Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance Flashcards
A heritable feature that varies among individuals, such as flower color, is called a
character
ach variant for a character, such as purple or white color for flowers, is called a
trait
Other advantages of using peas are their short generation time and the
large number of offspring from each mating
Each pea flower has both pollen-producing organs (stamens) and an
egg-bearing organ (carpel).
In nature, pea plants usually self-fertilize: Pollen grains from the stamens land on the carpel of the same flower, and sperm released from the pollen grains
fertilize eggs present in the carpel
To achieve cross-pollination of two plants, Mendel removed the immature stamens of a plant before they produced pollen and then dusted pollen from another
plant onto the altered flowers
Each resulting zygote then developed into a plant embryo encased in a
seed (pea).
figure 14.2 research method
over many generations of self-pollination, these plants had produced only the same variety as the parent plant.
true-breeding
plant with purple flowers is true-breeding if the seeds produced by self-pollination in successive generations all give rise to plants that also have
purple flowers.
In a typical breeding experiment, Mendel cross-pollinated two contrasting, true-breeding pea varieties—for example, purple-flowered plants and white-flowered plants (see Figure 14.2). This mating, or crossing, of two true-breeding varieties is called
hybridization
The true-breeding parents are referred to as the
P generation (parental generation)
their hybrid offspring are the
F1 generation (first filial generation
Allowing these F1 hybrids to self-pollinate (or to cross-pollinate with other F1 hybrids) produces an
F2 generation
Mendel usually followed traits for at least the
P,F1 , F2 and generations
Mendel’s quantitative analysis of the F2 plants from thousands of genetic crosses like these allowed him to deduce two fundamental principles of heredity, now called the
law of segregation and the law of independent assortment.
in mendel’s terminology, purple flower color is a dominant trait, and white flower color is a
recessive trait
the reappearance of white-flowered plants in the generation was evidence that the heritable factor causing white flowers had not been ______________________ by coexisting with the purple-flower factor in the hybrids.
diluted or destroyed
figure 14.3
For example, when Mendel crossed a true-breeding variety that produced smooth, round pea seeds with one that produced wrinkled seeds, all the hybrids produced round seeds; this is the
dominant trait for seed shape
figure 14.1
Mendel developed a model to explain the 3:1 inheritance pattern that he consistently observed among the offspring in his pea experiments. We describe four related concepts making up this model, the fourth of which is the
law of segregation.
First in model
alternative versions of genes account for variations in inherited characters
The gene for flower color in pea plants, for example, exists in two versions, one for purple flowers and the other for white flowers. These alternative versions of a gene are called
alleles
Today, we can relate this concept to chromosomes and DNA. As shown in Figure 14.4, each gene is a sequence of nucleotides at a specific place, or locus, along a particular
chromosome
The DNA at that locus, however, can vary slightly in its
nucleotide sequence
This variation in information content can affect the function of the encoded protein and thus an inherited character of the
organism
The purple-flower allele and the white-flower allele are two ________________________ possible at the flower-color locus on a pea plant’s chromosomes.
DNA sequence variations
figure 14.4
second in model
for each character, an organism inherits two copies (that is, two alleles) of a gene, one from each parent
Each somatic cell in a diploid organism has two sets of chromosomes, one set inherited
from each parent
Thus, a genetic locus is actually represented_______________________, once on each homolog of a specific pair of chromosomes
twice in a diploid cell
The two alleles at a particular locus may be ______________, as in the true-breeding plants of Mendel’s P generation. Or the alleles may _________, as in the hybrids
identical, differ
third in model
if the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance.
Accordingly, Mendel’s plants had purple flowers because the allele for that trait is dominant and the allele for white flowers is
recessive.
The fourth and final part of Mendel’s model,
the law of segregation
the law of segregation, states that
the two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes.
Thus, an egg or a sperm gets only ___________________ that are present in the somatic cells of the organism making the gamete.
one of the two alleles
In terms of chromosomes, this segregation corresponds to the distribution of copies of the two members of a pair of __________________________ to different gametes in meiosis
homologous chromosomes
Because it is the only allele that can be passed on to offspring, the offspring always look like their parents; this explains why these plants are
true-breeding.
But if different alleles are present, as in the F1 hybrids, then
50% of the gametes receive the dominant allele and 50% receive the recessive allele.
a handy diagrammatic device for predicting the allele composition of offspring from a cross between individuals of known genetic makeup.
Punnett square
Notice that we use a _______________to symbolize a dominant allele and a
capital letter, lowercase letter for a recessive allele
figure 14.5, animation mendel’s cross of one character: flower color
An organism that has a pair of identical alleles for a gene encoding a character is called a _______________ and is said to be _______________ for that gene.
homozygote, homozygous
In the parental generation in Figure 14.5, the purple-flowered pea plant is homozygous for the dominant allele (PP), while the white plant is
homozygous for the recessive allele (pp).
If we cross dominant homozygotes with recessive homozygotes, every offspring will have ___________________—Pp in the case of the F1 hybrids of our flower-color experiment
two different alleles
An organism that has two different alleles for a gene is called a ___________________ and is said to be _______________ for that gene.
heterozygote, heterozygous
Unlike homozygotes, heterozygotes produce gametes with different alleles, so they are not
true-breeding
Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition. Therefore, we distinguish between an organism’s appearance or observable traits, called its ______________, and its genetic makeup, its
phenotype, genotype.
Note that the term phenotype refers to ___________________ as well as traits that relate directly to appearance
physiological traits
figure 14.6, animation simplified crosses of one character in humans, animation cross of one character in “ MendAliens”
Given a purple-flowered pea plant, we cannot tell if it is homozygous (PP) or heterozygous (Pp) because both genotypes result in the same purple phenotype. To determine the genotype, we can cross this plant with a white-flowered plant (pp), which will make only
gametes with the recessive allele (p).
If all the offspring of the cross have purple flowers, then the purple-flowered mystery plant must be homozygous for the dominant allele, because a cross produces all Pp offspring. But if both the purple and the white phenotypes appear among the offspring, then the purple-flowered parent must be
heterozygous.
Breeding an organism of unknown genotype with a recessive homozygote is called a
testcross because it can reveal the genotype of that organism.
figure 14.7 Research Method, animation Testcross in “MendAliens”
Mendel derived the law of segregation from experiments in which he followed only a single character, such as flower color. All the F1 progeny produced in his crosses of true-breeding parents were _________________, meaning that they were heterozygous for the one particular character being followed in the cross.
monohybrids
We refer to a cross between such heterozygotes as a
monohybrid cross
individuals heterozygous for the two characters being followed in the cross (YyRr
dihybrids
a cross between two organisms that are each heterozygous for both of the characters being followed (or the self-pollination of a plant that is heterozygous for both characters)
dihybrid cross
figure 14.8, animation Mendel’s cross of two characters
The results of Mendel’s dihybrid experiments are the basis for what we now call the
law of independent assortment,
law of independent assortment, which states that
two or more genes assort independently—that is, each pair of alleles segregates independently of any other pair of alleles—during gamete formation.
This law applies only to genes (allele pairs) located on different
chromosomes (that is, on chromosomes that are not homologous) or, alternatively, to genes that are very far apart on the same chromosome
animation a simplified cross of two characters in humans, animation crosses of two characters, animation independent assortment
