Ch. 15: Chromosomal Inheritance

Ch. 15: Chromosomal Inheritance

Physical Basis of Mendelian Inheritance

The chromosomal theory of inheritance explains how genes are located on chromosomes and how their behavior during meiosis accounts for inheritance patterns. This theory integrates Mendel's laws with the physical movement of chromosomes.

• Chromosomal Theory of Inheritance: Genes are located on chromosomes, and the behavior of chromosomes during meiosis explains inheritance patterns.

• Independent Assortment: Genes located on different chromosomes segregate independently during gamete formation.

• Thomas Hunt Morgan: Provided evidence for the chromosomal theory using fruit flies (Drosophila melanogaster).

Wild Type: The phenotype most commonly observed in natural populations.

Mutant Phenotype: Traits that differ from the wild type due to alleles assumed to have originated by mutation.

• Mendel’s Laws: The law of segregation and the law of independent assortment are explained by the behavior of chromosomes during meiosis.

Sex-Linked Genes

Sex-linked genes are located on sex chromosomes (X and Y). In humans, females have two X chromosomes, and males have one X and one Y chromosome. The inheritance of sex-linked traits differs between males and females.

• X-Linked Genes: Genes found on the X chromosome. Males are more likely to express recessive X-linked traits because they have only one X chromosome.

• Y-Linked Genes: Genes found on the Y chromosome. These are passed from father to son.

• Examples: Color blindness, hemophilia, and Duchenne muscular dystrophy are X-linked disorders.

Systems of Sex Determination

• XY System: Found in mammals. Females are XX, males are XY.

• ZW System: Found in birds, some fish, and insects. Females are ZW, males are ZZ.

• XO System: Found in some insects. Females are XX, males are XO.

• Haplo-Diploid System: Found in bees and ants. Females develop from fertilized (diploid) eggs, males from unfertilized (haploid) eggs.

X-Inactivation in Mammals

• One of the two X chromosomes in female mammals becomes inactivated during early embryonic development, forming a Barr body.

• This ensures dosage compensation between males and females.

Linked Genes

Linked genes are genes located close together on the same chromosome and tend to be inherited together. Genetic recombination can separate linked genes through crossing over during meiosis.

• Genetic Recombination: The production of offspring with combinations of traits differing from those found in either parent.

• Recombinant Frequency: The percentage of recombinant offspring; used to map gene locations on chromosomes.

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

Genetic Map: An ordered list of genetic loci along a chromosome. The probability that a crossover will occur between two genes is proportional to the distance between them.

• Map Units: 1 map unit = 1% recombination frequency.

Genetic Disorders from Chromosomal Alterations

Chromosomal mutations and errors during meiosis can lead to genetic disorders. These include changes in chromosome number or structure.

• Nondisjunction: Failure of homologous chromosomes or sister chromatids to separate properly during meiosis, resulting in gametes with abnormal chromosome numbers.

• Aneuploidy: The presence of an abnormal number of chromosomes in a cell (e.g., trisomy or monosomy).

• Polyploidy: The condition of having more than two complete sets of chromosomes (common in plants).

Chromosomal Structure Alterations

Structural changes in chromosomes can also cause genetic disorders.

• Deletion: Loss of a chromosome segment.

• Duplication: Repetition of a chromosome segment.

• Inversion: Reversal of a chromosome segment.

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

Human Conditions

• Down Syndrome: Caused by trisomy 21 (an extra chromosome 21). Symptoms include developmental delays, characteristic facial features, and increased risk of heart defects.

• Klinefelter Syndrome: Males with an extra X chromosome (XXY). Symptoms include sterility and some female characteristics.

• Turner Syndrome: Females with only one X chromosome (XO). Symptoms include short stature and infertility.

• Cri du Chat Syndrome: Caused by deletion of part of chromosome 5. Symptoms include intellectual disability and a cat-like cry in infants.

• Chronic Myelogenous Leukemia (CML): Caused by reciprocal translocation between chromosomes 22 and 9, producing the Philadelphia chromosome.

Exceptions to Mendelian Inheritance

Some inheritance patterns do not follow Mendel’s laws strictly. These include genomic imprinting and extranuclear inheritance.

• Genomic Imprinting: Expression of an allele depends on whether it is inherited from the mother or father.

• Extranuclear (Cytoplasmic) Genes: Genes located outside the nucleus, such as in mitochondria or chloroplasts.

Table: Types of Chromosomal Alterations and Associated Disorders

Alteration Type	Description	Example Disorder
Deletion	Loss of a chromosome segment	Cri du Chat Syndrome
Duplication	Repetition of a chromosome segment	Some developmental disorders
Inversion	Reversal of a chromosome segment	May affect phenotype
Translocation	Movement of a segment to a nonhomologous chromosome	Chronic Myelogenous Leukemia (CML)

Key Equations

• Recombination Frequency:

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

• Map Units:

$1 \text{ map unit} = 1\% \text{ recombination frequency}$

*Additional info: Expanded explanations and examples were added for clarity and completeness, including definitions and context for genetic disorders and chromosomal alterations.*