Genetics Lecture 4: Gene Segregation

Lecture 4: Gene Segregation and Interaction

• Factor: A particle or unit in an organism responsible for inheritance and expression of a characteristic.
• Gene: Mendel's factor, a segment of DNA determining inheritance and expression of a characteristic.
• Alleles/Allelomorphs: Two or more alternative forms of a gene. Examples include round (R) and wrinkled (r) seed shapes in pea plants. Alleles occupy the same locus on homologous chromosomes.
• Dominant: The trait that appears in the F1 generation.
• Recessive: The trait suppressed in the F1 generation.
• Genotype: The genetic makeup (e.g., RR or Rr) of an individual.
• Phenotype: The observable characteristics (e.g., round or wrinkled seeds).
• Homozygous: An individual with identical alleles (e.g., RR or rr).
• Heterozygous: An individual with dissimilar alleles (e.g., Rr). Also referred to as a hybrid.
• Parent Generation (P): The initial parents involved in a cross.
• F1 Generation: The offspring from the P generation cross.
• Inbreeding: When F1 individuals cross.
• F2 Generation: The offspring from the self-hybridization or inbreeding of F1 individuals.

Monohybrid Cross

• Monohybrid cross: The cross between two parents differing in a single pair of contrasting characteristics. Offspring in the F1 generation are heterozygous for one trait only.
• Monohybrid ratio: The phenotypic ratio of 3 dominant to 1 recessive, obtained in the F2 generation, from a monohybrid cross. This ratio is 3:1.

Dihybrid Cross

• Dihybrid Cross: The cross between two parents differing in two pairs of contrasting characteristics. The F1 offspring are described as dihybrid or double heterozygous.

Mendel's Hybridization Experiments

• Mendel tested seven characteristics with pea plants by hybridizing different varieties showing contrasting traits. (e.g., tall x short; green-seeded x yellow-seeded).
• Parental plants were pure-breeding (homozygous) line.
• F1 generation produced all round seeds.
• Self-fertilization of F1 plants gave a mix of round and wrinkled seeds in F2 generation.

Results of Mendel's Crosses

• Details of characteristic traits, observed ratio in F2 offspring. (e.g., round seeds:wrinkled seeds, 2.96:1)

Law of Dominance

• In a hybrid union, the allele expressing itself phenotypically is the dominant allele. The other allele is recessive.
• The hybrid individual shows only the dominant characteristic.
• This is Mendel's first law of inheritance

Law of Segregation

• The law states that the unit of hereditary characteristics occur in pairs. During meiosis, these separate, and only one member of the pair goes to each gamete.
• The combination of the male and female gametes during fertilization restores the diploid chromosome number.
• The inheritance process depends on chance - whether a particular gamete gets the dominant or recessive allele. (Example: 3:1 ratio of yellow to green seeds).

Law of Independent Assortment

• This law applies when considering two or more genes simultaneously.
• Genes for different characteristics are inherited independently.
• Alleles from different gene pairs separate independently and combine randomly during meiosis.
• Results of dihybrid crosses support this law.

Formulas

• Used in calculating probabilities based on the number of gene pairs

Chi-Square Test

• Statistical test to check if the observed data fits a predicted Mendelian ratio.
• A formula for calculating the chi-square value.
• Degrees of freedom calculation
• The decision to accept or reject the hypothesis (null hypothesis) based on comparing the calculated chi squared value to a critical chi squared value.

Explore the principles of gene segregation and interaction in this quiz based on Lecture 4. Learn about key concepts such as alleles, dominant and recessive traits, and the differences between genotype and phenotype. Test your understanding of genetic inheritance through Mendel's framework.