Mendel, DNA to RNA to Proteins Flashcards
the “blending” hypothesis
The idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green).
the “particulate” hypothesis
The idea that parents pass on discrete heritable units (genes).
character and trait
A heritable feature that varies among individuals (such as flower color).
Each variant for a character, such as purple or white color for flowers.
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments.
Other advantages of using peas? (3)
Short generation time.
Large numbers of offspring.
Mating could be controlled; plants could be allowed to self-pollinate or could be cross-pollinated.
true-breeding
Mendel chose to track only those characters that occurred in two distinct alternative forms.
He also started with varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate).
hybridization
In a typical experiment, Mendel mated two contrasting, true-breeding varieties.
P generation
The true-breeding parents.
F1 generation
The hybrid offspring of the P generation.
F2 generation
It’s produced when F1 individuals self-pollinate or cross-pollinate with other F1 hybrids.
The Law of Segregation (7)
When Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants, all of the F1 hybrids were purple.
When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white.
Mendel discovered a ratio of about three purple flowers to one white flower in the F2 generation.
Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids.
Mendel called the purple flower color a dominant trait and the white flower color a recessive trait.
The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation.
Heritable factor” is what we now call a gene.
Mendel’s Model
Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring.
Four related concepts make up this model.
These concepts can be related to what we now know about genes and chromosomes.
Mendel’s Model: 1
Alternative versions of genes account for variations in inherited characters.
For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers.
alleles
Alternative versions of a gene.
Each gene resides at a specific locus on a specific chromosome.
Mendel’s Model: 2
For each character, an organism inherits two alleles, one from each parent.
The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation.
Or the two alleles at a locus may differ, as in the F1 hybrids.
Mendel’s Model: 3
If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance.
In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant.
Mendel’s Model: 4
The law of segregation: the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes.
Thus, an egg or a sperm gets only one of the two alleles that are present in the organism.
This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis.
Punnett square
The model accounts for the 3:1 ratio observed in the F2 generation of Mendel’s crosses.
Possible combinations of sperm and egg can be shown using a Punnett square.
A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele.
homozygote and heterozygote
An organism with two identical alleles for a character.
It is said to be homozygous for the gene controlling that character.
An organism with two different alleles for a gene is a heterozygote and is said to be heterozygous for the gene controlling that character.
Unlike homozygotes, heterozygotes are not true-breeding.
An organism’s traits do not always reveal its genetic composition.
phenotype and genotype
Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup.
In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes.
