Mendel and the Chromosome Theory of Inheritance

Introduction to Chromosome Theory

The Chromosome Theory of Inheritance connects Mendel's laws of genetics to the behavior of chromosomes during meiosis. Thomas Hunt Morgan's experiments with fruit flies (Drosophila melanogaster) provided key evidence for this theory.

• Chromosome Theory: Genes are located on chromosomes, which segregate and assort independently during meiosis.

• Model Organism: Drosophila melanogaster is widely used in genetic studies due to its short life cycle and easily observable traits.

Morgan's Experiments: Eye Color in Fruit Flies

Morgan studied the inheritance of eye color in fruit flies, discovering that red eyes are the "wild-type" (most common) and white eyes are "mutant" (less common, resulting from mutation).

• Wild-type: The most common phenotype in a population.

• Mutant: A phenotype resulting from a genetic mutation, less common in the population.

• Dominance: Red eyes are dominant to white eyes.

• Reciprocal Crosses: Swapping the phenotypes of male and female parents can yield different results, indicating sex-linked inheritance.

Sex Chromosomes and Sex-Linked Inheritance

Segregation of Sex Chromosomes

Sex chromosomes (X and Y) segregate during meiosis, resulting in gametes that carry either an X or a Y chromosome. This determines the sex of the offspring.

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

• 50% of sperm contain an X chromosome; 50% contain a Y chromosome.

Autosomal vs. Sex-Linked Inheritance

Genes on non-sex chromosomes are autosomal, while those on sex chromosomes exhibit sex-linked inheritance.

• Autosomal Inheritance: Both males and females have two copies of each autosomal gene.

• X-linked Inheritance: Genes located on the X chromosome; males (XY) have one copy, females (XX) have two.

• Y-linked Inheritance: Genes located on the Y chromosome; only males inherit these traits (rare).

• Morgan proposed that the gene for red/white eye color in fruit flies is located on the X chromosome.

Reciprocal Crosses and X-Linked Traits

Reciprocal crosses help confirm X-linked inheritance by showing different outcomes depending on which parent carries the trait.

• Draw X and Y chromosomes, indicating the location of the X-linked allele.

• For X-linked traits, the allele is present only on the X chromosome, not the Y.

• In fruit flies, the letter used for the allele refers to the mutant form (capital letter still indicates dominance).

Punnett Square Practice: X-Linked Cross

Consider a cross between a heterozygous female (XRXr) and a red-eyed male (XRY):

• Genotypes: Female: XRXr; Male: XRY

• Punnett Square:

• Probability of white-eyed offspring: 1 out of 4 (25%)

• Probability daughters have white eyes: 0%

• Probability sons have white eyes: 50%

Extending Mendel’s Rules

Gene Linkage

Linkage refers to the tendency of genes located on the same chromosome to be inherited together. This is different from sex-linkage, which involves genes on sex chromosomes.

• Linked Genes: Genes close together on the same chromosome are often inherited together.

• Sex-Linkage: Genes located on sex chromosomes (X or Y).

Crossing Over and Recombinant Chromosomes

Crossing over during Prophase I of meiosis can separate linked genes, resulting in recombinant chromosomes and new genetic combinations.

• Crossing Over: Exchange of genetic material between homologous chromosomes.

• Recombinant Chromosomes: Chromosomes with new combinations of alleles.

• Punnett Squares cannot predict outcomes when crossing over occurs, as they assume independent assortment.

Genetic Mapping

The frequency of crossing over between genes is used to estimate their distance apart on a chromosome, measured in map units (centimorgans).

• Genetic Map: Diagram showing the relative positions of genes on a chromosome.

• Genes farther apart are more likely to undergo crossing over.

• For example, genes 20 map units apart have a 20% chance of producing recombinant offspring.

Other Extensions to Mendel’s Rules

Multiple Alleles and ABO Blood Groups

Some genes have more than two alleles, such as the I gene for human ABO blood types.

• Multiple Allelism: More than two alleles exist for a gene in a population.

• ABO Blood Types: Three alleles (IA, IB, i) produce four phenotypes (A, B, AB, O).

Incomplete Dominance

In incomplete dominance, heterozygotes express an intermediate phenotype between the two homozygotes.

• Example: In snapdragons, crossing red and white flowers produces pink offspring.

Pleiotropy

Pleiotropy occurs when a single gene affects multiple traits.

• Example: Marfan syndrome is caused by mutations in one gene, but affects connective tissue, heart, and eyes.

Environmental Effects on Traits

Some traits are influenced by environmental factors as well as genetics.

• Example: Phenylketonuria (PKU) is a genetic disorder that can be managed with a low-phenylalanine diet.

• Other traits influenced by environment include height and intelligence.

Quantitative Traits and Polygenic Inheritance

Quantitative traits, such as human height, are controlled by multiple genes (polygenic inheritance) and often show a continuous range of phenotypes.

• Polygenic Traits: Traits controlled by two or more genes.

• Distribution of quantitative traits often forms a normal curve in populations.

Applying Mendel’s Rules to Human Inheritance

Pedigree Analysis

Pedigrees are family trees used to study the inheritance of traits in humans, especially single-gene disorders.

• Symbols: Squares represent males, circles represent females; shaded shapes indicate affected individuals.

• Horizontal lines connect mating partners; vertical lines connect parents to offspring.

Autosomal and X-Linked Traits

Traits can be classified as autosomal dominant, autosomal recessive, X-linked dominant, or X-linked recessive.

• Autosomal Recessive: Trait appears only when both alleles are recessive; carriers are heterozygous and do not show the trait.

• Autosomal Dominant: Trait appears when at least one dominant allele is present.

• X-Linked Recessive: More common in males; inherited from mother.

• X-Linked Dominant: Trait appears in both males and females, but inheritance patterns differ.

Examples of Human Genetic Disorders

Pedigree Practice Problem

Freckles (F) is a dominant trait. A heterozygous woman (Ff) marries a man without freckles (ff). They have three children: one daughter with freckles, one daughter without freckles, and one son without freckles.

• Pedigree: Draw a family tree with appropriate symbols and shading.

• Genotypes: Woman: Ff; Man: ff; Daughter with freckles: Ff; Daughter without freckles: ff; Son without freckles: ff.

Additional Resources

• For more information on human genetic diseases, visit: Genome.gov

• Video resources on Huntington's Disease and other genetic disorders are available online.