Genetic Inheritance Study Guide

Mendel's Three Laws

• Law of Segregation: During gamete formation, the two alleles responsible for a trait separate from each other. Each gamete carries only one allele for each trait.
• Law of Independent Assortment: Alleles for different traits are distributed to sex cells (& offspring) independently of one another.
• Law of Dominance: In a heterozygote, one allele may conceal the presence of another allele.

Key Genetic Terms

• Homozygous: Having two identical alleles for a particular gene (e.g., AA or aa).
• Heterozygous: Having two different alleles for a particular gene (e.g., Aa).
• Dominant: An allele that expresses its phenotype even in the presence of a recessive allele.
• Recessive: An allele that only expresses its phenotype when homozygous (e.g., aa).
• Genotype: The genetic constitution of an individual; the alleles present (e.g., Aa, AA, aa).
• Phenotype: The observable traits or characteristics of an organism resulting from the genotype.
• Autosome: Any chromosome that is not a sex chromosome (e.g., chromosomes 1-22 in humans).
• Sex chromosome: Chromosomes that determine the sex of an organism (X and Y in humans).
• Co-dominance: A form of inheritance where both alleles are fully expressed (e.g., AB blood type).
• Incomplete dominance: A form of inheritance in which the phenotype is a blending of the two parental traits.
• Test Cross: A way to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual.

Punnett Squares

• Be proficient in creating Punnett Squares to visualize the genetic crosses between parents.
• Practice working backwards from offspring genotypes to determine possible parent genotypes.

Blood Types and Genotypes

• Possible genotypes for blood phenotypes:
  • Type A: AA or AO
  • Type B: BB or BO
  • Type AB: AB
  • Type O: OO

Eye Color Inheritance

• Brown eyes (B) are dominant over blue eyes (b).
  • BB: Brown
  • bb: Blue
  • Bb: Brown

Blood Type Scenario

• Baby 1 (Type O) has possible parents with the following blood types:
  • Mrs. Brown: Type B
  • Mr. Brown: Type AB
  • Mrs. Smith: Type B
  • Mr. Smith: Type B
• Draw separate Punnett squares for each couple to determine probable parentage for Baby 1.

Gene Inheritance Patterns

• Understand sex-linked traits and how they differ in inheritance patterns for males and females.
• Detailed reasons why patterns of gene inheritance can be complex (e.g., multiple alleles, polygenic traits, environmental interactions).

Genetic Disorders

• Distinction between autosomal recessive and dominant genetic disorders.
• Examples include:
  • Autosomal Recessive: Cystic Fibrosis, Sickle Cell Anemia
  • Autosomal Dominant: Huntington's Disease, Marfan Syndrome

Chromosome Structure Changes

• Types of changes that can occur:
  • Deletion: Loss of a chromosome segment.
  • Duplication: Additional copies of a chromosome segment.
  • Inversion: A segment of a chromosome is reversed.
  • Translocation: A segment of one chromosome is transferred to another chromosome.

Nondisjunction and Aneuploidy

• Nondisjunction: Failure of homologous chromosomes or sister chromatids to separate during meiosis.
• Aneuploidy: An abnormal number of chromosomes (e.g., Down syndrome).

Requires Understanding of:

• Monosomy: Missing one chromosome from the pair.
• Trisomy: Having an extra chromosome (e.g., Trisomy 21).

Disorders Related to Nondisjunction

• Disorders include:
  • Down syndrome: Trisomy 21
  • Klinefelter syndrome: XXY male
  • Turner syndrome: X (monosomy of X chromosome)
  • Triple X syndrome: XXX female
  • XYY males: Additional Y chromosome leading to possible physical effects.

Phenotype and Genotype Distinction

• The physical characteristics of an organism (phenotype) are influenced by its genetic makeup (genotype).

Punnett Squares and Genetic Crosses

• A tool to predict genotype and phenotype ratios of offspring based on parental genotypes.
• For traits influenced by multiple alleles or dominance patterns (e.g., AB blood type or co-dominance), Punnett squares help visualize outcomes.

Sex-Linked Traits and Inheritance

• These traits are typically located on the X chromosome. Males (XY) have only one allele for these traits, making them more susceptible to express recessive traits such as color blindness.