Biology: Chromosomes and Inheritance

Variation in Traits

• Continuous variation: small differences in characteristics (e.g., human height).

• Discontinuous variation: distinct traits (e.g., one of two traits).

Mendelian Genetics

• Gregor Mendel: foundational experiments on inheritance through garden peas.

• Pea plants self-fertilize with both male and female reproductive organs.

• Cross-breeding: mating true-breeding individuals with different traits.

Mendel's Pea Plant Traits

• Identified seven traits in pea plants.

• Traits: variations in physical characteristics;

  • Example: Flower color - dominant (purple) vs. recessive (white).

• Observed F1 and F2 generations to track traits:

  • F1: all purple flower offspring from mixed parents.

  • F2: 75% purple, 25% white from self-pollinated hybrids.

Dominant and Recessive Traits

• Dominant traits: appear unchanged in hybrids.

• Recessive traits: may disappear but reappear in subsequent generations.

• Genotype: genetic makeup; Phenotype: observable traits.

  • Alleles: versions of a gene located at the same locus on chromosomes.

Rules of Inheritance

Rule 1: Law of Dominance

• In heterozygotes, one trait masks another; dominant trait expressed.

Rule 2: Law of Segregation

• Alleles segregate during gamete formation, leading to equal inheritance likelihood.

• Results in a 3:1 ratio of trait expression in offspring.

Rule 3: Law of Independent Assortment

• Genes for different traits segregate independently during gamete formation.

Mendelian Traits Summary

• Two alleles per gene; maintain integrity across generations.

• Dominance can mask recessive traits; recessive alleles can be carried.

Inheritance Patterns

• Incomplete dominance: heterozygous phenotype is intermediate.

• Codominance: both alleles express equally without masking each other.

Human Chromosomes

• 23 pairs; 23rd pair determines biological sex.

• Sex-linked traits: associated with X or Y chromosomes; X-linked traits may not be present on Y chromosome.