Heredity Flashcards

Key Terms

• Gene: A basic unit or sequence of genetic material that encodes a trait.

• Locus: A gene's specific location within a genome.

• Allele: A variant form of a gene.

  • Wild type: The normal, most common version of an allele.

  • Mutant: An allele with an altered DNA sequence, potentially affecting the gene's phenotype.

• Genotype: The specific combination of alleles an individual carries for a particular gene (e.g., Aa).

• Phenotype: The observable characteristics of an individual, resulting from the expression of their genotype (e.g., genotype Aa might produce brown eyes, while aa produces blue eyes).

• Homologous chromosomes: A pair of chromosomes, one maternal and one paternal, that contain the same genes in the same locations.

• Hemizygous: Having only one copy of a gene instead of the usual two, such as the sex chromosomes in males (XY).

• Heterozygous: Carrying two different alleles for a gene on homologous chromosomes, one dominant and one recessive (e.g., Aa).

• Homozygous: Carrying two identical alleles for a gene on homologous chromosomes.

  • Homozygous Dominant: Possessing two copies of the dominant allele (e.g., BB).

  • Homozygous Recessive: Possessing two copies of the recessive allele (e.g., bb).

Mendel's Laws of Inheritance

• The Law of Segregation: During Anaphase 1 of meiosis, homologous chromosomes separate, resulting in haploid gametes that contain only one allele per gene.

• The Law of Independent Assortment: The lining up and separation of one pair of homologous chromosomes during meiosis does not influence the lining up and separation of any other pair of homologous chromosomes, provided the genes are on different chromosomes.

• The Law of Dominance: A dominant allele will mask the effect of a recessive allele when present in a heterozygous individual.

Punnett Squares & Test Crosses

• Punnett squares: Diagrams that show all possible allelic combinations of gametes in a cross between two individuals with known genotypes.

• Test Cross: A genetic cross between a homozygous recessive individual and an individual of unknown genotype to determine the latter's genotype for a specific trait. The unknown individual is crossed with a homozygous recessive individual. The offspring phenotypes reveal the unknown parent’s genotype.

• Monohybrid cross: A cross that tests a single gene.

• Dihybrid cross: A cross that tests two different genes simultaneously.

• Test cross generations:

  • P1: Parental generation

  • F1: First generation of offspring

  • F2: Second generation of offspring

Patterns of Inheritance

• Epistasis: One gene affects the phenotypic expression of another independently inherited gene. For example, baldness where one gene determines if an individual can express hair color and a second gene determines the hair color if the first gene allows.

• Multiple Alleles: More than two alleles exist for a particular gene within a population (e.g., the A, B, and O alleles in the human ABO blood type system). This does not mean an individual can have more than two alleles, just that there are more than two allele options in the population..

• Polygenic Inheritance: Many genes interact to shape a single phenotype, often resulting in continuous variation (e.g., height, skin color). The more genes involved, the more continuous the variation.

• Pleiotropy: A single gene controls the expression of multiple phenotypic traits (e.g., a single gene in a plant might control its height, color, and texture).

• Incomplete Dominance: A blending of alleles, producing a unique heterozygous phenotype. For example, (R \text{ red}) x (W \text{ White}) = (RW \text{ pink}) .

• Codominance: Both alleles are completely expressed in the heterozygous condition. For example, (R \text{ Red}) x (W \text{ White}) = (RW \text{ Red & White speckled})

Phenotypic Expression

• Sex-linked genes: Genes located on a sex chromosome (usually the X chromosome).

• Sex-influenced genes: Genes whose expression can be influenced by the sex of the individual carrying the trait. For example, males and females might carry the same genes for hair, but males might be bald while females are not.

• Linked genes: Genes that reside close together on a chromosome, making them less likely to be separated by recombination during meiosis, and therefore more likely to be inherited together.

Linkage Map

• A linkage map uses recombination frequency to show the relative positions of genes on a chromosome.

• The closer together the genes are, the less likely they are to be separated by recombination.

X-inactivation

• During embryonic development in female mammals, one of the two X chromosomes is randomly inactivated, forming a highly condensed chromosome called a Barr body. This ensures that females, like males, have only one functional copy of the X chromosome in each cell.

Aneuploidy

• The occurrence of an abnormal number (extra or missing) of chromosomes, often caused by nondisjunction.

  • Trisomy 21 (Down syndrome): Having three copies of chromosome 21 instead of the normal two copies.

Nondisjunction

• The failure of chromosomes or chromatids to separate properly during mitosis or meiosis, which can result in gametes with too many or too few chromosomes.

Phenotype Variability

• Penetrance: The proportion of individuals with a specific genotype who will express the corresponding phenotype. If penetrance is less than 100%, some individuals with the genotype will not show the phenotype.

• Expressivity: The variation in the severity or nature of a phenotype for a specific genotype. Individuals with the same genotype can exhibit different degrees of the phenotype.

Inheritance Patterns of Genetic Disorders

• Autosomal dominant: A single copy of the mutated gene is enough to express the condition. Affected individuals typically have at least one affected parent. Cannot skip generations and affects males and females equally.

• Autosomal recessive: Two copies of the mutated gene must be present to express the condition. Individuals often have unaffected parents who are carriers. Can skip generations, affects males and females equally, and two unaffected (heterozygous) parents can have affected offspring.

• X-linked dominant: A single copy of the mutation of a gene on the X chromosome is enough to cause the condition in both males and females. Cannot skip generations and affects males and females equally. Affected fathers will always have affected daughters.

• X-linked recessive: Two copies of the mutated gene on the X chromosomes are required to cause the condition in females; one copy will cause the condition in males. Can skip generations. Affects males more commonly than females and affected mothers always have affected sons. Normal fathers never have affected daughters.

• Y-linked: Genes located on the Y chromosome cause the condition/trait. Only affects males.

Chromosomal Aberrations

• Changes in chromosome number or structure.

  • Inversions: A chromosome segment is rearranged in the reverse of its original orientation.

  • Deletions: A chromosome segment is missing or deleted.

  • Translocations: A chromosome segment is moved to another chromosome; it can be reciprocal (exchange of segments) or nonreciprocal (one-way transfer).

  • Duplication: A chromosome segment is repeated on the same chromosome.

Other

• Colchicine: Arrests mitosis by interfering with mitotic spindle formation; it can prevent cells from replicating and has anticancer effects

• Proto-oncogenes: Stimulate normal growth; if mutated become oncogenes (cancer-causing genes)

• Tumor suppressor genes: Make proteins that help control cell growth; if mutated may lead to cancer

Aneuploidy Numerical Examples

• Monosomy: 2n - 1 = 45 (Missing chromosome)

• Trisomy: 2n + 1 = 47 (Extra chromosome)