chromosomal inheritance

The Chromosomal Basis of Inheritance

The Gene Hunt

  • Early 20th Century Significance:

    • Mendel's Rediscovery: Mendel's concepts of heredity were rediscovered but remained abstract.

    • Rejection of Mendel's Laws: Many scientists dismissed Mendel's laws until a physical basis could be demonstrated.

    • Current Understanding: Today, it is known that genes are located on chromosomes.

The Chromosomal Theory of Inheritance

  • Chromosome Behavior in Meiosis:

    • Chromosomes undergo two main processes during meiosis:

    • Segregation: The behavior of homologous chromosomes accounts for the Law of Segregation.

    • Independent Assortment: The behavior of non-homologous chromosomes accounts for the Law of Independent Assortment.

F1 Generation Example

  • Parental Generation (P): Yellow round seeds represented by the genotypes G: YyRr.

  • Meiosis Stages: Exhibits segregation in metaphase I and II during gamete formation.

  • Laws Demonstrated:

    • Law of Segregation: The two alleles for each gene separate during gamete formation.

    • Law of Independent Assortment: Alleles on non-homologous chromosomes assort independently.

  • Punnett Square Analysis: Distribution of alleles during gamete formation for genes Y and R showed multiple combinations.

Thomas Hunt Morgan

  • Background: Thomas Hunt Morgan was a professor at Columbia University, known for his work in genetics, particularly using the fruit fly as a model organism.

  • Contributions: Morgan provided crucial evidence that Mendel’s genes are found on chromosomes and identified the first specific gene - eye color in fruit flies.

The Fruit Fly Model Organism

  • Scientific Name: Drosophila melanogaster.

  • Advantages as Model Organism:

    • High offspring count.

    • Short generation time, approximately every two weeks.

    • Possesses only four pairs of chromosomes.

The First Mutant in Drosophila

  • Morgan's Experiments:

    • Bred fruit flies searching for mutations.

    • Phenotypes Identified:

    • Wild Type (wt): Normal red-eyed flies denoted as w+.

    • Mutant: White-eyed flies described by w.

White Eyed Flies Inheritance

  • Cross Study: White-eyed males (w) crossed with red-eyed females (w+).

    • F1 Generation Results: All progeny exhibit the red-eye phenotype (w+).

  • F2 Generation Results: The ratio observed was 3:1 for red (w+) to white (w) eyes, conforming to Mendelian predictions.

Sex-linked Inheritance in White Eyed Flies

  • Observation: Only male offspring expressed the white-eyed phenotype.

    • Interpretation: The mutant allele is located on the X chromosome:

    • Females require two recessive alleles (ww) for white eyes.

    • Males only need one mutated allele (w0) because they have only one X chromosome.

The Chromosomal Basis of Sex

  • Sex Chromosomes in Mammals: Two types found in mammals:

    • X Chromosome: Large.

    • Y Chromosome: Much smaller with only a small homologous region with X.

  • Genetic Compositions:

    • Females typically possess two X chromosomes (XX).

    • Males generally have one X and one Y chromosome (XY).

The Y Chromosome Roles

  • Size: Significantly smaller than X. Contains only 78 genes.

  • Key Genes:

    • SRY (Sex-determining Region on Y): Responsible for testes development.

    • Default development leads to ovaries unless SRY is present.

Sex-Linked Genes Overview

  • Definition: Genes located on sex chromosomes, specifically the X or Y chromosome.

  • Inheritance Data:

    • Fathers pass X chromosomes solely to daughters.

    • Mothers pass X chromosomes to both daughters and sons, facilitating different inheritance patterns.

Patterns of Inheritance for Sex-Linked Genes

  • Recessive X-Linked Traits:

    • Females (XX) require two copies of the allele (homozygous) to express the phenotype.

    • Males (XY) require only one copy (hemizygous), leading to higher prevalence of X-linked disorders in males.

    • Example Disorders: Color blindness, muscular dystrophy, hemophilia.

Cinnabar Eyes Trait in Fruit Flies

  • Sex-linked Recessive Trait: For a female with cinnabar eyes crossed with a wild-type (wt) male, the possible percentage of F1 males exhibiting cinnabar eyes calculated as follows:

    • Options: 0%, 25%, 50%, 75%, 100%.

Linked Genes Principles

  • Definition of Linked Genes: Genes located close to each other on the same chromosome, often inherited together contrary to independent assortment principles.

  • Morgan's Investigations: Studied two characters:

    • Body color (gray - b+ or black - b)

    • Wing size (normal - vg+ or vestigial - vg)

Inheritance Observations from Linked Genes Study

  • Observed distinct proportions of phenotypes differing from independent assortment predictions:

    • Typical outcomes from unlinked genes versus observed traits showed significant discrepancies in ratios.

Nonparental Phenotypes in Linked Genes

  • Definition: Distinctiveness in genetic combinations leading to phenotypes not present in either parent.

  • Cause: Genetic recombination occurring through crossing over during prophase I of meiosis, influencing linkage between genes.

Recombination of Unlinked Genes

  • Parental Types vs. Recombinant Types:

    • Parental types: Offspring that resemble parents phenotypically.

    • Recombinant types: Offspring displaying non-parental phenotypes indicating crossover results.

    • Standard frequency of recombination for genes on different chromosomes is 50 %.

Recombination of Linked Genes Insights

  • Incomplete Linkage: Morgan found some recombinant phenotypes when examining linkage, proposing crossover during prophase.

  • Animation Example: Illustrates crossing over during meiosis leading to genetic diversity.

Recombinant Chromosomes and Testcrosses

  • F1 dihybrid Testcross Results: Generation outcomes fluctuate based on the genetic makeup from crossbreeding.

  • Recombination Frequency Calculation:

    • Determined by the ratio of recombinant phenotypes to total offspring, yielding significant insights into gene linkage.

Linkage Maps and Genetic Distance

  • Alfred Sturtevant's Contribution: Developed genetic maps based on recombination frequency.

  • Map Units Concept:

    • One map unit or centimorgan represents a 1% recombination frequency.

    • Provides relative distances and order but not precise locations of genes.

Chromosomal Disorders Overview

  • Types of Chromosomal Alterations in Humans:

    • Abnormal chromosome numbers and structural changes,

    • Can result in spontaneous abortions or numerous developmental disorders.

Aneuploidy Definitions and Types

  • Nondisjunction Explanation: Refers to the failure of homologous chromosome pairs to separate correctly during meiosis, leading to aneuploidy which describes an abnormal chromosome number.

  • Types of Aneuploidy:

    • Monosomic: Zygote with one less chromosome (2n - 1).

    • Trisomic: Zygote with an extra chromosome (2n + 1).

    • Aneuploidy is a major cause of pregnancy loss, occurring in 10-25 % of pregnancies.

Polyploidy Discussions

  • Definition: Condition where an organism has more than two sets of chromosomes.

    • Triploidy (3n) and Tetraploidy (4n) are common types.

    • More prevalent in plants than animals, generally displaying a more normal appearance than aneuploid organisms.

Alterations of Chromosome Structure Overview

  • Types of Chromosomal Breakage Changes:

    1. Deletion: A segment is removed.

    2. Duplication: A segment is duplicated, creating multiple copies.

    3. Inversion: A segment is reversed.

    4. Translocation: A segment moves from one chromosome to a non-homologous chromosome.

Mendelian Exceptions Comprehension

  • Nuclear and Extra-nuclear Factors:

    • Analysis of how parental sex factors influence inheritance patterns.

    • Involves concepts such as genomic imprinting and the inheritance of organelle genes.

Genomic Imprinting Concept

  • Definition: Phenotype expression is dependent on the parental origin of alleles due to gene silencing mechanisms.

    • Occurs via methylation affecting gene expression, particularly involved in embryonic development genes like insulin-like growth factor 2 (Igf2).

Genomic Imprinting Examples**

  • Homozygous Normal vs. Mutant Igf2 Expression: Demonstrates phenotypic outcomes varying in size depending on parental allele contributions.

Inheritance of Organelle Genes Principles

  • Location and Nature: Extra-nuclear genes are found in organelles like mitochondria and chloroplasts and typically exhibit maternal inheritance due to zygote cytoplasm originating from the egg.

  • Example Phenotype: Inheritance of leaf color variation in plants can be attributed to maternal gene contributions.