Chromosomal Basis of Inheritance Quiz

Chromosomal Basis of Inheritance

• Chromosome Theory of Inheritance: Mendelian genes have fixed locations (loci) on chromosomes, which segregate and assort independently during inheritance.

• Law of Segregation: Chromosomes are distributed equally to haploid gametes during reproduction, ensuring genetic diversity.

• Law of Independent Assortment: Traits are inherited independently; alleles of different genes segregate into gametes without influencing one another.

Genetic Studies and Model Organisms

• Thomas Hunt Morgan's Work: Utilized fruit flies (Drosophila melanogaster) due to rapid reproduction, low cost, and manageable chromosome number (four chromosomes).

• Eye Color in Fruit Flies: Higher incidence of white-eyed males than females indicated that the gene for eye color is located on the X chromosome.

Sex Determination Systems

• Various Sex Determination Systems: Include X-Y system in mammals, Z-W system in birds, X-O system in some insects, and haplo-diploid system in bees and ants.

• SRY Gene: Located on the Y chromosome, this gene is crucial for the development of testes and male sexual characteristics.

Sex-Linked Genes and Disorders

• Definition of Sex-Linked Genes: Genes found on sex chromosomes; most often refers to genes on the X chromosome.

• Examples of Sex-Linked Disorders: Duchenne muscular dystrophy, hemophilia, colorblindness.

• Duchenne Muscular Dystrophy: Characterized by muscle weakening and coordination loss, often fatal by early adulthood. Caused by lack of dystrophin protein.

• Hemophilia: An X-linked recessive disorder that leads to difficulty in blood clotting, historically associated with royal families.

X Chromosome Inactivation

• Barr Body: An inactive X chromosome found in females, which condenses to ensure dosage equality of X-linked genes.

• Process of X Inactivation: The inactive X chromosome is methylated, resulting in the formation of a Barr body during early embryonic development.

• Calico Cats: Female cats inherit one X chromosome with an allele for black and one for orange fur, leading to a tortoiseshell pattern.

Linked Genes and Genetic Mapping

• Linked Genes: Genes that are physically close on the same chromosome; tend to be inherited together rather than assorting independently.

• Parental Combination: Refers to the combination of linked genes that are inherited without recombination.

• Recombinants: Offspring that exhibit new genetic combinations due to crossing over during prophase I of meiosis.

• Alfred H. Sturtevant: A student of Morgan who created linkage maps based on recombination frequencies to estimate gene locations on chromosomes.

• Linkage Map: A representation showing the relative positions of genes based on recombination frequencies; measured in map units.

Chromosomal Abnormalities

• Nondisjunction: Failure of homologous chromosomes or sister chromatids to separate properly during cell division, causing aneuploidy.

• Aneuploidy: An abnormal number of chromosomes in a zygote, resulting from nondisjunction events.

• Monosomy: Condition where a cell is missing one chromosome; e.g., Turner syndrome.

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

Syndromes and Genetic Disorders

• Down Syndrome: Result of having an extra 21st chromosome, characterized by intellectual disabilities, unique facial features, and various health issues.

• Klinefelter Syndrome: Males with an extra X chromosome leading to various physical and developmental issues.

• Turner Syndrome: Female condition with a single X chromosome, often resulting in several physical abnormalities and infertility.

• Cri du Chat Syndrome: Deletion on chromosome 5, resulting in characteristic crying, small head, and developmental delays.

• Chronic Myelogenous Leukemia: Result of genetic mutations involving chromosomes 9 and 22, characterized by overproduction of white blood cells.

Genetic Alterations and Imprinting

• Genomic Imprinting: A process where only one allele of an imprinted gene is expressed depending on parental origin, affecting gene expression patterns.

• Extranuclear Genes: Genes inherited exclusively from the mother, such as mitochondrial DNA, passed from the egg cytoplasm.

• Diseases from Mitochondrial DNA: Include mitochondrial myopathy and Leber's hereditary optic neuropathy, affecting energy production and leading to muscle and vision problems respectively.

Mechanisms of Chromosomal Changes

• Chromosomal Deletion: Loss of a chromosomal fragment, potentially leading to loss of gene function.

• Chromosomal Duplication: Addition of an extra gene, which may disrupt normal function and lead to disease.

• Chromosomal Inversion: Reversal of the gene order on a chromosome, possibly impacting gene expression.

• Chromosomal Translocation: A chromosomal segment breaks off and attaches to a non-homologous chromosome, leading to genetic imbalances.