Mendel's Laws of Inheritance are the building blocks of genetics. They explain how traits pass from parents to offspring through genes and alleles. Understanding these laws helps us predict inheritance patterns and genetic diversity in populations.
These laws cover segregation, independent assortment, and dominance. They show how genes separate during reproduction, combine randomly, and interact to create visible traits. This knowledge is key to grasping how genetic info shapes living things.
Mendel's Laws of Inheritance
Foundations of Classical Genetics
- Mendel's laws of inheritance form the foundation of classical genetics based on experiments with pea plants in the mid-19th century
- Mendel's principles apply to genes located on different chromosomes or far apart on the same chromosome
- Exceptions include linked genes which tend to be inherited together due to their proximity on the same chromosome
Law of Segregation
- States each organism possesses two alleles for each trait which separate during gamete formation
- During meiosis homologous chromosomes separate ensuring each gamete receives only one allele for each gene
- Explains how genetic variation arises in offspring
- Example: In pea plants, alleles for seed shape (round or wrinkled) separate during gamete formation
Law of Independent Assortment
- States alleles of different genes segregate independently during gamete formation
- Occurs due to random alignment of homologous chromosomes during metaphase I of meiosis
- Results in new combinations of alleles in offspring
- Example: Alleles for pea seed color (yellow or green) assort independently from alleles for seed shape
Dominance, Recessiveness, and Incomplete Dominance
Allelic Relationships
- Dominance refers to relationship between alleles where one allele's effect masks the contribution of the other allele
- Dominant allele expressed in phenotype when present in either one or two copies (heterozygous or homozygous)
- Recessive alleles only expressed in phenotype when present in two copies (homozygous recessive)
- Dominant/recessive relationship specific to each gene and can vary between species or populations
- Example: In humans, brown eye color allele dominant over blue eye color allele
Incomplete Dominance
- Occurs when neither allele completely dominant resulting in blended or intermediate phenotype in heterozygotes
- Heterozygous phenotype distinct from both homozygous phenotypes showing partial expression of each allele
- Example: In snapdragons, red flower allele and white flower allele produce pink flowers in heterozygotes
Codominance
- Form of allelic interaction where both alleles in heterozygote fully expressed
- Results in phenotype displaying characteristics of both alleles simultaneously
- Example: In cattle, red coat color and white coat color alleles produce roan coat pattern in heterozygotes
Genotype vs Phenotype
Genetic Makeup and Observable Traits
- Genotype refers to genetic makeup of an organism specifically alleles present for particular gene or set of genes
- Phenotype represents observable physical or biochemical characteristics resulting from interaction of genotype with environment
- Genotype provides genetic instructions while phenotype manifests those instructions physically
- Not all genotypic differences result in phenotypic differences due to factors like dominance, gene interactions, and environmental influences
- Example: Two individuals with different genotypes for eye color (Bb and BB) may have same brown-eyed phenotype
Genotype-Phenotype Relationship
- Multiple genotypes can sometimes produce same phenotype as in case of dominant alleles masking recessive alleles
- Environmental factors influence phenotype expression leading to phenotypic plasticity in some organisms
- Relationship between genotype and phenotype central to understanding how genetic information translates into observable traits
- Example: Plant height genotype may result in different phenotypes based on environmental factors (sunlight, nutrients)
Monohybrid and Dihybrid Crosses
Monohybrid Crosses
- Involves study of inheritance for single gene with two alleles
- Punnett squares predict possible genotypes and phenotypes of offspring from given cross
- Monohybrid cross between two heterozygous parents yields expected phenotypic ratio of 3:1 (dominant:recessive)
- Genotypic ratio in monohybrid cross between heterozygous parents 1:2:1 (homozygous dominant:heterozygous:homozygous recessive)
- Example: Cross between two heterozygous tall pea plants (Tt x Tt) produces 3/4 tall offspring and 1/4 short offspring
Dihybrid Crosses
- Examines two genes simultaneously
- Dihybrid cross between parents heterozygous for both traits yields phenotypic ratio of 9:3:3:1 when traits independently assorting
- Demonstrates principle of independent assortment showing alleles for different genes segregate independently
- Example: Cross between plants heterozygous for both seed color and shape (YyRr x YyRr) produces 9/16 yellow round, 3/16 yellow wrinkled, 3/16 green round, and 1/16 green wrinkled seeds
Test Crosses
- Involves mating individual with unknown genotype to homozygous recessive individual
- Used to determine genotype of unknown parent
- Allows differentiation between homozygous dominant and heterozygous individuals
- Example: Crossing a tall pea plant (TT or Tt) with a short pea plant (tt) to determine if tall plant is homozygous or heterozygous