Chapter 12: The Chromosomal Basis of Inheritance – Study Guide

Chapter 12: The Chromosomal Basis of Inheritance

Key Concepts

• 12.1 Mendelian inheritance has its physical basis in the behavior of chromosomes

• 12.2 Sex-linked genes exhibit unique patterns of inheritance

• 12.3 Linked genes tend to be inherited together because they are located near each other on the same chromosome

• 12.4 Alterations of chromosome number or structure cause some genetic disorders

Mendelian Inheritance and Chromosomes

Physical Basis of Mendelian Inheritance

Mendelian inheritance is explained by the behavior of chromosomes during meiosis. Homologous chromosomes may have the same or different alleles, which is possible due to the independent assortment and segregation of chromosomes.

• Homologous Chromosomes: Chromosome pairs, one from each parent, that have the same genes but may carry different alleles.

• Wild Type: The most common phenotype in a population; the standard or 'normal' form.

• Mutant Phenotype: Any phenotype that differs from the wild type due to genetic mutation.

• Example: Thomas Hunt Morgan's work with Drosophila melanogaster (fruit flies) demonstrated that specific genes are located on specific chromosomes.

Genotype Notation Example:

• Fly homozygous for red eyes: XX

• Fly heterozygous for red eyes: Xx

• Fly homozygous for white eyes: xx

Sex-Linked Genes and Inheritance Patterns

Sex Chromosomes and Sex-Linked Traits

Sex-linked genes are located on sex chromosomes (X and Y). These genes exhibit unique inheritance patterns, especially in humans and other mammals.

• SRY Gene: Located on the Y chromosome; responsible for male sex determination.

• A gene located on a sex chromosome, typically the X chromosome.Sex-Linked Gene:

• Examples of Sex-Linked Disorders:

  1. Color blindness

  2. Duchenne muscular dystrophy

• Punnett Squares: Used to predict inheritance patterns of sex-linked traits.

Genotype	Phenotype
Male with normal vision	XY
Female carrier of color-blindness	XCXc
Female with color-blindness	XcXc

Linked Genes and Genetic Mapping

Gene Linkage and Crossing Over

Linked genes are located close together on the same chromosome and tend to be inherited together. Crossing over during meiosis can separate linked genes, creating new combinations.

• Linked Genes: Genes located near each other on the same chromosome.

• Crossing Over: The exchange of genetic material between homologous chromosomes during meiosis.

• Linkage Map: A diagram showing the relative positions of genes on a chromosome, based on recombination frequencies.

• Recombination Frequency: The percentage of recombinant offspring; used to estimate the distance between genes.

Equation:

• $\text{Recombination Frequency} = \frac{\text{Number of Recombinant Offspring}}{\text{Total Number of Offspring}} \times 100$

Gene Pair	Recombination Frequency (%)	Map Units (cM)
A-B	35	35
B-C	10	10
C-D	15	15
D-B	25	25

Alterations of Chromosome Number and Structure

Genetic Disorders from Chromosomal Changes

Alterations in chromosome number or structure can lead to genetic disorders. These changes include nondisjunction, aneuploidy, trisomy, and polyploidy.

• Nondisjunction: Failure of chromosomes to separate properly during meiosis.

• Aneuploidy: Abnormal number of chromosomes (e.g., monosomy, trisomy).

• Trisomy: Presence of an extra chromosome (e.g., Down syndrome, trisomy 21).

• Polyploidy: More than two complete sets of chromosomes.

• Down Syndrome: Caused by trisomy 21; characterized by intellectual disability, distinct facial features, and heart defects.

Syndrome	Sex	Physical Traits
XXY	Male	Klinefelter syndrome: tall stature, reduced fertility
XXX	Female	Triple X syndrome: often no physical symptoms
X0	Female	Turner syndrome: short stature, infertility
XYV	Male	Additional info: May have mild physical differences

Chromosome structure can be altered by deletion, duplication, inversion, or translocation. These changes can disrupt gene function and lead to disease.

• Deletion: Loss of a chromosome segment.

• Duplication: Repetition of a chromosome segment.

• Inversion: Reversal of a chromosome segment.

• Translocation: Movement of a segment to a nonhomologous chromosome.

Summary Table: Chromosomal Alterations and Disorders

Type of Alteration	Definition	Example
Nondisjunction	Failure of chromosomes to separate	Down syndrome
Aneuploidy	Abnormal chromosome number	Turner syndrome
Trisomy	Three copies of a chromosome	Trisomy 21
Polyploidy	More than two sets of chromosomes	Plants (e.g., wheat)

Additional info:

• Chromosomal theory of inheritance links Mendel’s laws to physical chromosome behavior.

• Sex-linked traits are more common in males due to the presence of only one X chromosome.

• Gene mapping is a key tool in genetics for understanding inheritance patterns and locating disease genes.