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Honors Biology
Table of Contents
Unit 10 Overview: Heredity and Mendelian Genetics Principles
10.1 Mendel's Laws and Probability
10.2 Extensions of Mendelian Genetics
10.3 Linkage, Crossing Over, and Chromosomal Mapping
10.4 Human Genetics and Pedigree Analysis

Mendelian genetics isn't always straightforward. Sometimes, genes interact in complex ways, leading to unexpected inheritance patterns. This topic explores how dominance, multiple alleles, and gene interactions can shake up genetic outcomes.

Beyond simple dominant and recessive traits, we'll see how genes can blend, work together, or influence each other. We'll also look at how traits can be linked to sex chromosomes, adding another layer to genetic inheritance.

Dominance Patterns

Incomplete Dominance and Codominance

  • Incomplete dominance occurs when one allele is not completely dominant over the other resulting in a phenotype that is a blend of both alleles (red and white flowers producing pink offspring)
  • Codominance happens when both alleles are expressed equally in the phenotype
  • In codominance, the heterozygous phenotype shows characteristics of both alleles simultaneously (AB blood type, roan coat color in cattle)
  • Neither allele masks the expression of the other in codominance, unlike complete dominance where the dominant allele fully masks the recessive

Multiple Alleles

  • Multiple alleles refer to the presence of three or more alleles for a particular gene in a population
  • Multiple alleles arise through mutations and provide increased genetic variation
  • Although an individual can only possess two alleles for a gene, multiple alleles can exist within a population (ABO blood types, coat color in rabbits)
  • The inheritance of multiple alleles follows the same basic principles of Mendelian genetics, with the added complexity of additional possible genotypes and phenotypes

Gene Interactions

Pleiotropy and Epistasis

  • Pleiotropy occurs when a single gene influences multiple phenotypic traits
  • Pleiotropic genes can have wide-reaching effects on an organism's phenotype (sickle cell anemia affects red blood cells, spleen, and kidneys)
  • Epistasis is an interaction between genes at different loci, where one gene influences the expression of another
  • In epistasis, the phenotypic expression of one gene depends on the presence of one or more modifier genes (coat color in mice, fruit color in summer squash)

Polygenic Inheritance

  • Polygenic inheritance involves the cumulative effects of multiple genes on a single phenotypic trait
  • Traits controlled by polygenic inheritance often exhibit a continuous range of phenotypes rather than distinct categories (skin color, height, intelligence)
  • The additive effect of multiple genes contributes to the wide variation observed in polygenic traits
  • Environmental factors can also influence the expression of polygenic traits, leading to further phenotypic variation

Inheritance Patterns

Sex-Linked Inheritance

  • Sex-linked inheritance involves genes located on the sex chromosomes (X and Y in mammals)
  • X-linked traits are more common than Y-linked traits due to the larger size and greater number of genes on the X chromosome
  • Males are more likely to express recessive X-linked traits because they only have one X chromosome (hemophilia, color blindness)
  • Females can be carriers of recessive X-linked alleles without expressing the trait, as they have two X chromosomes
  • Y-linked traits are passed from father to son and are always expressed in males (hairy ears, webbed toes)
  • Sex-linked inheritance patterns deviate from the typical Mendelian ratios observed in autosomal inheritance

Key Terms to Review (17)

Gene sequencing: Gene sequencing is the process of determining the precise order of nucleotides within a gene. This technique is essential for understanding genetic information and how it influences traits, making it a significant tool in the study of genetic variation and inheritance patterns that extend beyond simple Mendelian genetics.
Cross-breeding: Cross-breeding is the practice of mating two genetically different individuals to produce offspring with a combination of traits from both parents. This method can introduce new genetic variations and enhance specific characteristics, making it a vital tool in genetics, agriculture, and animal breeding.
Continuous variation: Continuous variation refers to a range of phenotypes in a population that are not distinct or separate, but instead show a gradual transition from one trait to another. This concept highlights how traits can exhibit a spectrum of characteristics due to the influence of multiple genes and environmental factors, illustrating a complex interaction in the inheritance and expression of traits.
Recombinant frequency: Recombinant frequency is the proportion of offspring that exhibit recombinant phenotypes compared to the total number of offspring produced. It serves as a measure of genetic linkage, indicating how far apart two genes are on a chromosome based on the likelihood of crossing over occurring between them during meiosis.
Discrete traits: Discrete traits are characteristics that are determined by specific alleles and can be categorized into distinct groups without intermediate forms. These traits often follow Mendelian inheritance patterns, where they are influenced by single genes, resulting in clear-cut phenotypes such as color, shape, or presence/absence of a trait. Understanding discrete traits is crucial for exploring genetic variation and inheritance patterns in organisms.
Drosophila melanogaster: Drosophila melanogaster, commonly known as the fruit fly, is a species of small fly that has been widely used as a model organism in genetics and developmental biology. It is significant in the study of Mendelian genetics due to its rapid life cycle, simple genome, and the ease with which it can be genetically manipulated, making it an invaluable resource for understanding genetic inheritance and mutations.
Autosomal dominant: Autosomal dominant is a mode of inheritance where only one copy of a mutated gene from an affected parent is sufficient for a person to inherit and express a genetic disorder. This means that each child of an affected individual has a 50% chance of inheriting the disorder, regardless of the sex of the parent or child. Understanding this inheritance pattern is crucial for analyzing genetic disorders, mapping traits, and studying human genetics.
Linkage disequilibrium: Linkage disequilibrium refers to the non-random association of alleles at different loci in a given population. This means that certain combinations of alleles occur together more or less frequently than would be expected if the alleles were segregating independently. It is crucial in understanding how genes are inherited together and can affect traits, as well as in mapping the genetic basis of diseases and traits by revealing how closely genes are located on the same chromosome.
X-linked recessive: X-linked recessive refers to a mode of genetic inheritance where a gene responsible for a trait or disorder is located on the X chromosome and requires two copies of the mutated gene in females (XX) or one copy in males (XY) for the phenotype to be expressed. This type of inheritance results in a higher prevalence of the trait or disorder among males due to their single X chromosome, making them more susceptible to conditions caused by mutations on that chromosome.
Polygenic inheritance: Polygenic inheritance is a form of genetic inheritance where multiple genes, often located on different chromosomes, collectively influence a single trait or characteristic in an organism. This results in a continuous range of phenotypes, rather than distinct categories, for traits such as height, skin color, and weight. It highlights the complexity of genetic expression beyond the simple dominant-recessive relationships observed in Mendelian genetics.
Epistasis: Epistasis is a genetic phenomenon where the expression of one gene is suppressed or modified by one or more other genes. This interaction can significantly alter phenotypic outcomes, meaning that the overall traits an organism displays can be affected by the interplay of multiple gene products rather than just a single gene's effect. Understanding epistasis is crucial for exploring how complex traits and genetic diseases can arise from multiple genes working together.
Pleiotropy: Pleiotropy is the phenomenon where a single gene influences multiple, seemingly unrelated phenotypic traits. This concept challenges the classic Mendelian view of one gene determining one trait, highlighting the complexity of genetic interactions and the interconnectedness of biological systems. It is important in understanding genetic diseases and traits that manifest through various symptoms or characteristics due to a single genetic change.
Multiple alleles: Multiple alleles refer to the presence of three or more alternative forms of a gene that can occupy the same locus on a chromosome. This concept extends Mendelian genetics by illustrating that traits can be influenced by more than just two alleles, leading to a variety of phenotypes in a population. Instead of a simple dominant-recessive inheritance pattern, multiple alleles create more complex inheritance patterns and allow for greater genetic diversity.
Codominance: Codominance is a genetic situation where both alleles in a heterozygote are fully expressed, resulting in offspring with a phenotype that is neither dominant nor recessive. This means that both traits contribute to the organism's appearance, leading to a unique phenotype that showcases both characteristics distinctly, rather than blending them together. Codominance provides a clear example of how multiple alleles can interact in inheritance patterns and contributes to the diversity seen in traits.
Incomplete dominance: Incomplete dominance is a genetic phenomenon where the phenotype of a heterozygote is an intermediate blend of the phenotypes of the homozygous individuals. This means that neither allele is completely dominant over the other, resulting in a third phenotype that reflects a mix of both traits. This concept expands on traditional Mendelian genetics by demonstrating that inheritance can be more complex than simple dominant and recessive relationships.
Sex-linked inheritance: Sex-linked inheritance refers to the pattern of genetic transmission of traits that are associated with genes located on sex chromosomes, primarily the X and Y chromosomes. This type of inheritance explains why certain genetic disorders, such as hemophilia and color blindness, are more prevalent in males than females, as males have only one X chromosome while females have two. Understanding sex-linked inheritance helps explain variations in traits and diseases across different sexes.
Punnett Square: A Punnett square is a diagram used in genetics to predict the genotypes and phenotypes of offspring resulting from a cross between two parent organisms. This tool helps visualize how alleles are inherited, allowing for the analysis of dominant and recessive traits, and lays the foundation for understanding inheritance patterns and probabilities in organisms.