Glossary

4.3 Mendelian inheritance and population genetics

3 min read•july 25, 2024

Mendelian inheritance forms the foundation of modern genetics. 's pea plant experiments revealed key principles like dominance, segregation, and independent assortment, which explain how traits are passed from parents to offspring.

These concepts are crucial for understanding genetic inheritance patterns. Punnett squares and probability calculations help predict offspring genotypes and phenotypes, while population genetics explores how allele frequencies change over time in groups of organisms.

Mendelian Inheritance

Principles of Mendelian inheritance

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  • Gregor Mendel's experiments with pea plants laid foundation of modern genetics through controlled breeding and statistical analysis
  • Dominance and recessiveness explain how dominant alleles mask effects of recessive alleles while recessive traits only appear in homozygous recessive individuals (tall vs short pea plants)
  • states each parent passes one allele for each trait to offspring during gamete formation (round vs wrinkled pea seeds)
  • describes genes for different traits inherited independently applies to genes on different chromosomes (flower color and seed shape in peas)
  • Punnett squares predict offspring genotypes and phenotypes by visually representing possible allele combinations
  • Monohybrid crosses involve one trait with two alleles used to study inheritance of single characteristics (pea plant height)
  • Dihybrid crosses involve two traits with two alleles each used to study inheritance of multiple characteristics simultaneously (pea seed color and shape)

Probability in genetic crosses

  • Probability in genetics uses product rule to multiply probabilities of independent events and sum rule to add probabilities of mutually exclusive events
  • probabilities yield 1/41/4 : 1/21/2 : 1/41/4 ratio for homozygous dominant heterozygous and homozygous recessive genotypes (AaBb x AaBb cross)
  • probabilities result in 9:3:3:19:3:3:1 phenotypic ratio for two independently assorting genes (round yellow vs wrinkled green peas)
  • Test crosses determine if an organism is homozygous or heterozygous for a dominant trait by crossing with homozygous recessive individual
  • Pedigree analysis traces inheritance patterns through family trees helps identify carriers of recessive alleles
  • Calculating probabilities for specific traits in populations based on allele frequencies and mating patterns predicts likelihood of certain genotypes or phenotypes

Concepts in population genetics

  • represents proportion of a specific allele in a population expressed as p and q for two alleles of a gene
  • describes proportion of individuals with a specific in a population calculated as p2p^2 2pq2pq and q2q^2 for homozygous dominant heterozygous and homozygous recessive genotypes
  • Hardy-Weinberg equilibrium describes genetic equilibrium in a population under specific conditions:
    1. No
    2. No migration
    3. Large population size
    4. Random mating
    5. No
  • p2+2pq+q2=1p^2 + 2pq + q^2 = 1 and p+q=1p + q = 1 used to calculate allele and genotype frequencies
  • Applications of Hardy-Weinberg equilibrium include predicting allele and genotype frequencies and detecting evolutionary change in populations (ABO blood type frequencies)

Factors affecting allele frequencies

  • Natural selection causes differential reproduction based on heritable traits includes directional stabilizing and disruptive selection (antibiotic resistance in bacteria)
  • leads to random changes in allele frequencies more pronounced in small populations includes founder effect and population bottleneck (cheetah population)
  • Mutation introduces new alleles or modifies existing ones serves as source of genetic variation includes point mutations insertions deletions chromosomal mutations (sickle cell anemia)
  • transfers alleles between populations through migration can introduce new alleles or change existing frequencies (pesticide resistance in insects)
  • increases homozygosity in a population through mating between closely related individuals (purebred dog breeds)
  • changes genotype frequencies without changing allele frequencies through non-random mating based on phenotypic similarities (height in humans)
  • changes allele frequency due to linkage with a selected gene (coat color in mice)
  • causes preferential transmission of certain alleles during meiosis (t-haplotype in mice)

Key Terms to Review (20)

Allele frequency: Allele frequency refers to how often a specific allele appears in a population compared to all alleles for that gene. It plays a crucial role in understanding genetic variation, evolutionary processes, and the genetic structure of populations, influencing how traits are inherited and how populations adapt over time.
Assortative mating: Assortative mating is a mating pattern where individuals select partners based on specific traits or characteristics, leading to non-random mating within a population. This behavior can influence the genetic structure of populations by increasing the frequency of certain traits and reducing genetic diversity. It can also contribute to reproductive isolation, which is essential in understanding processes like speciation.
Dihybrid cross: A dihybrid cross is a genetic cross between individuals that examines the inheritance of two different traits, each represented by two alleles. This type of cross helps illustrate the principle of independent assortment, which states that the alleles for different traits segregate independently during gamete formation. Understanding dihybrid crosses is essential for grasping basic Mendelian genetics and how traits are inherited in populations.
Dominant allele: A dominant allele is a variant of a gene that can mask or override the expression of another allele at the same locus. In Mendelian inheritance, when an organism has two different alleles for a particular trait, the dominant allele is the one that is expressed in the phenotype, while the recessive allele remains hidden unless two copies are present. This concept plays a crucial role in population genetics by influencing how traits are inherited and how they can spread through populations over generations.
Gene flow: Gene flow refers to the transfer of genetic material between populations through processes such as migration, interbreeding, or the movement of gametes. This exchange of genetic information is crucial for maintaining genetic diversity within populations and can impact evolutionary trajectories by introducing new alleles or modifying allele frequencies over time.
Genetic drift: Genetic drift is the random fluctuation in allele frequencies within a population due to chance events, leading to changes in genetic variation over time. This process can significantly impact small populations where random events can lead to large changes in allele frequencies, affecting evolution and the overall genetic diversity of populations.
Genetic Hitchhiking: Genetic hitchhiking refers to the phenomenon where an allele increases in frequency in a population because it is located near a beneficial allele that is being favored by natural selection. This occurs when the advantageous allele enhances the survival or reproductive success of individuals carrying it, causing neighboring alleles on the same chromosome to also be carried along, even if they are neutral or deleterious. This process illustrates the link between genetic variation and population genetics, as it highlights how selection can affect not just specific traits but entire segments of the genome.
Genotype: Genotype refers to the genetic constitution of an organism, represented by the specific alleles that it carries for a particular trait. This term is fundamental in understanding how traits are inherited and expressed, playing a crucial role in Mendelian inheritance, where dominant and recessive alleles interact to determine phenotypic outcomes. In population genetics, genotypes help researchers study genetic variation and evolutionary processes within populations.
Genotype frequency: Genotype frequency refers to the proportion of different genotypes present in a population. This concept is crucial for understanding how genetic variation is distributed within populations, as it provides insight into the genetic structure and potential evolutionary dynamics of that population over time.
Gregor Mendel: Gregor Mendel was a 19th-century Austrian scientist known as the father of genetics due to his pioneering work on inheritance patterns in pea plants. His experiments laid the groundwork for understanding how traits are passed from one generation to the next, connecting heredity with evolution and helping explain population changes over time. Mendel's laws of inheritance reveal how variation within populations can lead to evolutionary changes, making his work critical in linking genetics to natural selection.
Hardy-Weinberg Equation: The Hardy-Weinberg Equation is a mathematical model that describes the genetic variation of a population at equilibrium, predicting the frequency of alleles and genotypes across generations. This equation is foundational in population genetics, illustrating how allele frequencies remain constant from one generation to the next in the absence of evolutionary influences. It connects to Mendelian inheritance by demonstrating how inherited traits are distributed in a population under ideal conditions.
Inbreeding: Inbreeding is the mating of individuals who are closely related genetically, which can lead to an increase in the expression of recessive traits and a decrease in genetic diversity. This practice is significant because it can influence population dynamics, health, and adaptability of species, affecting how genetic traits are passed on through generations.
Law of independent assortment: The law of independent assortment states that alleles for different traits segregate independently of one another during the formation of gametes. This principle is fundamental to understanding how genetic variation occurs through the distribution of alleles, leading to diverse combinations in offspring. It helps explain how traits are inherited separately from one another, contributing to the genetic diversity seen in populations.
Law of segregation: The law of segregation is a fundamental principle of genetics stating that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization. This concept explains how offspring inherit one allele from each parent, maintaining genetic diversity in a population. It connects to inheritance patterns and the distribution of traits in populations, underpinning the principles of Mendelian inheritance.
Meiotic drive: Meiotic drive is a phenomenon where certain alleles or genes manipulate the process of meiosis to increase their own transmission to the next generation, often at the expense of other alleles. This results in an unequal representation of alleles in the offspring, leading to a distortion of Mendelian inheritance patterns. Such mechanisms can affect population genetics by altering allele frequencies in populations and driving evolutionary changes.
Monohybrid Cross: A monohybrid cross is a genetic mixing of two organisms that are both heterozygous for a single trait, allowing the study of inheritance patterns of one specific characteristic. This type of cross is foundational in Mendelian genetics as it reveals how traits are passed from parents to offspring through alleles. By examining the phenotypic ratios of the offspring, researchers can gain insights into dominant and recessive traits, ultimately helping to understand basic principles of heredity.
Mutation: A mutation is a change in the nucleotide sequence of an organism's DNA, which can lead to alterations in the organism's traits. These changes can occur naturally or be induced by environmental factors and play a crucial role in the genetic diversity of populations, influencing evolution and adaptation.
Natural Selection: Natural selection is the process through which organisms better adapted to their environment tend to survive and produce more offspring. This concept is a key mechanism of evolution, linking genetic variation, adaptation, and the survival of the fittest in the dynamic interplay of species and their environments.
Phenotype: Phenotype refers to the observable physical and biological traits of an organism, resulting from the interaction of its genetic makeup (genotype) and environmental factors. These traits can include characteristics such as height, eye color, and behavior, and they play a crucial role in understanding how organisms adapt to their environments and evolve over time.
Recessive allele: A recessive allele is a version of a gene that does not manifest its trait in the presence of a dominant allele. In Mendelian inheritance, for an organism to express a trait associated with a recessive allele, it must possess two copies of that allele, one inherited from each parent. This concept is fundamental in understanding how traits are passed down through generations and how genetic variation is represented in populations.
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