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Phenotype: The outward appearance of the individual. It is the product of interactions between genes, and between the Genotype and the environment.

Natural Selection- Concept

JoVE 10632

Fitness

Widespread variation of phenotypes in natural populations provides the raw material for evolution, which is the change in the inherited traits of populations over successive generations. Natural selection is one of the main mechanisms of evolution and requires variable traits to be heritable and associated with differential survival and/or reproductive success. Phenotypes that…

 Lab Bio

Genetics of Organisms- Concept

JoVE 10557

Mendelian Genetics

Evolution is caused by changes in the genetic composition of populations. In the field of population genetics, scientists model this process as changes in the frequency of alleles at individual genetic loci. This simple representation of how evolution occurs dates to Gregor Mendel’s analysis of trait inheritance patterns in pea plants, first presented in 1865.…

 Lab Bio

Genetic Lingo

JoVE 10771

An organism is diploid if it inherits two variants, or alleles, of each gene, one from each parent. These two alleles constitute the genotype for a given gene. The term genotype is also used to refer to an organism’s complete set of genes. A diploid organism with two identical alleles has a homozygous genotype, whereas two different alleles indicates a heterozygous genotype. Observable traits arising from genotypes are called phenotypes, which can also be influenced by environmental factors. An allele is dominant if only one copy is needed to manifest an associated phenotype and recessive if two copies are required for phenotypic expression. Diploid organisms, including humans, most other animals, and many plants, have a duplicate set of chromosomes in somatic (non-sex) cells. These chromosome duplicates are homologous, with similar lengths, centromere positions, and gene locations. Diploid organisms inherit two versions of each gene, one from each parent. These two gene variants, or alleles, are situated at the same relative locus, or position, on two homologous chromosomes. Each chromosome contains many genetic loci, and there are often several possible alleles of a given gene. The two alleles inherited by a diploid organism constitute its genotype at the locus. The term genotype also refers to an organism’s total set of g

 Core: Biology

Types of Selection

JoVE 10959

Natural selection influences the frequencies of particular alleles and phenotypes within populations in several different ways. Primarily, natural selection can be directional, stabilizing, or disruptive. Directional selection favors one extreme trait and shifts the population towards that phenotype while selecting against individuals displaying alternate traits. Stabilizing selection favors an intermediate trait with a narrow range of variation. Deviation from the optimal phenotype towards an extreme one is unfavorable. Finally, disruptive selection favors both extremes of a phenotype, while intermediate phenotypes are selected against. Directional selection favors one extreme of a phenotype. For example, in sockeye salmon, research has shown that directional selection is favoring seasonally earlier migration. This is thought to be due to predation pressure from fisheries, as fishing increases later in the migration season. Thus, fish arriving and spawning earlier may have a better chance of reaching their destination to reproduce before being caught by fishermen. When a particular non-extreme phenotype is favored, this is referred to as stabilizing selection. For example, across many species of birds, clutch size (the number of eggs in a single brood) is kept within an optimal window. Lapwings and golden plovers typically lay four eggs. This optimiza

 Core: Biology

Natural Selection - Student Protocol

JoVE 10593

Simulating Natural Selection
To begin, cut the white, black, and gray pipe cleaners into quarters, which should produce a total of 100 pieces that are 3 inches long.
Then, cut the green pipe cleaners in the quantities and lengths specified in the video, taking care to minimize waste. This approximates a normal distribution of lengths and should…

 Lab Bio

Pedigree Analysis

JoVE 10775

A pedigree is a diagram displaying a family’s history of a trait. Analyzing pedigrees can reveal (1) whether a trait is dominant or recessive, (2) the type of chromosome, autosomal or sex, a trait is linked to, (3) genotypes of family members, and (4) probabilities of phenotypes in future generations. For families with a history of autosomal or sex-linked diseases, this information can be crucial to family planning. In various plant and animal species, scientists study the inheritance of phenotypes, or traits, using carefully controlled mating experiments called crosses. For example, monohybrid crosses can establish trait dominance or recessiveness, and test crosses can determine the genotype (homozygous or heterozygous) of an organism exhibiting a known dominant phenotype. Humans, however, cannot be ethically or feasibly crossed. Therefore, researchers analyze pedigrees, or family trees, to understand how human traits and diseases are inherited. Pedigrees display a family’s history of a trait across generations and family members. Using the same principles that apply to crosses to analyze reproductive events that have already occurred, information about trait heritability can be inferred. On a typical pedigree, squares represent males and circles represent females. Shaded squares or circles signify the presence of a trait of intere

 Core: Biology

Law of Segregation

JoVE 10978

When crossing pea plants, Mendel noticed that one of the parental traits would sometimes disappear in the first generation of offspring, called the F1 generation, and could reappear in the next generation (F2). He concluded that one of the traits must be dominant over the other, thereby causing masking of one trait in the F1 generation. When he crossed the F1 plants, he found that 75% of the offspring in the F2 generation had the dominant phenotype, while 25% had the recessive phenotype. Mendel’s model to explain this result had four parts. First, alternative versions of genes, called alleles, account for differences in traits. Second, an organism inherits two copies of each gene, one from each parent. Third, the presence of a dominant allele masks the recessive allele. Fourth, the two alleles for a trait are separated during gamete formation. This last part of the model is called the Law of Segregation. If a parent has two different alleles, or is heterozygous, these alleles will be equally and randomly separated during gamete formation. Scientists now understand that the separation of chromosomes during meiosis accounts for the segregation of parental alleles. Let us think about the cross between a purple flowering plant (genotype PP) and a white flowering plant (pp). The r

 Core: Biology

Punnett Squares

JoVE 10772

A Punnett square displays the possible genotypes offspring can inherit from two parental genotypes. If a trait’s inheritance pattern (e.g., dominant or recessive) is known, Punnett squares can also be used to determine the probability of inheriting a phenotype. Punnett squares are applicable in situations where trait inheritance is determined by a single gene locus and traits are independently inherited. However, they cannot predict trait probabilities for more complex genetic inheritance scenarios. Punnett squares are visual representations that display possible offspring genotypes resulting from a cross between two parental genotypes. They can depict inheritance of one or multiple phenotypes, or traits, although other tools are more appropriate for investigating the inheritance of more than two traits. Punnett squares can be used to determine the likelihood of offspring inheriting a specific genotype, or pair of alleles causing a particular characteristic (i.e., phenotype or trait), provided that the phenotype is caused by a single gene locus and is independently assorted during meiosis. In other words, Punnett squares are useful for determining inheritance probabilities in scenarios where the likelihood of inheriting one trait does not affect the probability of inheriting another. Although there are many exceptions to these assumptions (e.

 Core: Biology
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