Chapter 14 Notes Flashcards Preview

Biology 1406 > Chapter 14 Notes > Flashcards

Flashcards in Chapter 14 Notes Deck (139):
1

Mendel discovered the

basic principles of heredity by breeding garden peas in carefully planned experiments

2

Advantages of pea plants for genetic study

-There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits
-Mating can be controlled (via cross pollination)
-Each flower has sperm-producing organs (stamens) and an egg-producing organ (carpel)
-Cross-pollination (fertilization between different plants) involves dusting one plant with pollen from another

3

Mendel chose to track only those characters that occurred in

two distinct alternative forms.

4

He also used varieties that were true-breeding

(plants that produce offspring of the same variety when they self-pollinate)

5

In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called

hybridization

6

The true-breeding parents are the

P generation

7

The hybrid offspring of the P generation are called the

F1 generation

8

When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the

F2 generation is produced

9

When Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants,

all of the F1 hybrids were purple

10

When Mendel crossed the F1 hybrids,

many of the F2 plants had purple flowers, but some had white

11

Mendel discovered a ratio of about

three to one, purple to white flowers, in the F2 generation

12

Mendel reasoned that

only the purple flower factor was affecting flower color in the F1 hybrids

13

Mendel called the purple flower color

a dominant trait and the white flower color a recessive trait

14

The factor for white flowers was not diluted or destroyed because

it reappeared in the F2 generation

15

Mendel observed the same pattern of inheritance in

six other pea plant characters, each represented by two traits

16

What Mendel called a “heritable factor” is what we now call

a gene

17

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

18

These alternative versions of a gene are now called

alleles

19

Each gene resides at a

specific locus on a specific chromosome

20

For each character, an organism inherits two alleles, one from each parent.

Mendel made this deduction without knowing about the role of chromosomes

21

The two alleles at a particular locus may be

identical, as in the true-breeding plants of Mendel’s P generation

22

Alternatively, the two alleles at a locus may

differ, as in the F1 hybrids

23

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

24

(now known as the law of segregation):

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

25

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.

26

Mendel’s segregation model accounts for the 3:1 ratio he observed in

the F2 generation of his numerous crosses

27

The possible combinations of sperm and egg can be shown using a Punnett square,

a diagram for predicting the results of a genetic cross between individuals of known genetic makeup

28

A capital letter represents a dominant allele, and a

lowercase letter represents a recessive allele

-Potential gametes are on the outside of the square

29

Punnett Square Steps

Step 1- Write the genotypes of the parents
Step 2- Determine the gametes each parent can produce
Step 3- Place the gametes on the outside of the punnett square
Step 4- Fill in the boxes

30

An organism with two identical alleles for a character is said to be

homozygous for the gene controlling that character

31

An organism that has two different alleles for a gene is said to be

heterozygous for the gene controlling that character

32

Unlike homozygotes, heterozygotes are not

true-breeding

33

Because of the different effects of dominant and recessive alleles,

an organism’s traits do not always reveal its genetic composition

34

Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its

genotype, or genetic makeup

35

In the example of flower color in pea plants,

PP and Pp plants have the same phenotype (purple) but different genotypes

36

How can we tell the genotype of an individual with the dominant phenotype?

Such an individual could be either homozygous dominant or heterozygous

37

The answer is to carry out a testcross:

breeding the mystery individual with a homozygous recessive individual

38

If any offspring display the recessive phenotype,

the mystery parent must be heterozygous

39

all kids dominant =

AA (homozygous)

40

1/2 kids dominant
1/2 kids recessive =

Aa (heterozygous)

41

Phenotype

physical appearance (like the color)

42

Genotype

genetic makeup (like the Bb, BB, bb letter stuff)

43

BB genotype
(two capital letters) =

homozygous dominant

44

bb genotype
(two lower case letters) =

homozygous recessive

45

Bb genotype
(one capital and one lower case letter) =

heterozygous

46

Mendel derived the law of segregation by

following a single character

47

The F1 offspring produced in this cross were monohybrids,

individuals that are heterozygous for one character

48

A cross between such heterozygotes is called a

monohybrid cross

49

Mendel identified his second law of inheritance by

following two characters at the same time

50

Crossing two true-breeding parents differing in two characters produces

dihybrids in the F1 generation, heterozygous for both characters

51

A dihybrid cross, a cross between F1 dihybrids, can determine

whether two characters are transmitted to offspring as a package or independently

52

Using a dihybrid cross, Mendel developed the

law of independent assortment

53

The law of independent assortment states that

each pair of alleles segregates independently of each other pair of alleles during gamete formation

54

Strictly speaking, this law (law of independent assortment) applies only to genes on different,

nonhomologous chromosomes or those far apart on the same chromosome

55

Genes located near each other on the same chromosome tend to be

inherited together

56

Mendel’s laws of segregation and independent assortment reflect the

rules of probability

57

When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss

In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles

58

The multiplication rule states that

the probability that two or more independent events will occur together is the product of their individual probabilities –AND-

59

The addition rule states that

the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities –OR-

60

We can apply the multiplication and addition rules to predict

the outcome of crosses involving multiple characters

61

In calculating the chances for various genotypes,

each character is considered separately, and then the individual probabilities are multiplied

62

What’s the probability of getting an individual with the genotype AABbCc? From a cross of AaBbCc x AaBbCc

-1st- Break each trait apart
AA – probability ¼
Bb – (Bb =1/4 OR bB = ¼) probability ½
Cc – (Cc = ¼ OR cC = ¼) probability ½
-Now decide to add or multiply? AND = multiply
¼ x ½ x ½ = 1/16 ANSWER

63

The relationship between genotype and phenotype is rarely as simple as

in the pea plant characters Mendel studied

64

Many heritable characters are not determined by only one gene with two alleles

However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

65

Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:

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

66

Complete dominance occurs when

phenotypes of the heterozygote and dominant homozygote are identical

67

In incomplete dominance,

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

((cross two traits and there will be a mixture of something. like cross red and white and get pink))

68

In codominance,

two dominant alleles affect the phenotype in separate, distinguishable ways
Ex: AB blood type

((like there being two color stripes on something))

69

A dominant allele does not subdue a recessive allele; alleles don’t interact that way

Alleles are simply variations in a gene’s nucleotide sequence

For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype

70

Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain

-At the organismal level, the allele is recessive
-At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant
-At the molecular level, the alleles are codominant

71

Dominant alleles are not necessarily more common in populations than recessive alleles

-For example, one baby out of 400 in the United States is born with extra fingers or toes
-The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage
-In this example, the recessive allele is far more prevalent than the population’s dominant allele

72

Most genes exist in populations in more than two allelic forms

(multiple alleles)

For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.

73

The enzyme encoded by the IA allele adds the A carbohydrate,

whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

74

multiple alleles

when you have more than 2 alleles of choices

75

Most genes have multiple phenotypic effects, a property called

pleiotropy

76

For example, pleiotropic alleles are responsible for the

multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

77

pleiotropy

one gene affects many traits

78

Some traits may be determined by two or more genes

Epistasis
Polygenic inheritance

79

In epistasis,

a gene at one locus alters the phenotypic expression of a gene at a second locus

80

(epistasis)
For example, in Labrador retrievers and many other mammals, coat color depends on two genes

-One gene determines the pigment color (with alleles B for black and b for brown)
-The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair

81

Quantitative characters are those

that vary in the population along a continuum

82

Quantitative variation usually indicates polygenic inheritance

an additive effect of two or more genes on a single phenotype

83

Skin color in humans is an example of

polygenic inheritance

((eye color is also an example of this))

84

polygenic inheritance

many genes influence one trait

85

Another departure from Mendelian genetics arises when the

phenotype for a character depends on environment as well as genotype

86

The norm of reaction is the

phenotypic range of a genotype influenced by the environment

-For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity

87

Norms of reaction are generally broadest for polygenic characters
Such characters are called multifactorial because

genetic and environmental factors collectively influence phenotype

88

An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior.

An organism’s phenotype reflects its overall genotype and unique environmental history.

89

Humans are not good subjects for genetic research

Generation time is too long
Parents produce relatively few offspring
Breeding experiments are unacceptable

-However, basic Mendelian genetics endures as the foundation of human genetics

90

A pedigree is a

family tree that describes the interrelationships of parents and children across generations

91

Inheritance patterns of particular traits can be traced and described using

pedigrees

92

Pedigrees can also be used to make predictions about future offspring.

We can use the multiplication and addition rules to predict the probability of specific phenotypes.

93

Many genetic disorders are inherited in

a recessive manner

These range from relatively mild to life-threatening

94

Recessive inherited disorders

-Albinism
-Cystic Fibrosis
-Sickle-cell syndrom/disease

95

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

AA and Aa don’t have the disorder
aa does have the disorder

96

Carriers are

heterozygous individuals who carry the recessive allele but are phenotypically normal; most individuals with recessive disorders are born to carrier parents (Aa)

97

Most people who have recessive disorders are born to

two carrier parents

98

Albinism is a recessive condition characterized by

a lack of pigmentation in skin and hair

99

If a recessive allele that causes a disease is rare, then

the chance of two carriers meeting and mating is low

100

Consanguineous matings (i.e., matings between close relatives) increase

the chance of mating between two carriers of the same rare allele

-Most societies and cultures have laws or taboos against marriages between close relatives

101

Cystic fibrosis is the

most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent

102

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

103

Cystic fibrosis Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

Chronic bronchitis, foul stools, recurrent bacterial infections

104

If children with cystic fibrosis remain untreated, the children die before their 5th birthday

Treatment options include:
-Physical chest clearing
-Daily antibiotic regimens
-Life expectancy is about 30 years

105

Sickle-cell disease affects

one out of 400 African-Americans

106

Sickle-cell disease is caused by

the substitution of a single amino acid in the hemoglobin protein in red blood cells

107

In homozygous individuals,

all hemoglobin is abnormal (sickle-cell)

108

Sickle-cell disease symptoms include

physical weakness, pain, organ damage, and even paralysis

109

Sickle-cell disease and cystic fibrosis are examples of

pleiotropy

110

In sickle-cell disease

hh- full blown disease
Hh – sickle-cell trait
HH - normal
Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms

111

About one out of ten African Americans has sickle cell trait, an unusually high frequency of an allele with detrimental effects in homozygotes

Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous

112

Some human disorders are caused by

dominant alleles

113

Dominant alleles that cause a lethal disease are

rare and arise by mutation

114

Achondroplasia is a form of

dwarfism caused by a rare dominant allele

DD – lethal
Dd- dwarfism
dd – normal height

This is an example where the recessive allele is more common than the dominant allele

115

The timing of onset of a disease significantly affects its

inheritance

116

Huntington’s disease is

a degenerative disease of the nervous system

117

Huntington's disease has no obvious phenotypic effects until the individual is

about 35 to 40 years of age
Hh - affected
hh-normal

Once the deterioration of the nervous system begins the condition is irreversible and fatal

118

Many diseases, such as heart disease, diabetes, alcoholism, mental illnesses, and cancer have both

genetic and environmental components

119

Little is understood about the

genetic contribution to most multifactorial diseases

120

Genetic counselors can provide information to prospective parents concerned about

a family history for a specific disease

121

Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders.

Probabilities are predicted on the most accurate information at the time; predicted probabilities may change as new information is available.

122

For a growing number of diseases, tests are available that

identify carriers and help define the odds more accurately

123

In amniocentesis,

the liquid that bathes the fetus is removed and tested

124

In chorionic villus sampling (CVS),

a sample of the placenta is removed and tested

125

Other techniques, such as

ultrasound and fetoscopy, allow fetal health to be assessed visually in utero

126

Some genetic disorders can be detected at birth by

simple tests that are now routinely performed in most hospitals in the United States
-PKU
--Can’t metabolize phenylalanine – diet

127

Huntington's disease and Achondroplasia are

dominantly inherited disorders

128

gene-

a heritable unit that determines a character; can exist in different forms

129

allele-

an alternative version of a gene

130

character-

a heritable feature that varies among individuals

131

trait-

a variant for a character

132

dominant allele-

determines phenotype in a heterozygote

133

recessive allele-

has no effect on phenotype in a heterozygote

134

genotype-

the genetic makeup of an individual

135

phenotype-

an organism's appearance or observable traits

136

homozygous-

having two identical alleles for a gene

137

heterozygous-

having two different alleles for a gene

138

testcross-

a cross between an individual with an unknown genotype and a homozygous recessive individual

139

monohybrid cross-

a cross between individuals heterozygous for a single character