2.3 Probability and Pedigree Analysis

Genetic inheritance follows patterns that can be predicted using probability. Mendel's laws of segregation and independent assortment form the basis for understanding how traits are passed down. Punnett squares help visualize these probabilities, showing possible offspring genotypes.

Pedigree analysis is a powerful tool for tracing genetic traits through families. By examining patterns of inheritance, we can identify autosomal dominant, autosomal recessive, and sex-linked traits. This knowledge helps predict the likelihood of inheriting specific genetic conditions.

Probability in Genetic Crosses

Probability in genetic inheritance

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• Mendel's laws of inheritance form the foundation for predicting trait inheritance
  • Law of segregation: Alleles separate during gamete formation resulting in gametes carrying only one allele for each gene (e.g., A or a)
  • Law of independent assortment: Alleles of different genes sort independently during gamete formation leading to new combinations of traits (e.g., AaBb, aaBB)
• Probability of inheriting a specific genotype depends on the type of genetic cross
  • Monohybrid cross: Probability of homozygous dominant $P(AA) = 1/4$, heterozygous $P(Aa) = 1/2$, and homozygous recessive $P(aa) = 1/4$
  • Dihybrid cross: Probability of double homozygous dominant $P(AABB) = 1/16$, double heterozygous $P(AaBb) = 1/4$, and double homozygous recessive $P(aabb) = 1/16$
• Probability of inheriting a specific phenotype is influenced by the dominance relationship between alleles
  • Dominant trait: Probability of expressing a dominant phenotype $P(A\_) = 3/4$ (e.g., purple flowers in pea plants)
  • Recessive trait: Probability of expressing a recessive phenotype $P(aa) = 1/4$ (e.g., white flowers in pea plants)

Punnett squares for inheritance visualization

• Punnett squares provide a visual representation of possible genotypes and their probabilities in a genetic cross
  • 2x2 square for monohybrid cross (e.g., Aa x Aa)
    1. Gametes from each parent are placed on the top and left side of the square
    2. Genotypes of offspring are determined by combining the alleles from each parent
  • 4x4 square for dihybrid cross (e.g., AaBb x AaBb)
    1. Gametes from each parent are placed on the top and left side of the square, considering all possible combinations of alleles
    2. Genotypes of offspring are determined by combining the alleles from each parent for both genes

Pedigree Analysis

Pedigree symbols and construction

• Pedigree symbols represent individuals and their characteristics
  • Squares denote males, while circles denote females
  • Shaded symbols indicate affected individuals, while unshaded symbols represent unaffected individuals
  • Half-shaded symbols represent carriers of recessive disorders
• Relationships between individuals are depicted using lines
  • Horizontal lines represent mating relationships (e.g., marriage)
  • Vertical lines show parent-offspring relationships
  • Siblings are offspring connected to the same parents

Generations in pedigree charts

• Generations are labeled with Roman numerals (I, II, III, etc.) from the top of the pedigree to the bottom
  • Generation I represents the oldest ancestors in the pedigree
  • Subsequent generations (II, III, etc.) represent the offspring of the previous generation

Characteristics of autosomal dominant inheritance

1. Affected individuals appear in every generation (e.g., Huntington's disease)
2. Affected offspring must have at least one affected parent
3. Male-to-male transmission is observed, as the gene is located on an autosome

Patterns of autosomal recessive inheritance

1. Affected individuals may skip generations, as carriers are unaffected (e.g., cystic fibrosis)
2. Unaffected parents can have affected offspring if both are carriers
3. Affected offspring are more likely from consanguineous marriages, which increase the chance of inheriting two recessive alleles

Features of X-linked inheritance

1. More affected males than females, as males only need one recessive allele to be affected (e.g., hemophilia)
2. No male-to-male transmission, as fathers pass the Y chromosome to their sons
3. Affected males inherit the trait from carrier or affected mothers

Traits of Y-linked inheritance

1. Only males are affected, as the gene is located on the Y chromosome (e.g., male infertility)
2. All affected males inherit the trait from their fathers, as the Y chromosome is passed from father to son

Carrier probability in recessive disorders

• Autosomal recessive disorders carrier probability
  • Carrier frequency in the general population varies by disorder (e.g., 1 in 25 for cystic fibrosis in Caucasians)
  • Probability of being a carrier based on family history
    1. One affected offspring: Both parents are obligate carriers (100% probability)
    2. Consanguineous marriage: Increased carrier probability due to shared ancestors

X-linked recessive carrier probability

• X-linked recessive disorders carrier probability in females
  • Carrier frequency in females is higher than in males (e.g., 1 in 5,000 females are carriers of Duchenne muscular dystrophy)
  • Probability of being a carrier based on family history
    1. Affected male offspring: Mother is an obligate carrier (100% probability)
    2. Affected female offspring: Mother is a carrier or affected, father is unaffected (50% chance if mother is a carrier)

Penetrance and expressivity in genetics

• Incomplete penetrance: Not all individuals with the genotype express the phenotype (e.g., BRCA1 mutations in breast cancer)
• Variable expressivity: Varying severity of the phenotype among affected individuals (e.g., neurofibromatosis type 1)

Genetic heterogeneity and phenotypes

• Genetic heterogeneity: Different genetic causes for the same phenotype (e.g., hearing loss can be caused by mutations in different genes)
• Locus heterogeneity: Mutations in different genes cause the same phenotype
• Allelic heterogeneity: Different mutations within the same gene cause the same phenotype

Environmental factors in genetic expression

• Environmental factors can influence the expression of genetic traits (e.g., diet affecting phenylketonuria severity)
• Gene-environment interactions: The interplay between genetic predisposition and environmental triggers (e.g., smoking and lung cancer risk)

Genetic testing for inheritance confirmation

• Genetic testing can help:
  1. Confirm the presence of specific genetic variants (e.g., CFTR mutations in cystic fibrosis)
  2. Determine carrier status for recessive disorders
  3. Predict the likelihood of developing a genetic condition (e.g., Huntington's disease)
  4. Guide family planning decisions and genetic counseling