28 terms

Genetics- Mendel's Principles

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Distinguish between characteristics and traits.
Mendel studied the characteristics of pea plants such as seed shape and flower color. He observed that a characteristic may appear in different forms, such as yellow or green. These forms are different traits of the same characteristic.
Distinguish between genes and alleles.
In our modern terms (what we now call Mendelian genetics), one gene determines one characteristic, such as flower color in pea plants. An allele is a variant of a gene that produces a specific trait, such as purple flowers.
Explain the relationship between a genotype and a phenotype.
The phenotype of an organism is the outward appearance or physical manifestation of a genetic trait. The phenotype is an expression of an organism's genotype, the specific set of alleles present in an organism. An organism may have a homozygous or heterozygous genotype — that is, it may have two of the same allele or two different alleles. The organism will show the phenotype associated with the allele coding for the dominant trait, if at least one of these alleles is present. If both alleles code for the recessive trait, then the organism will show the recessive phenotype.
Summarize Mendel's experimental approach, and his major conclusions.
In the mid-1800s, Gregor Mendel conducted a series of meticulous experiments that contradicted the prevailing theory of blended inheritance. He bred pea plants that varied in appearance and recorded the appearances of plants in each succeeding generation. He observed that different forms of each trait, which we now call alleles, segregated into gametes. This is the principle of segregation. He also observed that some alleles coded for a dominant trait, and these dominant traits were observed even in the presence of the allele coding for a recessive trait. This is the principle of dominance. Although meiosis had not yet been discovered, Mendel predicted it conceptually, by observing that multiple traits from each parent sorted independently into offspring, creating new combinations of traits that did not exist in either parent. This is the principle of independent assortment.
Explain how independent assortment leads to different genetic combinations in offspring.
By assessing first single traits, then two traits at a time, Mendel determined that the alleles for different genes segregate independently when gametes form. This is the principle of independent assortment. Because the alleles for different genes are segregating independently, the gametes that will produce the offspring may have a different combination of alleles than the gametes that created the parent. As a result of independent assortment, some offspring show different combinations of phenotypes than either parent.
allele
One version of a gene.
characteristic
A heritable feature of an organism, such as color and shape.
dependent assortment
Occurs when alleles tend to assort in pairs or groups, rather than independently; another term for allelic linkage.
dihybrid cross
Cross between two organisms that are each heterozygous for two distinct genes.
dominant
Describes a trait that is expressed (manifested as a phenotype) even when only one copy of the allele encoding the trait is present in the genotype.
F1 generation
First filial offspring of the parents; the first generation of offspring.
F2 generation
Second filial generation; offspring of the F1 generation.
gene
Segment of DNA with instructions for protein synthesis.
genotype
Alleles present in an organism.
heritable trait
Characteristic that is passed from parent to offspring through the gametes.
heterozygous
Having two different alleles for a trait present.
homozygous
Both alleles present are identical.
monohybrid cross
Cross between two organisms, each of which is heterozygous for the trait.
P generation
Parental generation.
phenotype
Physical or physiological characteristic present; also defined as the manifestation of the genes in an organism.
principle of dominance
States that some traits are dominant and mask recessive traits.
principle of independent assortment
Each pair of alleles segregates separately of any other pair of alleles when gametes are formed.
principle of segregation
States that the two alleles for each gene separate from each other when gametes form, so that each gamete contains one allele of each gene.
Punnett square
Table method used to assess probabilities of outcomes in genetics.
recessive
Describes a trait that is masked by the dominant trait and is only expressed in the homozygous condition.
segregation
The two alleles for each trait separate when gametes form.
trait
Form of a characteristic represented.
true-breeding
Describes organisms that when bred only have offspring with the same phenotype as the parents; the organism is homozygous at the genotypes of interest.