Mendelian Genetics and Chromosomes

Mendelian Genetics and Chromosomes

• Mendel's work, which established the fundamental principles of inheritance, was based on experiments with pea plants.
• Gametes (sex cells) contain only one allele for each gene.
• Gametes fuse (fertilize) to produce a zygote with two alleles for each gene.
• The "Law of Segregation" describes how alleles separate during gamete formation, ensuring each gamete receives only one allele from each parent.
• The "Law of Independent Assortment" states that different genes are inherited independently of each other, meaning the allele for one trait doesn't influence the inheritance of another.
• Incomplete dominance occurs when heterozygotes exhibit an intermediate phenotype between the two homozygous phenotypes.
• A trait is the version of a characteristic.
• A character is a genetically encoded feature, like flower color.
• Dominance is where one allele masks (hides) the effect of another allele.

Mendel's Hypotheses

• Alternative gene versions (alleles) exist and account for variation in traits.
• Each organism inherits two alleles for each gene (one from each parent).
• Homozygous means having two identical alleles, heterozygous means having two different alleles.
• Gametes carry only one allele for each gene.
• Dominant alleles mask the effect of recessive alleles.

Law of Segregation

• Alleles for a heritable character segregate during gamete formation and end up in different gametes.
• A Punnett square is a diagram used to predict allelic combinations in offspring.
• Dominant alleles are written with capital letters and recessive with lowercase letters.

Genotype vs. Phenotype

• Genotype represents the genetic makeup of an organism.
• Phenotype represents the observable traits of an organism.
• Genotype can be masked, and the phenotype can be determined by multiple genes.

Law of Independent Assortment

• Two or more genes segregate independently of each other during gamete formation.
• Dihybrid crosses involve the inheritance of two traits simultaneously.
• The phenotypic ratio in dihybrid crosses is typically 9:3:3:1.

Summary of Mendel's Laws

• The law of segregation describes how alleles separate during gamete formation.
• The law of independent assortment illustrates how different genes are inherited independently.

More Complex Cases of Inheritance

• Incomplete dominance: Heterozygotes exhibit a phenotype intermediate between the two homozygous phenotypes.
• Pleiotropy: A single gene affects multiple traits.
• Polygenic inheritance: Multiple genes influence a single trait.
• Nature vs nurture: Environmental factors affect the phenotype.
• Epistasis: One gene affects the expression of another gene.

Multiple Alleles

• More than two alleles for a gene exist.
• Examples include ABO blood groups
• Different combinations of alleles result in various phenotypes.

Epistasis

• One gene affects the expression of another.
• An example is Labrador color, where the genotype of E affects the expression of B.

Genetic Linkage and Chromosomes

• Linked genes are located close together on the same chromosome.
• Linked genes tend to be inherited together.
• Linkage is imperfect, meaning that genes can sometimes separate.
• Recombination frequency is used to map the location of genes on chromosomes.

Chromosomal Basis for Mendel's Laws

• Alleles for a gene are located on homologous chromosomes.
• Alleles segregate during meiosis.
• Non-homologous chromosomes assort independently during meiosis.

Human Genetic Diseases

• Pedigree analysis is used to trace inheritance patterns in families.
• Recessive disorders: Two copies of a recessive allele are required for the disorder to manifest.
• Example: Albinism
• Dominant disorders: One copy of a dominant allele is sufficient for the disorder to manifest.
• Example: Achondroplasia

Chromosomal Non-Disjunction

• Non-disjunction is when chromosomes fail to separate during meiosis, resulting in gametes with an abnormal number of chromosomes.
• Aneuploidy: Having an abnormal number of chromosomes.
• Trisomy: Having three copies of a particular chromosome.
• Down syndrome is an example of a trisomy.

Summary of Lecture

• Mendel's laws govern inheritance patterns.
• Complex patterns of inheritance involve multiple genes and interactions.