Bio - Heredity

Bio-Heredity

Incomplete Dominance

  • Definition: One allele is not completely expressed over its paired allele.
  • Result: The heterozygous genotype exhibits an intermediate phenotype (Example: Red x White = Pink).

Codominance

  • Definition: Heterozygous genotype expresses both alleles.
  • Result: For example, Red x White = Red-White spots.

Epistasis

  • Definition: One gene affects the expression of a different gene, impacting phenotypes and traits.

Pleiotropy

  • Definition: One gene is responsible for multiple traits.

Polygenic Inheritance

  • Definition: Many genes are responsible for a single trait, contributing to continuous variation in the phenotype.

Gene Defects

Haploinsufficiency
  • Definition: One copy of a gene is lost or nonfunctional, and the expression of the remaining copy is insufficient to produce a normal phenotype.
  • Result: This leads to an intermediate phenotype.
HaploSufficiency
  • Definition: The remaining copy of the gene is sufficient to result in a normal phenotype.
Proto-oncogenes
  • Definition: Genes that can mutate into oncogenes (cancer-causing genes) due to gain-of-function mutations.
  • Characteristics: These mutations may result in excessive protein production or the creation of overactive proteins, leading to cancerous growth.
One Hit Hypothesis
  • Concept: A gain-of-function mutation in one copy of the gene is sufficient to turn it into an oncogene.
Tumor Suppressor Genes
  • Definition: Genes that suppress cancerous growth.
  • Characteristics: Tumor suppressor genes can become cancerous due to loss-of-function mutations.
Two Hit Hypothesis
  • Concept: A loss-of-function mutation in both copies of a tumor suppressor gene is necessary to cause cancer.
  • Note: Such genes are often haplosufficient.
P53 Gene
  • Definition: A crucial tumor-suppressor gene known as the "guardian of the cell."
  • Function: It is upregulated to prevent cells from becoming cancerous.
P21 Gene
  • Definition: A tumor-suppressor gene that inhibits phosphorylation activity, which helps to decrease runaway cell division.
Retinoblastoma Gene (RB)
  • Definition: A tumor-suppressor gene coding for a retinoblastoma protein.
  • Function: Prevents excessive cell growth during interphase.
Null Alleles
  • Definition: Mutations that result in alleles lacking normal function.
  • Context: Tumor suppressor genes acquire null alleles when they become cancer-causing.

Mendel's Laws

Law of Dominance

  • Definition: Dominant alleles mask the expression of recessive alleles.

Law of Segregation

  • Definition: Homologous chromosomes separate during meiosis (specifically during anaphase I) allowing one allele to be passed down.
  • Example: An individual with genotype Aa will produce gametes containing either A or a alleles.

Law of Independent Segregation

  • Definition: Homologous chromosomes line up independently during metaphase I of meiosis, leading to random separation of alleles.
  • Result: Increases genetic variability. (Note: This independent segregation does not occur in metaphase II.)
Nondisjunction
  • Definition: An improper segregation of chromosome pairs during anaphase.
Outcomes of Nondisjunction
  • Single nondisjunction results in different abnormal chromosome counts in daughter cells:
    • Homologous chromosomes → 24, 24, 22, 22 chromosomes in diploid daughter cells.
    • Sister chromatids during meiosis I → 46 chromosomes in parent cells.
    • Sister chromatids during meiosis II → 46 chromosomes in parent cells leading to haploid daughter cells showing aneuploidy.
Specific Cases of Nondisjunction
  • Aneuploidy: Abnormal number of chromosomes in daughter cells.
    • Trisomy: 3 copies of a chromosome.
    • Monosomy: 1 copy of a chromosome.
    • Disomy: Normal diploid cell (2 copies of each chromosome).

Genetic Processes in Meiosis

Crossing Over

  • Definition: A process occurring in prophase I of meiosis, promoting genetic diversity.
  • Mechanism: Homologous chromosomes join to form tetrads and exchange genetic material at chiasmas.
Gametes
  • Recombinant Gametes: Gametes that receive genetically unique chromatids, exhibiting new combinations of alleles.
  • Non-recombinant Gametes: Gametes that receive chromatids that match those of the parents.

Mapping Units (m.u.)

  • Definition: A mapping unit represents a 1% chance of recombination between two genes.
    • Recombination frequencies less than 50% indicate linked genes.
    • A recombination frequency of 0.5 corresponds to unlinked genes, which assort independently, with a 50% chance of recombination.

Sex-Linked Traits

  • Definition: Traits associated with genes located on sex chromosomes.
  • Most disorders related to sex chromosomes are X-linked.

Types of X-Linked Traits

X-Linked Dominant
  • Definition: Dominant inheritance occurring on the X chromosome.
  • Result: Any offspring that inherits the affected allele will have the disorder.
X-Linked Recessive
  • Definition: Recessive inheritance occurring on the X chromosome.
  • Characteristics: Males require only one affected allele to express the disorder. Females, having two X chromosomes, require one affected allele each to exhibit the disorder.

Y-Linked Traits

  • Definition: Traits inherited through the Y chromosome.
  • Characteristics: This inheritance can only be passed from father to son and will always be expressed in males due to having only one Y chromosome.

Pedigree Charts

  • Purpose: Tools for tracking inherited traits through multiple generations.
    • Females are represented by circles.
    • Males are represented by squares.
    • Individuals affected by the trait are shaded; unaffected individuals are unshaded.

Epigenetics

Definition of Epigenetics

  • Concept: Epigenetic changes do not alter the genetic code but involve the regulation of gene expression.

Mechanisms of Epigenetics

DNA Methylation
  • Definition: A modification that suppresses gene activity.
  • Mechanism: Methyl groups are added to DNA, impeding transcription factors from binding to the gene and initiating transcription.
Histone Acetylation
  • Definition: A process that promotes gene activation.
  • Effect: Results in the formation of euchromatin (easily accessible DNA), allowing for gene activation.
Histone Deacetylation
  • Definition: A modification that suppresses gene activity.
  • Effect: Leads to heterochromatin formation (hard-to-access DNA), thus resulting in gene suppression.
Histone Methylation
  • Definition: A complex process that can either upregulate or downregulate gene expression.
  • Dependency: The effect depends on the location and number of methyl groups added to histones.