Genetics: Phenotype and Genotype

• Genetics is the study of heredity and variation in living organisms.
• It explores how traits are passed from parents to offspring.
• Inheritance patterns describe how genes are transmitted from one generation to the next.
• Gregor Mendel's work with pea plants laid the foundation for understanding inheritance.
• He proposed that traits are determined by discrete units called genes.
• Genes exist in pairs, with one allele inherited from each parent.
• Alleles are different forms of a gene.

Phenotype and Genotype

• Phenotype refers to the observable characteristics of an organism.
• Examples of phenotypes include eye color, height, and blood type.
• Phenotype is influenced by both genotype and environmental factors.
• Genotype is the genetic makeup of an organism.
• It describes the specific alleles an individual possesses for a particular gene.
• A homozygous genotype has two identical alleles for a gene (e.g., AA or aa).
• A heterozygous genotype has two different alleles for a gene (e.g., Aa).

Dominant and Recessive Traits

• Dominant alleles express their trait even when paired with a recessive allele.
• Recessive alleles only express their trait when paired with another recessive allele.
• In heterozygous individuals (Aa), the dominant allele (A) masks the expression of the recessive allele (a).
• The phenotype of a heterozygous individual will display the dominant trait.
• Dominant traits are not necessarily more common than recessive traits in a population.
• Trait prevalence depends on allele frequencies and selection pressures.
• The concept of dominance and recessiveness can sometimes be more complex than the simple models taught.
• Some traits exhibit incomplete dominance, where the heterozygous phenotype is a blend of the two homozygous phenotypes.
• Codominance occurs when both alleles are equally expressed in the heterozygous phenotype.
• Human ABO blood type is an example of codominance.
• Multiple alleles exist for some genes, increasing the possible genotypes and phenotypes.
• Human ABO blood type is determined by three alleles: A, B, and O.

Inheritance Patterns

• Monohybrid crosses involve the inheritance of a single gene.
• Punnett squares are used to predict the possible genotypes and phenotypes of offspring.
• The phenotypic ratio in the F2 generation of a monohybrid cross with dominant and recessive alleles is typically 3:1.
• The genotypic ratio in the F2 generation is typically 1:2:1 (homozygous dominant: heterozygous: homozygous recessive).
• Dihybrid crosses involve the inheritance of two genes.
• If the genes are unlinked, they assort independently during gamete formation.
• The phenotypic ratio in the F2 generation of a dihybrid cross with two unlinked genes, each with a dominant and recessive allele, is typically 9:3:3:1.
• Test crosses are used to determine the genotype of an individual with a dominant phenotype.
• The individual with the dominant phenotype is crossed with a homozygous recessive individual.
• The offspring phenotypes reveal whether the dominant individual is homozygous dominant or heterozygous.
• Sex-linked traits are genes located on the sex chromosomes (X and Y in humans).
• Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
• X-linked recessive traits are more common in males because they only have one X chromosome.
• If a male inherits an X chromosome with a recessive allele, he will express the trait.
• Females must inherit two copies of the recessive allele to express the X-linked recessive trait.
• Examples of X-linked recessive traits include color blindness and hemophilia.
• Y-linked traits are only found in males, as they are located on the Y chromosome.

Variation

• Variation refers to the differences in characteristics among individuals within a population.
• Genetic variation arises from mutations, gene flow, and sexual reproduction.
• Mutations are changes in the DNA sequence.
• Mutations can be spontaneous or caused by environmental factors.
• Mutations can be harmful, beneficial, or neutral.
• Gene flow is the movement of genes between populations.
• Sexual reproduction involves the combination of genetic material from two parents.
• Sexual reproduction leads to new combinations of alleles, increasing genetic variation.
• Crossing over (recombination) during meiosis shuffles genes on homologous chromosomes.
• Independent assortment of chromosomes during meiosis creates different combinations of chromosomes in gametes.
• Random fertilization further increases genetic variation.
• Environmental variation refers to differences in characteristics due to environmental factors.
• Nutrition, climate and exposure to toxins are examples of environmental factors.
• Many traits are influenced by both genetic and environmental factors.
• The relative contributions of genes and environment to a trait can be estimated using heritability studies.
• Continuous variation refers to traits that show a range of phenotypes (e.g., height).
• Continuous variation is often influenced by multiple genes (polygenic inheritance) and environmental factors.
• Discontinuous variation refers to traits with distinct, separate phenotypes (e.g., blood type).
• Discontinuous variation is often determined by a small number of genes with simple inheritance patterns.

Species Definition

• A species is typically defined as a group of organisms that can interbreed in nature and produce fertile offspring.
• The biological species concept defines what a species is.
• Biological species concept has some limitations.
• The biological species concept is difficult to apply to organisms that reproduce asexually.
• It cannot be used to classify extinct organisms.
• Hybridization can occur between some closely related species.
• Morphological species concept (based on physical characteristics) and the phylogenetic species concept (based on evolutionary history) are examples of other species concepts.
• Speciation is the process by which new species arise.
• Allopatric speciation (geographic isolation) and sympatric speciation (reproductive isolation within the same geographic area) are examples of mechanisms of speciation.
• Reproductive isolation mechanisms prevent gene flow between populations.
• Prezygotic isolation mechanisms prevent the formation of a zygote, this can be through habitat isolation, behavioral isolation, mechanical isolation, or gametic isolation.
• Postzygotic isolation mechanisms occur after the formation of a zygote, for example, reduced hybrid viability, reduced hybrid fertility, or hybrid breakdown.