Genetics Part 2 Study Guide: MCDB 1A

Patterns of Genetic Inheritance

Introduction to Genetic Inheritance

Genetic inheritance describes how traits and diseases are passed from parents to offspring through genes. The study of inheritance patterns helps explain the transmission of characteristics and the occurrence of genetic disorders in populations.

• Gene: A segment of DNA that codes for a specific protein or trait.

• Allele: Different forms of a gene found at the same locus on homologous chromosomes.

• Dominant allele: An allele that masks the expression of another allele at the same locus.

• Recessive allele: An allele whose effects are masked in the presence of a dominant allele.

• Genotype: The genetic makeup of an organism (e.g., AA, Aa, aa).

• Phenotype: The observable traits of an organism.

Dominant and Recessive Alleles

Traits are often determined by pairs of alleles, one inherited from each parent. Dominant alleles are expressed even if only one copy is present, while recessive alleles require two copies to be expressed.

• Example: In pea plants, the allele for purple flowers (P) is dominant over the allele for white flowers (p).

• Homozygous dominant: Both alleles are dominant (PP).

• Homozygous recessive: Both alleles are recessive (pp).

• Heterozygous: One dominant and one recessive allele (Pp).

Mendelian Laws of Inheritance

Gregor Mendel discovered the basic principles of inheritance by breeding garden peas. His experiments led to the formulation of two key laws:

• Law of Segregation: During gamete formation, the two alleles for each gene separate, so each gamete receives only one allele.

• Law of Independent Assortment: Alleles of different genes assort independently during gamete formation, leading to genetic variation.

Punnett Squares and Probability

Punnett squares are used to predict the probability of offspring genotypes and phenotypes.

• Product Rule: The probability of independent events occurring together is the product of their individual probabilities.

• Sum Rule: The probability of mutually exclusive events is the sum of their individual probabilities.

Example Equation:

$P(\text{A and B}) = P(A) \times P(B)$ $P(\text{A or B}) = P(A) + P(B)$

Genetic Diseases and Evolutionary Benefits

Some genetic diseases are caused by mutations in specific genes. The inheritance pattern of these diseases can be dominant, recessive, or more complex. In certain cases, carrying a disease allele can confer evolutionary advantages.

• Cystic Fibrosis (CF): Caused by mutations in the CFTR gene. Inherited in a recessive manner (cc genotype).

• Cholera Resistance: Some CFTR mutations provide partial protection against cholera, an infectious disease caused by Vibrio cholerae.

• Heterozygote Advantage: Individuals with one normal and one mutated allele (Cc) may have increased fitness in certain environments.

• Sickle Cell Disease: Caused by mutations in the HBB gene. Heterozygotes (AS) are protected against malaria.

Exceptions to Mendelian Inheritance

Not all traits follow simple Mendelian patterns. Some exceptions include:

• Incomplete Dominance: The heterozygote phenotype is intermediate between the two homozygotes.

• Codominance: Both alleles are fully expressed in the heterozygote (e.g., ABO blood groups).

• Epistasis: The expression of one gene affects the expression of another gene (e.g., coat color in Labrador retrievers).

• Polygenic Inheritance: Traits are influenced by multiple genes, often resulting in continuous variation (e.g., height, skin color).

Table: Types of Genetic Inheritance

Type	Description	Example
Complete Dominance	Dominant allele masks recessive allele	Purple vs. white flowers in peas
Incomplete Dominance	Heterozygote is intermediate	Pink snapdragon flowers
Codominance	Both alleles expressed	ABO blood groups
Epistasis	One gene affects another's expression	Labrador coat color
Polygenic	Multiple genes influence trait	Height, skin color

Pedigree Analysis

Pedigrees are diagrams that show the inheritance of traits in families. They help determine the mode of inheritance and predict the risk of genetic diseases.

• Shaded symbols: Individuals affected by the disease.

• Unshaded symbols: Unaffected individuals.

• Carrier: An individual who carries one copy of a recessive allele but does not show symptoms.

Genetic Testing and Counseling

Genetic testing can determine carrier status and predict the probability of offspring inheriting genetic diseases. Genetic counseling uses Mendelian genetics and probability rules to advise families.

• Carrier testing: Identifies individuals who carry disease alleles.

• Probability calculations: Used to estimate risk for offspring.

Meiosis and Independent Assortment

Meiosis is the process by which gametes are formed, reducing chromosome number by half and ensuring genetic diversity through independent assortment and crossing over.

• Meiosis I: Homologous chromosomes separate, leading to random assortment of alleles.

• Crossing Over: Exchange of genetic material between homologous chromosomes during Prophase I increases genetic variation.

• Number of combinations: For humans, $2^{23} = 8,388,608$ possible gamete combinations due to independent assortment.

Glossary of Key Terms

• Gene: Unit of heredity.

• Allele: Variant form of a gene.

• Genotype: Genetic constitution.

• Phenotype: Observable traits.

• Dominant: Expressed allele.

• Recessive: Masked allele.

• Heterozygote: Two different alleles.

• Homozygote: Two identical alleles.

• Epistasis: Gene interaction.

• Polygenic: Multiple genes affect trait.

Additional info: Some context and explanations have been expanded for clarity and completeness, including definitions, examples, and the table summarizing inheritance types.