Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance

Question 1

A heritable feature that varies among individuals, such as flower color, is called a

Answer 1

character

Question 2

ach variant for a character, such as purple or white color for flowers, is called a

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trait

Question 3

Other advantages of using peas are their short generation time and the

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large number of offspring from each mating

Question 4

Each pea flower has both pollen-producing organs (stamens) and an

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egg-bearing organ (carpel).

Question 5

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

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fertilize eggs present in the carpel

Question 6

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

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plant onto the altered flowers

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Each resulting zygote then developed into a plant embryo encased in a

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seed (pea).

Question 8

figure 14.2 research method

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over many generations of self-pollination, these plants had produced only the same variety as the parent plant.

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true-breeding

Question 10

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

Answer 10

purple flowers.

Question 11

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

Answer 11

hybridization

Question 12

The true-breeding parents are referred to as the

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P generation (parental generation)

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their hybrid offspring are the

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F1 generation (first filial generation

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Allowing these F1 hybrids to self-pollinate (or to cross-pollinate with other F1 hybrids) produces an

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F2 generation

Question 15

Mendel usually followed traits for at least the

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P,F1 , F2 and generations

Question 16

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

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law of segregation and the law of independent assortment.

Question 17

in mendel’s terminology, purple flower color is a dominant trait, and white flower color is a

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recessive trait

Question 18

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.

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diluted or destroyed

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figure 14.3

Question 20

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

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dominant trait for seed shape

Question 21

figure 14.1

Question 22

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

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law of segregation.

Question 23

First in model

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alternative versions of genes account for variations in inherited characters

Question 24

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

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alleles

Question 25

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

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chromosome

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The DNA at that locus, however, can vary slightly in its

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nucleotide sequence

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This variation in information content can affect the function of the encoded protein and thus an inherited character of the

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organism

Question 28

The purple-flower allele and the white-flower allele are two ________________________ possible at the flower-color locus on a pea plant’s chromosomes.

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DNA sequence variations

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figure 14.4

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second in model

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for each character, an organism inherits two copies (that is, two alleles) of a gene, one from each parent

Question 31

Each somatic cell in a diploid organism has two sets of chromosomes, one set inherited

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from each parent

Question 32

Thus, a genetic locus is actually represented_______________________, once on each homolog of a specific pair of chromosomes

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twice in a diploid cell

Question 33

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

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identical, differ

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third in model

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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.

Question 35

Accordingly, Mendel’s plants had purple flowers because the allele for that trait is dominant and the allele for white flowers is

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recessive.

Question 36

The fourth and final part of Mendel’s model,

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the law of segregation

Question 37

the law of segregation, states that

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the two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes.

Question 38

Thus, an egg or a sperm gets only ___________________ that are present in the somatic cells of the organism making the gamete.

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one of the two alleles

Question 39

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

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homologous chromosomes

Question 40

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

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true-breeding.

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But if different alleles are present, as in the F1 hybrids, then

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50% of the gametes receive the dominant allele and 50% receive the recessive allele.

Question 42

a handy diagrammatic device for predicting the allele composition of offspring from a cross between individuals of known genetic makeup.

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Punnett square

Question 43

Notice that we use a _______________to symbolize a dominant allele and a

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capital letter, lowercase letter for a recessive allele

Question 44

figure 14.5, animation mendel’s cross of one character: flower color

Question 45

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.

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homozygote, homozygous

Question 46

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

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homozygous for the recessive allele (pp).

Question 47

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

Answer 47

two different alleles

Question 48

An organism that has two different alleles for a gene is called a ___________________ and is said to be _______________ for that gene.

Answer 48

heterozygote, heterozygous

Question 49

Unlike homozygotes, heterozygotes produce gametes with different alleles, so they are not

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true-breeding

Question 50

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

Answer 50

phenotype, genotype.

Question 51

Note that the term phenotype refers to ___________________ as well as traits that relate directly to appearance

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physiological traits

Question 52

figure 14.6, animation simplified crosses of one character in humans, animation cross of one character in “ MendAliens”

Question 53

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

Answer 53

gametes with the recessive allele (p).

Question 54

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

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heterozygous.

Question 55

Breeding an organism of unknown genotype with a recessive homozygote is called a

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testcross because it can reveal the genotype of that organism.

Question 56

figure 14.7 Research Method, animation Testcross in “MendAliens”

Question 57

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.

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monohybrids

Question 58

We refer to a cross between such heterozygotes as a

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monohybrid cross

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individuals heterozygous for the two characters being followed in the cross (YyRr

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dihybrids

Question 60

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)

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dihybrid cross

Question 61

figure 14.8, animation Mendel’s cross of two characters

Question 62

The results of Mendel’s dihybrid experiments are the basis for what we now call the

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law of independent assortment,

Question 63

law of independent assortment, which states that

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two or more genes assort independently—that is, each pair of alleles segregates independently of any other pair of alleles—during gamete formation.

Question 64

This law applies only to genes (allele pairs) located on different

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chromosomes (that is, on chromosomes that are not homologous) or, alternatively, to genes that are very far apart on the same chromosome

Question 65

animation a simplified cross of two characters in humans, animation crosses of two characters, animation independent assortment