Chapter11- Mendels

Observing Patterns in Inherited Traits

Chapter 11

Impacts, Issues:The Color of Skin

Like most human traits, skin color has a genetic basis; more than 100 gene products affect the synthesis and deposition of melanins

11.1 Mendel, Pea Plants, and Inheritance Patterns

Recurring inheritance patterns are observable outcomes of sexual reproduction

Before the discovery of genes, it was thought that inherited traits resulted from a blend of parental characters

Mendel’s Experimental Approach

Mendel was a monk with training in plant breeding and mathematics

He studied the garden pea (Pisum sativum), which breeds true for a number of traits

Garden Pea Plant: Self Fertilization and Cross-Fertilization

Fig. 11-3, p. 170

carpel anther

A Garden pea flower, cut in half. Sperm form in pollen grains, which originate in male floral parts (anthers). Eggs develop, fertilization takes place, and seeds mature in female floral parts (carpels).

B Pollen from a plant that breeds true for purple flowers is brushed onto a floral bud of a plant that breeds true for white flowers. The white flower had its anthers snipped off. Artificial pollination is one way to ensure that a plant will not self-fertilize.

C Later, seeds develop inside pods of the cross-fertilized plant. An embryo in each seed develops into a mature pea plant.

D Each new plant’s flower color is indirect but observable evidence that hereditary material has been transmitted from the parent plants.

Animation: Crossing garden pea plants

Terms Used in Modern Genetics

Genes• Heritable units of information about traits• Parents transmit genes to offspring• Each gene has a specific locus on a

chromosome

Diploid cells (chromosome number 2n) have pairs of genes on homologous chromosomes

Terms Used in Modern Genetics

A mutation is a permanent change in a gene• May cause a trait to change• Alleles are different molecular forms of a gene

A hybrid has nonidentical alleles for a trait• Offspring of a cross between two individuals that

breed true for different forms of a trait are hybrids

Terms Used in Modern Genetics

An individual with nonidentical alleles of a gene is heterozygous for that gene

An individual with identical alleles of a gene is homozygous for that gene

Terms Used in Modern Genetics

An allele is dominant if its effect masks the effect of a recessive allele paired with it• Capital letters (A) signify dominant alleles;

lowercase letters (a) signify recessive alleles• Homozygous dominant (AA)• Homozygous recessive (aa)• Heterozygous (Aa)

Terms Used in Modern Genetics

Gene expression• The process by which information in a gene is

converted to a structural or functional part of a cell or body

• Expressed genes determine traits

Terms Used in Modern Genetics

Genotype• The particular alleles an individual carries

Phenotype• An individual’s observable traits

Terms Used in Modern Genetics

P stands for parents, F for filial (offspring)

F1: First generation offspring of parents

F2: Second generation offspring of parents

11.1 Key ConceptsWhere Modern Genetics Started

Gregor Mendel gathered the first experimental evidence of the genetic basis of inheritance

His meticulous work gave him clues that heritable traits are specified in units

The units, which are distributed into gametes in predictable patterns, were later identified as genes

11.2 Mendel’s Law of Segregation

Garden pea plants inherit two “units” of information for a trait, one from each parent

Testcrosses

Testcross• A method of determining if an individual is

heterozygous or homozygous dominant• An individual with unknown genotype is crossed

with one that is homozygous recessive (AA x aa) or (Aa x aa)

Monohybrid Experiments

Monohybrid experiments• Testcrosses that check for a dominance

relationship between two alleles at a single locus• May be crosses between true breeding

(homozygous) individuals (AA x aa), or between identical heterozygotes (Aa x Aa)

Mendel’s Monohybrid Experiments

Mendel used monohybrid experiments to find dominance relationships among pea plant traits• When he crossed plants that bred true for white

flowers with plants that bred true for purple flowers, all F1 plants had purple flowers

• When he crossed two F1 plants, ¾ of the F2 plants had purple flowers, ¼ had white flowers

Segregation of Alleles at a Gene Locus

Fig. 11-5, p. 172

homozygous dominant parent

homozygous recessive parent

(chromosomes duplicated before

meiosis)

meiosis I

meiosis II

(gametes) (gametes)

fertilization produces heterozygous offspring

Fig. 11-5, p. 172

homozygous dominant parent

homozygous recessive parent

(chromosomes duplicated before

meiosis)

meiosis I

meiosis II

(gametes) (gametes)

fertilization produces heterozygous offspring

Stepped Art

Mendel’s Monohybrid Experiments

Fig. 11-6, p. 172

Trait Studied

Dominant Form

Recessive Form

F2 Dominant-to- Recessive Ratio

Seed shape 5,474 round 1,850 wrinkled 2.98 to 1

Seed color 6,022 yellow 2,001 green 3.01 to 1

Pod shape 882 inflated 299 wrinkled 2.95 to 1

Pod color 428 green 152 yellow 2.82 to 1

Flower color 705 purple 224 white 3.15 to 1

Flower position 651 along stem 207 at tip 3.14 to 1

Stem length 787 tall 277 dwarf 2.84 to 1

Calculating Probabilities

Probability• A measure of the chance that a particular

outcome will occur

Punnett square• A grid used to calculate the probability of

genotypes and phenotypes in offspring

Construction of a Punnett Square

Phenotype Ratios in a Monohybrid Experiment

Phenotype Ratios in a Monohybrid Experiment

Fig. 11-7, p. 173

Fig. 11-7a, p. 173

female gametes

A a A a A a

A

a

A A A

Aa

A

AA

Aa

mal

e g

amet

es

a aa a Aa aa a

Aa

aa a

Aa

aa

A From left to right, step-by-step construction of a Punnett square. Circles signify gametes, and letters signify alleles: A is dominant; a is recessive. The genotypes of the resulting offspring are inside the squares.

Fig. 11-7b, p. 173

aaF1 offspring

True-breeding homozygous recessive parent plant

a aAa Aa

A Aa Aa

AA

A Aa Aa

True-breeding homozygous dominant parent plant

Aa Aa

B A cross between two plants that breed true for different forms of a trait produces F1 offspring that are identically heterozygous.

Fig. 11-7c, p. 173

AaF2 offspring

Heterozygous F1 offspring

A a

A AA Aa

AA Aa

a Aa aaAa

aaHeterozygous F1 offspring Aa

C A cross between the F1 offspring is the monohybrid experiment. The phenotype ratio of F2 offspring in this example is 3:1 (3 purple to 1 white).

Mendel’s Law of Segregation

Mendel observed a phenotype ratio of 3:1 in the F2 offspring of his monohybrid crosses

• Consistent with the probability of the aa genotype in the offspring of a heterozygous cross (Aa x Aa)

This is the basis of Mendel’s law of segregation• Diploid cells have pairs of genes on pairs of

homologous chromosomes • The two genes of each pair separate during

meiosis, and end up in different gametes

11.2 Key ConceptsInsights from Monohybrid Experiments

Some experiments yielded evidence of gene segregation: When one chromosome separates from its homologous partner during meiosis, the alleles on those chromosomes also separate and end up in different gametes

11.3 Mendel’s Law of Independent Assortment

Mendel’s law of independent assortment• Many genes are sorted into gametes

independently of other genes

Dihybrid Experiments

Dihybrid experiments• Tests for dominance relationships between

alleles at two loci • Individuals that breed true for two different traits

are crossed (AABB x aabb)

• F2 phenotype ratio is 9:3:3:1 (four phenotypes)

• Individually, each dominant trait has an F2 ratio of 3:1 – inheritance of one trait does not affect inheritance of the other

Independent Assortment at Meiosis

Fig. 11-8, p. 174

One of two possible alignments The only other possible alignment

a Chromosome alignments at metaphase I:

A A a a A A a a

B B b b b b B B

b The resulting alignments at metaphase II:

A A a a A A a a

B B b b b b B B

c Possible combinations of alleles in gametes:

B A A B b a a b b A A b B a a B

AB ab Ab aB

Fig. 11-8, p. 174

One of two possible alignments

a Chromosome alignments at metaphase I:

A A a a

B B b b

The only other possible alignment

A A a a

b b B B

b The resulting alignments at metaphase II:

A A a a

B B b b

A A a a

b b B B

c Possible combinations of alleles in gametes:

B A A B b a a b

AB ab

b A A b B a a B

Ab aB

Stepped Art

Mendel’s Dihybrid Experiments

Fig. 11-9a, p. 175

Fig. 11-9a, p. 175

P generation

parent plant homozygous

for purple flowers

and long stems

parent plant homozygous

for white flowers

and short stems

A Meiosis in homozygous individuals results in one kind of gamete. AABB aabb

B A cross between plants homozygous for two different traits yields one possible combination of gametes:

AB x ab

Fig. 11-9b, p. 175

Fig. 11-9b, p. 175

F2 generation

AaBb AaBb AaBbAll F1 offspring are AaBb, with purple flowers and tall stems.

C Meiosis in AaBb dihybrid plants results in four kinds of gametes:

AB Ab aB ab

These gametes can meet up in one of 16 possible wayswhen the dihybrids are crossed (AaBb X AaBb):

F1 generation

Fig. 11-9c, p. 175

Fig. 11-9c, p. 175

AB Ab aB ab

AB AABB AABb AaBB AaBb

Ab AABb AAbb AaBb Aabb

aB AaBB AaBb aaBB aaBb

ab AaBb Aabb aaBb aabb

D Out of 16 possible genetic outcomes of this dihybrid cross, 9 will result in plants that are purple-flowered and tall; 3, purple-flowered and short; 3, white-flowered and tall; and 1, white-flowered and short. The ratio of phenotypes of this dihybrid cross is 9:3:3:1.

Animation: Dihybrid cross

Mendel’s Law of Independent Assortment

Mendel’s dihybrid experiments showed that “units” specifying one trait segregated into gametes separately from “units” for other traits

Exception: Genes that have loci very close to one another on a chromosome tend to stay together during meiosis

11.3 Key ConceptsInsights from Dihybrid Experiments

Some experiments yielded evidence of independent assortment: Genes are typically distributed into gametes independently of other genes

11.4 Beyond Simple Dominance

Mendel focused on traits based on clearly dominant and recessive alleles; however, the expression patterns of genes for some traits are not as straightforward

Codominance in ABO Blood Types

Codominance• Two nonidentical alleles of a gene are both fully

expressed in heterozygotes, so neither is dominant or recessive

• May occur in multiple allele systems

Multiple allele systems• Genes with three or more alleles in a population• Example: ABO blood types

Codominance in ABO Blood Types

Fig. 11-10, p. 176

AA BB

Genotypes: AO AB BO OO

Phenotypes (Blood type): A AB B O

or or

Animation: Codominance: ABO blood types

Incomplete Dominance

Incomplete dominance• One allele is not fully dominant over its partner• The heterozygote’s phenotype is somewhere

between the two homozygotes, resulting in a 1:2:1 phenotype ratio in F2 offspring

Example: Snapdragon color• RR is red• Rr is pink• rr is white

Incomplete Dominance in Snapdragons

Fig. 11-11a, p. 176

Fig. 11-11a, p. 176

homozygous parent (RR)

xhomozygous parent (rr)

heterozygous F1 offspring (Rr)

A Cross a red-flowered with a white-flowered plant, and all of the F1 offspring will be pink.

Fig. 11-11b, p. 176

Fig. 11-11b, p. 176

R r

B Cross two F1 plants, and the three phenotypes of the F2 offspring will occur in a 1:2 :1 ratio:

R

RR Rr

r

Rr rr

Epistasis

Epistasis• Two or more gene products influence a trait• Typically, one gene product suppresses the effect

of another

Example: Coat color in dogs• Alleles B and b designate colors (black or brown)• Two recessive alleles ee suppress color

Epistasis in Coat Colors

Fig. 11-13a, p. 177

EB Eb eB eb

EB black black blackEEBB EEBb EeBB

blackEeBb

Eb black chocolate black chocolateEEBb EEbb EeBb Eebb

eB black black yellow yellowEeBB EeBb eeBB eeBb

EeBb Eebb eeBbeb black chocolate yellow yellow

eebb

Epistasis in Chicken Combs

Pleiotropy

Pleiotropy• One gene product

influences two or more traits

• Example: Some tall, thin athletes have Marfan syndrome, a potentially fatal genetic disorder

11.5 Linkage Groups

The farther apart two genes are on a chromosome, the more often crossing over occurs between them

Linkage group• All genes on one chromosome • Linked genes are very close together; crossing

over rarely occurs between them

Linkage and Crossing Over

Fig. 11-15, p. 178

Parental generation

AC

F1 offspring All AaCc

meiosis, gamete formation

Gametes

Most gametes have parental genotypes

A smaller number have recombinant genotypes

ac

X

Animation: Crossover review

The Distance Between Genes

The probability that a crossover event will separate alleles of two genes is proportional to the distance between those genes

11.6 Genes and the Environment

Expression of some genes is affected by environmental factors such as temperature, altitude, or chemical exposure

The result may be variation in traits

Effects of Temperature on Gene Expression

Enzyme tyrosinase, works at low temperatures

Animation: Coat color in the Himalayan rabbit

Effects of Altitude on Gene Expression

Fig. 11-17, p. 179

a Mature cutting at high elevation (3,060 meters above sea level)

60

Hei

gh

t (c

enti

met

ers)

0

b Mature cutting at mid-elevation (1,400 meters above sea level)

60

Hei

gh

t (c

enti

met

ers)

0

c Mature cutting at low elevation (30 meters above sea level)

60

Hei

gh

t (c

enti

met

ers)

0

Effects of Predation on Gene Expression

Predators of daphnias emit chemicals that trigger a different phenotype

Fig. 11-18a, p. 179

Fig. 11-18b, p. 179

11.7 Complex Variations in Traits

Individuals of most species vary in some of their shared traits

Many traits (such as eye color) show a continuous range of variation

Continuous Variation

Continuous variation• Traits with a range of small differences• The more factors that influence a trait, the more

continuous the distribution of phenotype

Bell curve• When continuous phenotypes are divided into

measurable categories and plotted as a bar chart, they form a bell-shaped curve

Continuous Variation and the Bell Curve

Fig. 11-19a, p. 180

Fig. 11-19b, p. 180

Fig. 11-19c, p. 180

Animation: Continuous variation in height

Regarding the Unexpected Phenotype

Phenotype results from complex interactions among gene products and the environment• Enzymes and other gene products control steps

of most metabolic pathways• Mutations, interactions among genes, and

environmental conditions may affect one or more steps

11.4-11.7 Key ConceptsVariations on Mendel’s Theme

Not all traits appear in Mendelian inheritance patterns• An allele may be partly dominant over a

nonidentical partner, or codominant with it• Multiple genes may influence a trait; some genes

influence many traits• The environments also influences gene

expression

Animation: Testcross

Animation: Coat color in Labrador retrievers

Animation: Comb shape in chickens

Animation: F2 ratios interaction

Animation: Genetic terms

Animation: Incomplete dominance

Animation: Monohybrid cross

Animation: Pleiotropic effects of Marfan syndrome

Video: Genetics of skin color