PowerLecture:PowerLecture:Chapter 11Chapter 11

Observing Patterns in Inherited Observing Patterns in Inherited TraitsTraits

Earlobe Variation Earlobe Variation

Whether a person has attached or Whether a person has attached or detached earlobes depends on a single detached earlobes depends on a single genegene

Attached earlobes: two copies of the Attached earlobes: two copies of the recessive allele for this generecessive allele for this gene

Detached earlobes: either one or two Detached earlobes: either one or two copies of the dominant allelecopies of the dominant allele

Early Ideas about Heredity Early Ideas about Heredity

People knew that sperm and eggs People knew that sperm and eggs transmitted information about traitstransmitted information about traits

Blending theoryBlending theory Problem:Problem:

Would expect variation to disappearWould expect variation to disappear Variation in traits persistsVariation in traits persists

Gregor MendelGregor Mendel

Strong background Strong background in plant breeding and in plant breeding and mathematicsmathematics

Using pea plants, Using pea plants, found indirect but found indirect but observable evidence observable evidence of how parents of how parents transmit genes to transmit genes to offspringoffspring

Gregor MendelGregor Mendel

The founder of modern geneticsThe founder of modern genetics

Fig. 11-2, p.170

a Garden pea flower, cut in half. Sperm form in pollen grains, which originate in male floral parts (stamens). 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 stamens snipped off. This is one way to assure cross-fertilization of plants.

c Later, seeds develop inside pods of the cross-fertilized plant. An embryo within 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.

Fig. 11-3, p.170

Gregor MendelGregor Mendel

Crossing garden pea plants

GenesGenes

Units of information about specific traitsUnits of information about specific traits

Passed from parents to offspringPassed from parents to offspring

Each has a specific location (locus) on a Each has a specific location (locus) on a chromosomechromosome

AllelesAlleles

Different molecular forms of a gene Different molecular forms of a gene

Arise by mutationArise by mutation

Dominant allele masks a recessive Dominant allele masks a recessive

allele that is paired with itallele that is paired with it

Allele CombinationsAllele Combinations

Homozygous Homozygous having two identical alleles at a locushaving two identical alleles at a locus AAAA or or aaaa

Heterozygous Heterozygous having two different alleles at a locushaving two different alleles at a locus AaAa

Genetic TermsGenetic Terms

A pair of homologous chromosomes

A gene locus

A pair of alleles

Three pairs of genes

Figure 11.4Page 171

A pair of homologous chromosomes, each in the unduplicated state (most often, one from a male parent and its partner from a female parent)

A gene locus (plural, loci), the location for a specific gene on a chromosome. Alleles are at corresponding loci on a pair of homologous chromosomes

A pair of alleles may be identical or nonidentical. They are represented in the text by letters such as D or d

Three pairs of genes (at three loci on this pair of homologous chromosomes); same thing as three pairs of alleles

Fig. 11-4, p.171

Genetic TermsGenetic Terms

Genetic terms

Genotype & PhenotypeGenotype & Phenotype

Genotype refers to particular genes an Genotype refers to particular genes an individual carriesindividual carries

Phenotype refers to an individual’s Phenotype refers to an individual’s observable traitsobservable traits

Cannot always determine genotype by Cannot always determine genotype by observing phenotypeobserving phenotype

Tracking GenerationsTracking Generations

Parental generation Parental generation PP

mates to produce mates to produce

First-generation offspring First-generation offspring FF11

mate to produce mate to produce

Second-generation offspring Second-generation offspring FF22

ProbabilityProbability

The chance that each outcome of a given The chance that each outcome of a given event will occur is proportional to the event will occur is proportional to the number of ways that event can be reachednumber of ways that event can be reached

Independent AssortmentIndependent Assortment

Mendel concluded that the two “units” for Mendel concluded that the two “units” for the first trait were to be assorted into the first trait were to be assorted into gametes independently of the two “units” gametes independently of the two “units” for the other traitfor the other trait

Members of each pair of homologous Members of each pair of homologous chromosomes are sorted into gametes at chromosomes are sorted into gametes at random during meiosis random during meiosis

Independent AssortmentIndependent Assortment

Metaphase I:

Metaphase II:

Gametes:

1/4 AB 1/4 ab 1/4 Ab 1/4 aB

A A A A

A A A A

AAAA

B B

B B

BB

B B

BBBB

a a a a

aa aa

aaaa

bb b b

bb b b

b b b b

OR

Tremendous VariationTremendous Variation

Number of genotypes possible in Number of genotypes possible in

offspring as a result of independent offspring as a result of independent

assortment and hybrid crossing is assortment and hybrid crossing is

33nn

((nn is the number of gene loci is the number of gene loci

at which the parents differ)at which the parents differ)

Impact of Mendel’s WorkImpact of Mendel’s Work

Mendel presented his results in 1865Mendel presented his results in 1865 Paper received little noticePaper received little notice Mendel discontinued his experiments in Mendel discontinued his experiments in

18711871 Paper rediscovered in 1900 Paper rediscovered in 1900

Dominance Relations Dominance Relations

Complete dominance Complete dominance

Incomplete dominanceIncomplete dominance

CodominanceCodominance

Incomplete Incomplete DominanceDominance

XHomozygous parent

Homozygous parent

All F1 are heterozygous

X

F2 shows three phenotypes in 1:2:1 ratio

Incomplete Dominance

Codominance: ABO blood types

ABO Blood TypeABO Blood Type

Incomplete dominance

Incomplete DominanceIncomplete Dominance

Codominance: ABO Blood Types Codominance: ABO Blood Types

Gene that controls ABO type codes for Gene that controls ABO type codes for enzyme that dictates structure of a enzyme that dictates structure of a glycolipid on blood cellsglycolipid on blood cells

Two alleles (Two alleles (IIAA and and IIBB) are codominant ) are codominant when pairedwhen paired

Third allele (Third allele (ii) is recessive to others) is recessive to others

ABO and TransfusionsABO and Transfusions

Recipient’s immune system will attack Recipient’s immune system will attack

blood cells that have an unfamiliar blood cells that have an unfamiliar

glycolipid on surfaceglycolipid on surface

Type O is universal donor because it has Type O is universal donor because it has

neither type A nor type B glycolipidneither type A nor type B glycolipid

Pleiotropy Pleiotropy

Alleles at a single locus may have effects Alleles at a single locus may have effects on two or more traitson two or more traits

Marfan syndrome - Mutation in gene for Marfan syndrome - Mutation in gene for fibrillin affects skeleton, cardiovascular fibrillin affects skeleton, cardiovascular system, lungs, eyes, and skin system, lungs, eyes, and skin

Epistasis Epistasis

Interaction between the products of gene Interaction between the products of gene pairspairs

Common among genes for hair color in Common among genes for hair color in mammalsmammals

Environmental Effects on Plant Environmental Effects on Plant PhenotypePhenotype

Hydrangea macrophyllaHydrangea macrophylla

Action of gene responsible for floral Action of gene responsible for floral color is influenced by soil aciditycolor is influenced by soil acidity

Flower color ranges from pink to blueFlower color ranges from pink to blue

Fig. 11-17a, p.179

Environmental Effects on Plant PhenotypeEnvironmental Effects on Plant Phenotype

Fig. 11-17b, p.179

Environmental Effects on Plant PhenotypeEnvironmental Effects on Plant Phenotype

Temperature Effects Temperature Effects on Phenotype on Phenotype

Rabbit is homozygous for Rabbit is homozygous for an allele that specifies a an allele that specifies a heat-sensitive version of an heat-sensitive version of an enzyme in melanin-enzyme in melanin-producing pathwayproducing pathway

Melanin is produced in Melanin is produced in cooler areas of bodycooler areas of body

Figure 11.16Page 179

Fig. 11-16, p.179

Continuous VariationContinuous Variation

A more or less continuous range of small A more or less continuous range of small differences in a given trait among differences in a given trait among individualsindividuals

The greater the number of genes and The greater the number of genes and

environmental factors that affect a trait, the environmental factors that affect a trait, the

more continuous the variation in versions more continuous the variation in versions

of that traitof that trait

Human VariationHuman Variation

Some human traits occur as a few discrete Some human traits occur as a few discrete typestypes Attached or detached earlobes Attached or detached earlobes Many genetic disordersMany genetic disorders

Other traits show continuous variationOther traits show continuous variation HeightHeight WeightWeight Eye colorEye color

Fig. 11-18, p.180

Continuous Continuous VariationVariation

Variation in human Variation in human eye coloreye color

The line of a bell-shaped curve reveals continuous variation in the population

Range of values for the trait

Nu

mb

er o

f in

div

idu

als

wit

hso

me

valu

e o

f th

e tr

ait

Fig. 11-19a, p.180

Describing Continuous Variation Describing Continuous Variation