Sanjana Madhavan - (PV) Ch15 - The Chromosomal Basis Of Inheritance - 10667771

Pedigrees

  • Definition: A pedigree is a chart or "family tree" that depicts familial relationships and tracks a phenotype across generations.

    • Symbols Used:

      • Circles (⚪) represent females.

      • Squares (🟦) represent males.

      • Shaded/colored shapes indicate individuals affected by the phenotype.

  • Structure: Horizontal lines connect mating parents while vertical lines connect parents to their offspring.

    • Reading Pedigrees: Typically read from top to bottom, starting from the first generation.

  • Practice Example: Identify relationships between individuals on a pedigree.

    • Types of Charts:

      1. Pedigree charts

      2. Karyotypes

      3. Punnett squares

      4. Bell-shaped curves

Autosomal Inheritance

  • Definition: Autosomal inheritance refers to the transmission of traits/disorders through non-sex chromosomes and can be observed across generations.

  • Types of Genetic Disorders:

    1. Autosomal Dominant Disorders:

      • Occur in individuals with at least one dominant allele (e.g., FF or Ff).

      • Dominant disorders appear in every generation.

    2. Autosomal Recessive Disorders:

      • Occur in individuals who are homozygous recessive (e.g., aa).

      • Recessive disorders tend to skip generations.

  • Practice Example: If a disease appears in every generation on a pedigree, it is likely caused by an autosomal dominant disorder.

  • Pedigree Exercise: Determine if a pedigree depicts an autosomal recessive or dominant trait based on provided characteristics.

Autosomal Inheritance: Practice Exercise

  • Objective: Determine the inheritance pattern in a given pedigree and list genotypes of specified individuals in the correct order.

  • ABO Blood Type Group:

    • Example of autosomal inheritance.

    • Exercise: Fill in genotypes (IA , IB , i) and corresponding phenotypes (A, B, AB, or O blood type) in a pedigree chart.

Sex-Linked Inheritance

  • Definition: Inheritance patterns involving genes located on sex chromosomes (X or Y).

    • Sex Determination:

      • Female: XX

      • Male: XY

  • Sex-Linked Genes: Genes located on either sex chromosome; X chromosome has approximately 1,100 x-linked genes, Y chromosome has about 100 y-linked genes.

    • 50% chance of offspring being female or male at fertilization.

  • X-Linked Inheritance:

    • Females have two X-linked alleles and can be homozygous or heterozygous.

    • Males have one X-linked allele and express whatever allele is present on their X chromosome.

  • Practical Example: The inheritance pattern of eye color in fruit flies helps illustrate sex-linked inheritance.

Sex-Linked Inheritance: Practice Exercises

  • Red-Eyed vs White-Eyed Fruit Flies: Cross a white-eyed female with a red-eyed male; all offspring are red-eyed females and white-eyed males. This indicates a sex-linked inheritance pattern.

  • Thomas Hunt Morgan's Experiment: Crossing red-eyed F1 generation flies results in an F2 generation with red and white-eyed flies, explaining that all white-eyed flies are male due to X-linked recessive inheritance.

  • X-Linked Recessive Disorder (e.g., Hemophilia):

    • Characterized by abnormal blood clotting and expressed when an individual has recessive alleles on X chromosomes.

    • Males require only one recessive allele to be affected.

Inheritance Pattern and X-Linked Disorders

  • Gender-Specific Transmission: Women with X-linked disorders pass genes to all daughters; men pass their X-linked disorder genes to all sons.

  • X-Linked Recessive Disorder in Pedigrees: Pedigrees show more males affected than females; females can be affected only if their father is affected and their mother is at least a carrier.

  • Pedigree Analysis Practice: Analyze the inheritance pattern of a given pedigree determining whether it is X-linked recessive, dominant, autosomal, or Y-linked.

X-Inactivation

  • Concept: Females inherit double the number of X-linked genes; however, they do not express both.

    • During early development, one of the X chromosomes is randomly inactivated, forming a Barr body (the inactive X-chromosome).

  • Consequences of X-Inactivation: This random process can lead to females expressing different alleles in different cells, as seen in calico cats where coloration depends on X-linked genes.

  • Practice: Identify what constitutes a Barr body and predict offspring colors from black female and orange male cats based on their X-linked fur color alleles.