Chapter 14 - Mendel and the Gene Idea

Question 1

How did Mendel discover the basic principles of heredity?

Answer 1

By breeding garden peas in carefully planned experiments

Question 2

What is a character?

Answer 2

A heritable feature that varies among individuals (such a flower color)

Question 3

What is a trait?

Answer 3

Each variant for a character, such as purple or white colors for flowers

Question 4

What is true-breeding?

Answer 4

Plants that produce offspring of the same variety when they self pollinate

Question 5

What is hybridization?

Answer 5

The mating, or crossing, of two true-breeding variets

Question 6

What is the P generation?

Answer 6

Parental generation; the true-breeding parents

Question 7

What is the F1 generation?

Answer 7

The hybrid offspring of the parental generation

Question 8

What is the F2 generation?

Answer 8

When the FI hybrids self-pollinate, they produce an F2 generation (second filial generation)

Question 9

Discuss the law of segregation as it relates to Mendel’s experiment?

Answer 9

• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple
• When Mendel cross the F1 hybrids, many of the F2 plants had purple flowers, but some had white
• Mendel discovered a ratio of about three to one purple to white flowers in the F2 generation
• Mendel reasoned that only the purple flower factor was affecting flower color in the FI 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
• Mendel observed the same pattern of inheritance in six other pea plant characteristics, each represented by two traits
• What Mendel called a “heritable factor” is now referred to as a gene

Question 10

What are alleles?

Answer 10

Alternative versions of a gene; account for variations in inherited characters (each gene resides at a specific locus on a specific chromosome)

Question 11

What are the four related concepts that made up Mendel’s model?

Answer 11

1. Alternative versions of genes account for variations in inherited characters (alleles)
2. For each character, an organism inherits two copies of a gene, 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 alleles may differ, as in the F1 hybrids)
3. 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
4. The law of segregation; 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 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

Question 12

What is a dominant allele?

Answer 12

An allele that is fully expressed in the phenotype of a heterozygote

Question 13

What is a recessive allele?

Answer 13

An allele whose phenotypic effect is not observed in a heterozygote

Question 14

What is the Punnet square?

Answer 14

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

(A capital letter represents a dominant allele and a lowercase letter represents a recessive allele)

Question 15

What is homozygous?

Answer 15

An organism that has a pair of identical alleles for a character; i.e. organisms that breed true with PP or pp

Question 16

What is heterozygous?

Answer 16

An organism that has two different alleles for a gene; heterozygotes produce gametes with different alleles, so they are not true-breeding; i.e. Pp

Question 17

What is phenotype?

Answer 17

An organism’s appearance or observable traits; i.e. the color purple or white

Question 18

What is genotype?

Answer 18

An organisms genetic makeup; i.e. PP or Pp or pp

Question 19

What is the testcross?

Answer 19

• An individual with the dominant phenotype can be either homozygous dominant or heterozygous
• To determine the genotype we can carry out a testcross; breeding the mystery individual with a homozygous recessive individual
• If any offspring display the recessive phenotype, the mystery parent must be heterozygous

Question 20

What are monohybrids?

Answer 20

Heterozygous for the one particular character being followed

Question 21

What is a monohybrid cross?

Answer 21

A cross between heterozygotes

Question 22

What are dihybrids?

Answer 22

Individuals heterozygous for the two characters being followed in the cross (YyRr)

Question 23

What is a dihybrid cross?

Answer 23

A cross between F1 dihybrids

Question 24

What is the law of independent assortment?

Answer 24

Two or more genes assort independently - that is, each pair of alleles segregates independently of each other pair of alleles - during gamete formation

• This law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome
• Genes located near each other on the same chromosome tend to be inherited together

Question 25

What laws govern Mendelian inheritance?

Answer 25

The laws of probability;

• Mendel’s laws of segregation and independent assortment reflect the rules of probability
• When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
• In the same way, alleles of one gene segregate into gametes independently of another gene’s gametes

Question 26

What is the multiplication rule?

Answer 26

States that the probability that two or more independent events will occur together is the product of their individual probabilities;

Probability in a F1 monohybrid cross can be determined using the multiplication rule

Question 27

What is the addition rule?

Answer 27

The probability that any one of two or more mutually exclusive events will occur is calculated by adding their individual probabilities;

The rule of addition can be used to figure out the probability than an F2 plant from a monohybrid cross will be heterozygous rather than homozygous

Question 28

How might inheritance of characters by a single gene deviate from simple Mendelian patterns?

Answer 28

• When alleles are not completely dominate or recessive
• When a gene has more than two alleles
• When a gene produces multiple phenotypes

Question 29

What is complete dominance?

Answer 29

Complete dominance occurs when the phenotypes of the heterozygote and dominant homozygote are identical

Question 30

What is incomplete dominance?

Answer 30

The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

Incomplete dominance in snapdragon color; When red snapdragons are crossed with white ones, the F1 hybrids have pink flowers. Segregation of alleles into gametes of the F1 plants results in an F2 generations with a 1:2:1 ratio for both genotype and phenotype because neither allele is dominant

Question 31

What is codominance?

Answer 31

The two alleles each affect the phenotype in separate, distinguishable ways

Question 32

What is pleiotrophy?

Answer 32

Most genes have multiple phenotypic effects; i.e. pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as sickle-cell and cystic fibrosis

Question 33

What is epistasis?

Answer 33

The phenotypic expression of a gene at one locus alters that of a gene at a second locus; i.e. in labrador retrievers and many other mammals, coat color depends on two genes - one gene determines the pigment color (B for black or b for brown) while the other gene (E for color or e for no color) determines whether the pigment will be deposited in the hair

Question 34

What are quantitative characters?

Answer 34

Those that vary in the population along a continuum

Question 35

What is polygenic inheritance?

Answer 35

Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotypic character (i.e. skin color, height; a recent study identified at least 180 genes that affect height)

Question 36

What is multifactorial?

Answer 36

Many factors, both genetic and environmental, collectively influence phenotype; “genotype generally is not associated with a rigidly defined phenotype, but rather with a range of phenotypic possibilities due to environmental influences”

• An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior
• An organism’s phenotype represents its overall genotype and unique environmental history

Question 37

What is pedigree?

Answer 37

A pedigree is a family tree describing the the traits of parents and children across generations; inheritance patterns of particular traits can be traced and described using pedigrees

Question 38

What are carriers?

Answer 38

Heterozygous individuals who carry the recessive allele but are phenotypically normal; most individuals with recessive disorders are born to carrier parents

Recessively inherited disorders show up only in individuals homozygous for the allele

Question 39

A cross between homozygous purple flowered and homozygous white flowered pea plants results in offspring with purple flowers. This demonstrates?

Answer 39

Dominance

Question 40

What was the most significant conclusion that Gregor Mendel drew from his experiments with pea plants?

Answer 40

Traits are inherited in discrete units, and are not the results of “blending”

Question 41

Two plants are crossed, resulting in offspring with a 3:1 ratio for a particular trait. This suggests?

Answer 41

That the parents were both heterozygous

Question 42

Two characters that appear in a 9:3:3:1 ratio in the F2 generation should have which of the following properties?

Answer 42

The genes controlling the characters obey the law of independent assortment

Question 43

When crossing an organism that is homozygous recessive for a single trait with a heterozygote, what is the chance of producing an offspring with the homozygous recessive phenotype?

Answer 43

50%

Question 44

Radish flowers may be red, purple, or white. A cross between a red flowered plant and a white flowered plant yields all purple offspring. The flower color trait in radishes is an example of which of the following?

Answer 44

Codominance

Question 45

What describes the ability of a single gene to have multiple phenotypic effects?

Answer 45

Pleiotropy

Question 46

What describes the ABO blood group system?

Answer 46

Multiple alleles

The ABO blood groups in humans are determined by three alleles of a single gene: IA, IB, and i. A person’s blood group may be one of four types; (6 genotypes and 4 phenotypes)

IAIA or IAi = A
IBIB or IBi = B
IAIB = AB
ii = O

*The two alleles can affect the phenotype in different ways = co-dominance

Question 47

Carriers of a genetic disorder…?

Answer 47

Are heterozygous for the gene that can cause the disorder

Question 48

Describe the inheritance and expression of cystic fibrosis, Tay-Sachs disease, and sickle-cell disease.

Answer 48

Cystic fibrosis - the cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes, leading to a buildup of chloride ions outside the cell; symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

Tay-Sachs disease - the brain cells of a child with Tay-Sachs disease cannot metabolize certain lipids because a crucial enzyme does not work properly; as these lipids accumulate in brain cells, the child begins to suffer seizures, blindness, and degeneration of motor and mental performance (only children who inherit two copies of the allele (homozygotes) have the disease)

Sickle-cell disease - the disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells; in homozygous individuals, all hemoglobin is abnormal; symptoms include: physical weakness, pain, organ damage, and even paralysis