Mendel's Laws of Inheritance

Patterns of Inheritance

• Traits are controlled by a pair of genes (alleles)
• Three main laws of inheritance:
  • Law of Dominance
  • Law of Segregation
  • Law of Independent Assortment

Mendel's Experiment

• Used garden pea plants (Pisum sativum) for easy cultivation and short generation time
• Performed cross-breeding experiments between true-breeding plants
• True-breeding plants have parents with a certain trait that consistently pass it to every offspring (homozygous)
• Monohybrid cross: mating individuals that differ in only one trait
• Performed reciprocal crosses: individuals pollinate one another
• Generations: P (parental), F1 (first generation offspring), F2 (second generation offspring)

Key Terms

• Phenotype: observable traits
• Genotype: set of alleles in an individual's chromosomes
• Homozygous: same allele of a gene on both homologous chromosomes
• Heterozygous: hybrid, different forms of an allele
• Allele: one of two or more alternative forms of a gene
• Punnett square: a square diagram used to predict the genotypes of a particular cross or breeding experiment
• Test cross: an experimental cross of an individual organism with a dominant phenotype but unknown genotype and an organism with a homozygous recessive genotype and phenotype

Laws of Inheritance

• Law of Dominance: in a heterozygous condition, the allele whose characters are expressed over the other allele is called the dominant allele
• Example: albinism, probability of a child having albinism

Monohybrid Testcross

• Example: wire-haired and smooth hair in dogs
• Example: short hair and long hair in rabbits

Polygenic Inheritance

• Example: fur color in Labrador

Nature vs Nurture

• Environment affects the expression of a gene, which in turn affects the phenotype
• Genotype + environment = phenotype

Inheritance Patterns in Humans

• Autosomal dominant
• Autosomal recessive
• X-linked dominant
• X-linked recessive

Pedigree Analysis

• Used to analyze the pattern of inheritance of a particular trait throughout a family
• Shows the presence or absence of a trait as it relates to the relationship among parents, offspring, and siblings
• By analyzing a pedigree, we can determine genotypes, identify phenotypes, and predict how a trait will be passed on in the future

Reading a Pedigree

• Determine whether the trait is dominant or recessive
• Determine if the chart shows an autosomal or sex-linked (usually X-linked) trait
• Autosomal traits: both males and females are equally likely to be affected
• X-linked traits: males are more commonly affected than females

Autosomal Dominant

• The gene in question is located on one of the autosomes (non-sex chromosomes)
• A single copy of the mutated gene (from one parent) is enough to cause the disorder
• Traits appear whether the individual is heterozygous or homozygous
• Affects both males and females
• Example: Down syndrome

Klinefelter Syndrome

• Occurs when males have an extra X chromosome (47, XXY)
• Can inhibit growth of genitalia, hormone production, and fertility

Turner Syndrome

• A condition that affects only females, resulting from one of the X chromosomes (sex chromosomes) being missing or partially missing (45, XO)
• Can cause a variety of medical and developmental problems, including short height, failure of ovaries to develop, and heart defects