Single-Gene Inheritance: Mendelian Genetics

• Single-gene inheritance is also know as Mendelian Genetics

Important Terms

• Genotype refers to the specific alleles present in an individual, such as homozygous (TT or tt) or heterozygous (Tt).
• Homozygous individuals carry two identical alleles for a gene, while heterozygous individuals carry two different alleles.
• Phenotype is the observable trait, such as tall or short for height.
• Wild type is the most common phenotype in a population.
• Mutant phenotype is the result of a change in the DNA sequence.

Gregor Mendel

• Gregor Mendel conducted research on plant breeding.
• Mendel described the units of inheritance and how they are passed from generation to generation.
• Mendel undertook his experiments from 1857 to 1863.
• He developed the foundational laws of inheritance without knowledge of DNA, cells, or chromosomes.
• Mendel employed controlled plant breeding, careful recordkeeping, and large sample sizes in his research.
• Mendel studied the transmission of seven traits in pea plants from 1857 to 1863.
• His work was not recognized during his lifetime.

True-Breeding Plants

• True-breeding plants produce offspring with the same trait as the parent when crossed with another plant of the same phenotype.
• Round-seeded parents produce all round-seeded offspring, and must be RR (true breeding).
• Yellow-seeded parents produce all yellow-seeded offspring, and must be YY.
• Short plants produce all short offspring, and must be tt.

Monohybrid Cross

• True-breeding plants with two forms of a trait are crossed in a monohybrid cross.
• Progeny show only one form of the trait, the observed trait is dominant.
• The masked trait is recessive.

Punnett Square

• Punnett Square represents specific alleles of genes in gametes and how they may combine

Test Cross

• A test cross identifies parents with an unknown genotype based on the phenotypic ratios of the offspring.

Alleles

• Mendel's units (or "elementen") are alleles.
• Alleles are versions of the same gene or DNA sequence
• Alleles differ in DNA sequence at one or more sites.

Dominant vs. Recessive Alleles

• Whether an allele is dominant or recessive is important in determining risk and critical in medical genetics
• Alleles reflect the characteristics or abundance of a protein
• Recessive traits have "loss of function," preventing production or activity of the normal protein, and include examples such as inborn errors of metabolism.
• Dominant traits have "gain of function, leading to a different function than normal activity, because the protein is abnormal and examples include Huntington's disease.
• Traits may now be determined at the cellular and molecular level, not just phenotypes and ratios.

Autosomal Inheritance

• Human autosomal traits are located on the non-sex chromosomes (1-22).
• They may be inherited as autosomal dominant or autosomal recessive.

Autosomal Dominant Traits

• Males and females can be affected, and male-to-male transmission can occur.
• Males and females transmit the trait with equal frequency.
• Successive generations are affected.
• Transmission stops after a generation in which no one is affected.

Autosomal Recessive Traits

• Only homozygous recessive individuals exhibit the affected phenotype.
• Males and females are equally affected and may transmit the trait unless it causes death before reproductive age.
• Parents of an affected individual are heterozygous or have the trait
• May skip generations.

Mendel's Law of Genetic Segregation

• Each plant possesses two units (alleles) for each trait
• Alleles separate in the formation of gametes
• Gametes contain one allele for each trait

Mendel's Law of Independent Assortment

• Mendel's Law of Independent Assortment describes how different genes independently separate from one another when reproductive cells develop.

Solving Problems

• To solve single-gene inheritance problems:
  • List genotypes and phenotypes for the trait.
  • Determine the genotypes of the parents
  • Determine possible alleles in the gametes.
  • Derive possible genotypes of offspring.
  • Repeat for successive generations

Probability

• Probability is the likelihood that an event will occur
• The probability that a coin will land heads up is ½
• The probability that a heterozygous individual (Bb) will produce a gamete with the B allele is 1/2

Independent Events

• Find the probability of simultaneous independent events by multiplying the probability of each event.
• If both parents are heterozygous (Bb), then the probability that a sperm with B allele = 1/2, the probability of an ovum with B allele = ½, so the probability of a BB child is ½ X ½ = ¼

Dependent/Mutually Exclusive Events

• The probability of dependent events can be found by summing the probability of each event.
• For example, if parents are heterozygous for a trait, R, the chance that their child carries at least one dominant R allele can be calculated by adding together the probability of child carrying RR = ¼ + probability of child carrying Rr = ½ = ¾

Pedigree Analysis

• Pedigree Analysis uses Symbolic representations of family relationships and inheritance of a trait
• Autosomal Recessive Inheritance: Albinism may occur, where affected individuals has parents who are affected or are carriers (heterozygotes)
• Autosomal Dominant Inheritance: males and females affected, with equal frequency occurs where an affected individual has an affected parent, unless he or she has a de novo mutation

Explore single-gene inheritance, also known as Mendelian Genetics, through key terms like genotype, phenotype, and the contributions of Gregor Mendel. Understand how traits are passed down through generations. Learn about homozygous and heterozygous genotypes.