Applied Genetics and Analysis

• Chromosomal Basis:


  • Humans have 23 pairs of chromosomes, including one pair of sex chromosomes.
  • Females typically have two X chromosomes (XX).
  • Males typically have one X and one Y chromosome (XY).
• Role of Y Chromosome:


  • The presence of the Y chromosome typically determines male development.
  • The SRY gene on the Y chromosome initiates male development.
• X and Y Chromosome Differences:

  • The X chromosome is larger and contains many more genes than the Y chromosome.
  • The X chromosome carries about 800-900 genes.
  • The Y chromosome carries only about 50-60 genes, many related to male characteristics.
• X-inactivation:

  • In females, one X chromosome in each cell is randomly inactivated to balance gene expression with males.
• Sex-Linked Inheritance


  • Genes located on sex chromosomes are called sex-linked genes.
  • Most sex-linked traits are found on the X chromosome
• Genetic Basis:


  • Caused by mutations in genes for blood clotting factors (e.g., F8 gene for hemophilia A).
  • The allele is represented as a superscript on the X chromosome (e.g., X^h).
• Inheritance Pattern:


  • Males (XY) are more commonly affected as they only need one copy of the mutated gene.
  • Females (XX) can be carriers if they have one mutated copy and one normal copy.
• Punnett Square Example:


  • Mother (carrier): X^HX^h
  • Father (unaffected): X^HY
  • Possible offspring: X^HX^H, X^HX^h, X^HY, X^hY
• Probability:


  • 50% chance of a son being affected
  • 50% chance of a daughter being a carrier
• Understanding these concepts is crucial for:

  • Predicting inheritance patterns of sex-linked traits
  • Genetic counseling
  • Diagnosing and managing sex-linked disorders
• Purpose of Pedigree Charts

  Pedigree charts are graphical representations used to track the inheritance of traits through generations within a family. 
• Identifying Patterns of Inheritance: 


  By analyzing the relationships and traits of family members, one can deduce whether a genetic disorder is inherited in an autosomal dominant, autosomal recessive, or sex-linked manner.
• 1. Understanding Genetic Disorders: 


  Pedigree charts can illustrate how certain genetic conditions are passed from one generation to the next, helping to identify carriers and affected individuals.
• • Symbols of Pedigree Charts:

  • Circles represent females.
  • Squares represent males.
  • Shaded symbols indicate affected individuals, while unshaded symbols indicate unaffected individuals.
• • Lines of Pedigree Charts:

  • Horizontal lines connect mating pairs.
  • Vertical lines connect parents to their offspring.
• Analyzing Inheritance Patterns

  1. Autosomal Dominant
  2. Autosomal Recessive
  3. Sex-Linked Disorders
• Autosomal Dominant:


  • The trait typically appears in every generation.
  • Affected individuals have at least one affected parent.
• Autosomal Recessive:

  • The trait may skip generations.
  • Affected individuals can have unaffected parents who are carriers.
• Sex-Linked Disorders:


  • Often more common in males due to the presence of only one X chromosome.
  • Affected males cannot pass the trait to their sons but can pass it to daughters (who may become carriers).
• Genetic Risks: 


  Marriages between close relatives increase the likelihood of offspring inheriting recessive genetic disorders due to shared alleles. If both parents carry a recessive allele for a disorder, their children have a higher risk of being affected.
• Social and Legal Implications: 


  Many societies have laws or cultural practices that discourage or prohibit marriages between close relatives to reduce the risk of genetic disorders and promote overall population health.
• 1. Genetic Counseling: 


  Understanding pedigree charts is crucial for genetic counseling, where potential risks can be assessed based on family history.
• Continuous Variation


  Refers to the range of phenotypes that occur in a population for a particular trait, where the traits do not fall into distinct categories but instead show a gradual change.
• Polygenic Inheritance


  1. Multiple Genes Involvement: Continuous variation is often due to polygenic inheritance, where multiple genes (polygenes) contribute to a single trait.
  2. Additive Effects: Each gene may have an additive effect on the phenotype, leading to a wide range of possible outcomes.
• Environmental Factors


  1. Influence on Expression: Environmental factors can also play a significant role in the expression of traits. For example, nutrition, sunlight, and climate can affect phenotypic outcomes.
  2. Interaction with Genotype: The interplay between genetic predisposition and environmental conditions results in the observed continuous variation.
• Example: Skin Color in Humans

  • Polygenic Trait: Human skin color is influenced by multiple genes that control the amount and type of melanin produced.
  • Range of Phenotypes: This results in a spectrum of skin colors from very light to very dark, rather than discrete categories.
• Distinguishing Between Continuous and Discrete Variables


  1. Continuous Variables: Traits that can take any value within a range (e.g., skin color, height). These traits are measured on a scale and show gradual changes.
  2. Discrete Variables: Traits that fall into distinct categories (e.g., ABO blood groups). These traits are typically counted and do not show intermediate values.
• Measures of Central Tendency


  To analyze continuous data effectively, measures of central tendency are used:

  1. Mean: The average value calculated by summing all observations and dividing by the number of observations.
  2. Median: The middle value when all observations are ordered from lowest to highest; it effectively represents the center of the data set.
  3. Mode: The most frequently occurring value(s) in the data set.
• HemophiliaBack:

  • X-linked recessive blood clotting disorder
  • Caused by mutations in F8 (Hemophilia A) or F9 (Hemophilia B) genes
  • Symptoms: prolonged bleeding, easy bruising
  • More common in males
  • Inheritance:
  • Affected males pass to all daughters (carriers), no sons
  • Carrier females have 50% chance of passing to children
  • Treatment: clotting factor replacement
  • Diagnosis: blood tests, genetic testings