Observable Patterns of Inheritance

Chapter 14

Earlobe Variation

• Whether a person is born with attached or detached earlobes depends on a single gene

• Gene has two molecular forms (alleles)

Earlobe Variation

• You inherited one allele for this gene from each parent

• Dominant allele specifies detached earlobes

• Recessive allele specifies attached lobes

Dominant & Recessive Alleles

• If you have attached earlobes, you inherited two copies of the recessive allele

• If you have detached earlobes, you may have either one or two copies of the dominant allele

Early Ideas About Heredity

• People knew that sperm and eggs transmitted information about traits

• Blending theory

• Problem:– Would expect variation to disappear– Variation in traits persists

Gregor Mendel

• Strong background in plant breeding and mathematics

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

The Garden Pea Plant

• Self-pollinating

• True breeding (different alleles not normally introduced)

• Can be experimentally cross-pollinated

Genes

• Units of information about specific traits

• Passed from parents to offspring

• Each has a specific location (locus) on a chromosome

Alleles

• Different molecular forms of a gene

• Arise by mutation

• Dominant allele masks a recessive

allele that is paired with it

Allele Combinations

• Homozygous Chromosomes– having two identical alleles at a locus– AA or aa

• Heterozygous Chromosomes– having two different alleles at a locus– Aa

Genetic TermsA pair of homologous chromosomes

A gene locus

A pair of alleles

Three pairs of genes

Genotype & Phenotype

• Genotype refers to particular genes an individual carries

• Phenotype refers to an individual’s observable traits

• Cannot always determine genotype by observing phenotype

Tracking Generations

• Parental generation P

mates to produce

• First-generation offspring F1

mate to produce

• Second-generation offspring F2

F1 Results of One Monohybrid Cross

F2 Results of Monohybrid Cross

Mendel’s Monohybrid Cross Results

787 tall 277 dwarf

651 long stem

207 at tip

705 purple 224 white

152 yellow428 green

299 wrinkled882 inflated

6,022 yellow 2,001 green

5,474 round 1,850 wrinkled

F2 plants showed dominant-to-recessive ratio that averaged 3:1

Mendel’s Theory of Segregation

• An individual inherits a unit of information (allele) about a trait from each parent

• During gamete formation, the alleles segregate from each other

Probability

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

Punnett Square of a Monohybrid Cross

Female gametes

Male gametes

A a

A

a Aa

AA Aa

aa

Dominant phenotype canarise 3 ways,recessive only one

Test Cross

• Individual that shows dominant (heterozygous or Homozygous dominant) phenotype is crossed with individual with recessive phenotype

• Examining offspring allows you to determine the genotype of the dominant individual

Punnett Squares of Test Crosses

Homozygous recessive

a a

A

a aa

Aa Aa

aa

Homozygous recessive

a a

A

A Aa

Aa Aa

Aa

Two phenotypes All dominant phenotype

Dihybrid Cross

Experimental cross between individuals that are homozygous for different

versions of two traits

A Dihybrid Cross - F1 Results

AABB aabbx

AaBb

AB AB ab ab

TRUE-BREEDING PARENTS:

GAMETES:

F1 HYBRID OFFSPRING:

purple flowers, tall

white flowers,dwarf

All purple-flowered, tall

F1 Results of Mendel’s Dihybrid Crosses

• All plants displayed the dominant form

of both traits

• We now know:

– All plants inherited one allele for each trait

from each parent

– All plants were heterozygous (AaBb)

Phenotypic Ratios in F2

Four Phenotypes:– Tall, purple-flowered (9/16)

– Tall, white-flowered (3/16)

– Dwarf, purple-flowered (3/16)

– Dwarf, white-flowered (1/16)

AaBb X AaBb

Explanation of Mendel’s Dihybrid Results

If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible

aB

AB

AB

abAb

Ab

aB

ab

1/4

1/4

1/4

1/4

1/4 1/4 1/4 1/4

AaBb aabbAabb aaBb

AABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBb aaBB aaBbAaBB

1/161/161/161/16

1/161/161/161/16

1/161/161/161/16

1/16 1/16 1/16 1/16

16 Allele Combinations in F2

aB

AB

AB

abAb

Ab

aB

ab

1/4

1/4

1/4

1/4

1/4 1/4 1/4 1/4

AaBb aabbAabb aaBb

AABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBb aaBB aaBbAaBB

1/161/161/161/16

1/161/161/161/16

1/161/161/161/16

1/16 1/16 1/16 1/16

Independent Assortment

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

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

Independent 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 Variation

Number of genotypes possible in

offspring as a result of independent

assortment and hybrid crossing is

3n

(n is the number of gene loci

at which the parents differ)

Impact of Mendel’s Work

• Mendel presented his results in 1865

• Paper received little notice

• Mendel discontinued his experiments in 1871

• Paper rediscovered in 1900 and finally appreciated

Dominance Relations

• Complete dominance

• Incomplete dominance– Heterozygote phenotype is somewhere

between that of two homozyotes

• Codominance– Non-identical alleles specify two

phenotypes that are both expressed in heterozygotes

Flower Color in Snapdragons: Incomplete Dominance

Red-flowered plant X White-flowered plant

Pink-flowered F1 plants

(homozygote) (homozygote)

(heterozygotes)

Flower Color in Snapdragons: Incomplete Dominance

Pink-flowered plant X Pink-flowered plant

White-, pink-, and red-flowered plants in a 1:2:1 ratio

(heterozygote) (heterozygote)

Flower Color in Snapdragons: Incomplete Dominance

• Red flowers - two alleles allow them to make a red pigment

• White flowers - two mutant alleles; can’t make red pigment

• Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment

Genetics of ABO Blood Types: Three Alleles: Codominance

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

• Two alleles (IA and IB) are codominant when paired

• Third allele (i) is recessive to others

ABO Blood Type:Allele Combinations

• Type A - IAIA or IAi

• Type B - IBIB or IBi

• Type AB - IAIB

• Type O - ii

ABO Blood Type: Glycolipids on Red Cells

• Type A - Glycolipid A on cell surface

• Type B - Glycolipid B on cell surface

• Type AB - Both glyocolipids A & B

• Type O - Neither glyocolipid A nor B

ABO and Transfusions

• Recipient’s immune system will attack

blood cells that have an unfamiliar

glycolipid on surface

• Type O is universal donor because it

has neither type A nor type B glycolipid

Pleitropy

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

• Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia

Genetics of Sickle-Cell Anemia

• Two alleles1) HbA

Encodes normal beta hemoglobin chain2) HbS

Mutant allele encodes defective chain

• HbS homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia

Pleiotrophic Effects of HbS/HbS

• At low oxygen levels, cells with only HbS hemoglobin “sickle” and stick together

• This impedes oxygen delivery and blood flow

• Over time, it causes damage throughout the body

Epistasis

• Interaction between the products of gene pairs

• Common among genes for hair color in mammals

Genetics of Coat Color in Labrador Retrievers

• Two genes involved- One gene influences melanin production

• Two alleles - B (black) is dominant over b (brown)

- Other gene influences melanin deposition• Two alleles - E promotes pigment deposition

and is dominant over e

Allele Combinations and Coat Color

• Black coat - Must have at least one

dominant allele at both loci

– BBEE, BbEe, BBEe, or BbEE

• Brown coat - bbEE, bbEe

• Yellow coat - Bbee, BbEE, bbee

Albinism

• Phenotype results when pathway for melanin production is completely blocked

• Genotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melanin-synthesizing pathway

Comb Shape in Poultry

Alleles at two loci (R and P) interact

• Walnut comb - RRPP, RRPp, RrPP, RrPp

• Rose comb - RRpp, Rrpp

• Pea comb - rrPP, rrPp

• Single comb - rrpp

Campodactyly: Unexpected Phenotypes

• Effect of allele varies:

– Bent fingers on both hands

– Bent fingers on one hand

– No effect

• Many factors affect gene expression

Continuous Variation

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

• The greater the number of genes and

environmental factors that affect a trait,

the more continuous the variation in

versions of that trait

Human Variation

• Some human traits occur as a few discrete types– Attached or detached earlobes – Many genetic disorders

• Other traits show continuous variation– Height– Weight– Eye color

Describing Continuous Variation

Range of values for the trait

Nu

mb

er o

f in

div

idu

als

wit

hso

me

valu

e o

f th

e t

rait

(line of bell-shaped curve indicates continuous variation in 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 t

rait

Temperature Effects on Phenotype

• Himalayan rabbits are Homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathway

• Melanin is produced in cooler areas of body

Environmental Effects on Plant Phenotype

• Hydrangea macrophylla

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

• Flower color ranges from pink to blue

Web Sites

• BSCI 124 Lecture Notes -- Mendelian Genetics

• Biology Project - site map

• Genome Project