts

• Probability of offspring genotype (Punnett square):

$P(\text{genotype}) = \frac{\text{number of ways to obtain genotype}}{\text{total number of possible outcomes}}$

• Karyotype: An image showing all chromosomes in order of size, used to detect chromosomal abnormalities.

Applications and Examples

• Understanding inheritance patterns helps diagnose and predict genetic disorders.

• Pedigree analysis is crucial for genetic counseling and studying hereditary diseases.

• Karyotyping is used in prenatal diagnosis and cancer genetics.