Ch. 9 Patterns of Inheritance. The science of genetics has ancient roots The blending hypothesis,...

Ch. 9 Patterns of Inheritance

The science of genetics has ancient roots

The blending hypothesis, was suggested in the 19th century by scientists studying plants but later rejected because it did not explain how traits that disappear in one generation can reappear in later generations.

© 2012 Pearson Education, Inc.

Experimental genetics began in an abbey garden

Heredity = the transmission of traits from one generation to the next.

Genetics = the scientific study of heredity.

Gregor Mendel began the field of genetics in the 1860s,deduced the principles of genetics by breeding garden peas, and relied upon a background of mathematics, physics, and chemistry.



In 1866, Mendel correctly argued that parents pass on to their offspring discrete “heritable factors” and stressed that the heritable factors (today called genes), retain their individuality generation after generation.

A heritable feature that varies among individuals, is called a character (flower color)

Each variant for a character, is a trait (purple or white flowers)



True-breeding varieties result when self-fertilization produces offspring all identical to the parent.

The offspring of two different varieties are hybrids.

The cross-fertilization is a genetic cross (hybridization)

True-breeding parental plants are the P generation.

Hybrid offspring are the F1 generation.

A cross of F1 plants produces an F2 generation.


Figure 9.2D_1

Character TraitsDominant Recessive

Flower color

Purple White

Flower position

Axial Terminal

Seed colorYellow Green

Seed shape

Round Wrinkled

Figure 9.2D_2

CharacterDominant Recessive

Traits

Pod shape

Inflated Constricted

Pod color

Green Yellow

Stem length

Tall Dwarf

Mendel’s law of segregation describes the inheritance of a single character

A monohybrid cross is a cross between two individuals differing in a single character

Mendel performed a monohybrid cross between a plant with purple flowers and a plant with white flowers.

– The F1 generation produced plants with purple flowers.

– A cross of F1 plants with each other produced an F2 generation with ¾ purple and ¼ white flowers.


Figure 9.3A_s3

The Experiment

P generation(true-breedingparents)

F1 generation

F2 generation

of plantshave purple flowers

of plantshave white flowers

Purpleflowers

Whiteflowers

All plants havepurple flowers

Fertilizationamong F1 plants(F1 F1)

34

14


The all-purple F1 generation did not produce light purple flowers, as predicted by the blending hypothesis.

Mendel needed to explain why white color seemed to disappear in the F1 generation and white color reappeared in one quarter of the F2 offspring.



Mendel developed four hypotheses (using modern terminology)

1. Alleles are alternative versions of genes that account for variations in inherited characters.

2. For each characteristic, an organism inherits two alleles (on homologs), one from each parent. The alleles can be the same or different.

– A homozygous genotype has identical alleles.

– A heterozygous genotype has two different alleles.


Figure 9.4

P

P

a

a

B

b

PP aa Bb

Dominantallele

Recessiveallele

Gene loci

Homologouschromosomes

Genotype:Heterozygous,with one dominantand one recessiveallele

Homozygousfor the recessiveallele

Homozygousfor the dominantallele


3. If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele.

– The phenotype is the appearance or expression of a trait.

– The genotype is the genetic makeup of a trait.

*The same phenotype may be determined by more than one genotype.



4. A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation.


The law of independent assortment is revealed by tracking two characters at once

A dihybrid cross is a mating of parental varieties that differ in two characters.

Mendel performed the following dihybrid cross with the following results:– P generation: round yellow seeds wrinkled green seeds

– F1 generation: all plants with round yellow seeds

– F2 generation:

– 9/16 had round yellow seeds

– 3/16 had wrinkled yellow seeds

– 3/16 had round green seeds

– 1/16 had wrinkled green seeds

The law of independent assortment is revealed by tracking two characters at once

Mendel needed to explain why the F2 offspring had new nonparental combinations of traits and a 9:3:3:1 phenotypic ratio.

Mendel suggested that the inheritance of one character has no effect on the inheritance of another, and that the dihybrid cross is the equivalent to two monohybrid crosses, and called this the law of independent assortment.


F1 generation

P generation

Gametes

RRYY rryy

RY ry

RrYy

41

Sperm

Eggs

Yellowround

Greenround

Yellowwrinkled

Greenwrinkled

The hypothesis of independent assortmentActual results; hypothesis supported

RY

RY

rY

rY

Ry

Ry ry

ry

RRYY RrYY RRYy RrYy

RrYY rrYY RrYy rrYy

RRYy RrYy RRyy Rryy

RrYy rrYy Rryy rryy

41

41

41

41

41

41

41

169

163

161

F1 generation RrYy

163

Geneticists can use the testcross to determine unknown genotypes

A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual.

A testcross can show whether the unknown genotype includes a recessive allele.

Mendel used testcrosses to verify that he had true-breeding genotypes.


What is the genotype of the black dog?

Two possibilities for the black dog:

Testcross

Genotypes

Gametes

Offspring All black 1 black : 1 chocolate

or

B_? bb

BbBB

B B

b b

b

Bb Bb bb

Mendel’s laws reflect the rules of probability

Using his strong background in mathematics, Mendel knew that the rules of mathematical probability affected the segregation of allele pairs during gamete formation and the re-forming of pairs at fertilization.


Mendel’s laws reflect the rules of probability

The probability of a specific event is the number of ways that event can occur out of the total possible outcomes.

Determining the probability of two independent events uses the rule of multiplication, in which the probability is the product of the probabilities for each event.

The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities, called the rule of addition.


Figure 9.7

F1 genotypes

Formationof eggs

Formationof sperm

Bb female Bb male

Sperm

F2 genotypes Eggs

B

B B B B

B

b

b

bb

b b

21

21

21

21

21

21

41

41

41

41

( )

Dominant Traits Recessive Traits

Freckles No freckles

Widow’s peak Straight hairline

Free earlobe Attached earlobe

Genetic traits in humans can be tracked through family pedigrees

The inheritance of human traits follows Mendel’s laws.

A pedigree shows the inheritance of a trait in a family through multiple generations, demonstrates dominant or recessive inheritance, and can also be used to deduce genotypes of family members.


First generation(grandparents)

Second generation(parents, aunts,and uncles)

Third generation(two sisters)

Female MaleAttachedFree

Ff Ff Ffff

FfFfff ff ff

ff

FForFf

FForFf

CONNECTION: Many inherited disorders in humans are controlled by a single gene

Inherited human disorders show either recessive inheritance or dominant inheritance

Recessive inheritance: two recessive alleles are needed to show disease, (heterozygous parents are carriers of the disease-causing allele)

Dominant inheritance: one dominant allele is needed to show disease and dominant lethal alleles are usually eliminated from the population.


Parents

Offspring

Sperm

Eggs

NormalDd

NormalDd

D

D

d

d

DDNormal

DdNormal(carrier)

DdNormal(carrier)

ddDeaf

Many inherited disorders in humans are controlled by a single gene

The most common fatal genetic disease in the United States is cystic fibrosis (CF), and an example of recessive inheritance (carried by about 1 in 31 Americans)

Dominant human disorders include achondroplasia, (results in dwarfism), and Huntington’s disease (degenerative disorder of the nervous system)


VARIATIONS ON MENDEL’S LAWS


Incomplete dominance results in intermediate phenotypes

Mendel’s pea crosses always looked like one of the parental varieties, called complete dominance.

In incomplete dominance, the appearance of F1 hybrids falls between the phenotypes of the two parental varieties.

Incomplete dominance: neither allele is dominant over the other and expression of both alleles occurs.

21

21

21

21

Figure 9.11A_3

F2 generation

Eggs

Sperm

R

R

r

r

RR rR

Rr rr

Many genes have more than two alleles in the population

In codominance, neither allele is dominant over the other and expression of both alleles is observed as a distinct phenotype in the heterozygous individual.

– AB blood type is an example of codominance.


Many genes have more than two alleles in the population

Although an individual can at most carry two different alleles for a particular gene, more than two alleles often exist in the wider population.

Human ABO blood group phenotypes involve three alleles for a single gene.

The four human blood groups, A, B, AB, and O, result from combinations of these three alleles.

The A and B alleles are both expressed in heterozygous individuals, a condition known as codominance.


Figure 9.12_1

Blood Group(Phenotype) Genotypes

Carbohydrates Presenton Red Blood Cells

A

B

AB

O

IAIA

orIAi

IBIB

orIBi

IAIB

ii

Carbohydrate A

Carbohydrate B

Carbohydrate A

and

Carbohydrate B

Neither

A single gene may affect many phenotypic characters

Pleiotropy occurs when one gene influences many characteristics.

Sickle-cell disease is a human example of pleiotropy. Because it

– affects the type of hemoglobin produced and the shape of red blood cells and

– causes anemia and organ damage, etc


Figure 9.13BAn individual homozygous for the sickle-cell allele

Produces sickle-cell (abnormal) hemoglobin

The abnormal hemoglobin crystallizes,causing red blood cells to become sickle-shaped

Sickled cell

The multiple effects of sickled cells

Damage to organs Other effects

Kidney failureHeart failureSpleen damageBrain damage (impaired mental function, paralysis)

Pain and feverJoint problemsPhysical weaknessAnemiaPneumonia and other infections

A single character may be influenced by many genes

Polygenic inheritance -a single phenotypic character results from the additive effects of two or more genes.

Human skin color is an example of polygenic inheritance.


Figure 9.14_1

P generation

F1 generation

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

Figure 9.14_2

F2 generation

Eggs

Sperm

81

81

81

81

81

81

81

81

81

81

81

81

81

81

81

81

641

646

6415

6420

641

646

6415

Figure 9.14_3

Skin color

Fra

ctio

n o

f p

op

ula

tio

n6420

6415

646

641

THE CHROMOSOMAL BASIS OF INHERITANCE


Chromosome behavior accounts for Mendel’s laws

The chromosome theory of inheritance states that genes occupy specific loci (positions) on chromosomes and chromosomes undergo segregation and independent assortment during meiosis.


Chromosome behavior accounts for Mendel’s laws

Mendel’s laws correlate with chromosome separation in meiosis.

– The law of segregation depends on separation of homologous chromosomes in anaphase I.

– The law of independent assortment depends on alternative orientations of chromosomes in metaphase I.


Figure 9.16_s3

F1 generation

41

41

41

41

All yellow round seeds(RrYy)

Meta-phase I

of meiosis

Anaphase I

Metaphase II

Fertilization

Gametes

F2 generation 9 :3 :3 :1

RY ry rY Ry

R

R

R

yy

y

rr

r

YY

Y

YY

Y

R

R R

r

rr

y

yy

Y

Y

y

y

R

R

r

r r

r

r

R

R

R

Y

Y

Y

y

y

y

SEX CHROMOSOMES AND SEX-LINKED GENES


Chromosomes determine sex in many species

Many animals have a pair of sex chromosomes, designated X and Y, that determine an individual’s sex.

In mammals, males have XY sex chromosomes, females have XX sex chromosomes


X

Y

Chromosomes determine sex in many species

In some animals, environmental temperature determines the sex.

– For some species of reptiles, the temperature at which the eggs are incubated during a specific period of development determines whether the embryo will develop into a male or female.


Sex-linked genes are located on either of the sex chromosomes.

The X chromosome carries many genes unrelated to sex.

The inheritance of white eye color in the fruit fly illustrates an X-linked recessive trait.

Sex-linked genes exhibit a unique pattern of inheritance


Figure 9.21A

Figure 9.21B

MaleFemale

Sperm

Eggs

R red-eye alleler white-eye allele

XR

Xr

XrYXRXR

XRXr XRY

Y

Figure 9.21C


MaleFemale

Sperm

Eggs

XR

xR

XRYXRXr

Y

Xr XrY

XRYXRXR

XrXR

Figure 9.21D


MaleFemale

Sperm

Eggs

XrY

XRXr

XrXrXr

XR

Xr Y

XrY

XRY

XRXr

CONNECTION: Human sex-linked disorders affect mostly males

A male receiving a single X-linked recessive allele from his mother will have the disorder.

A female must receive the allele from both parents to be affected.


CONNECTION: Human sex-linked disorders affect mostly males

Recessive and sex-linked human disorders include

– hemophilia, red-green color blindness, and Duchenne muscular dystrophy


Figure 9.22

Female MaleHemophilia

Carrier

NormalAlexis

Alexandra CzarNicholas IIof Russia

QueenVictoria

Alice Louis

Albert

Ch. 9 Patterns of Inheritance. The science of genetics has ancient roots The blending hypothesis,...

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