Observing Patterns in Inherited Traits

Chapter 10

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

Genetic TermsA pair of homologous chromosomes

A gene locus

A pair of alleles

Three pairs of genes

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 – having two identical alleles at a locus– AA or aa

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

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

Monohybrid Crosses

• Use F1 offspring of parents that breed true for different forms of a trait:(AA x aa = Aa)

• The experiment itself is a cross between two identical F1 heterozygotes, which are the “monohybrids” (Aa x Aa)

Monohybrid Crosses

fertilization produces heterozygous offspring

meiosis II

meiosis I

(chromosomes duplicated

before meiosis)

Homozygous dominant parent

Homozygous recessive parent

(gametes) (gametes)

Mendel’s Monohybrid Cross Results

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

Trait Studied Dominant Form

Recessive Form

F2 Dominant-to-Recessive Ratio

SEED SHAPE

SEED COLOR

POD SHAPE

POD COLOR

FLOWER COLOR

FLOWER POSITION

STEM LENGTH

2.96:1

3.01:1

2.95:1

2.82:1

3.15:1

3.14:1

2.84:1787 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

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

Dominant phenotype can arise 3 ways,recessive only 1

aA

aaAa

AaAAA

a

F1 Results of One Monohybrid Cross

F2 Results of Monohybrid Cross

Testcross

• Individual that shows dominant phenotype is crossed with individual with recessive phenotype

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

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

Dihybrid Cross

Experimental cross between individuals that are homozygous for different

versions of two traits

A Dihybrid Cross - F1 Results

homozygous dominant parent plant (purple flowers, tall stem)

AB abX

1 2

3

homozygous recessive parent plant (white flowers, short stem)

F1 OUTCOME

AABB aabb

All F1 plants are AaBb heterozygotes (purple flowers, tall stems)

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

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 AssortmentNucleus of a diploid (2n)reproductive cell with two pairs of homologouschromosomes

Possible alignmentsof the two homologouschromosomes duringmetaphase I of meiosis

The resulting alignments at metaphase II

Allelic combinationspossible in gametes

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

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

Genetics of ABO Blood Types: Three Alleles

• 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

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

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

Pleiotropy

• 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

Pleiotropic 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

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

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 traitNu

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f th

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rait

(line of bell-shaped curve indicates continuous variation in population)

Range of values for the traitN

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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 Yarrow Plants