In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are the P generation

• In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization

• The true-breeding parents are the P generation

• The hybrid offspring of the P generation are called the F1 generation

• When F1 individuals self-pollinate, the F2 generation is produced

The Law of Segregation

• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple

• When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white

• Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

• Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids

• Mendel called the purple flower color a dominant trait and white flower color a recessive trait

• Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits

• What Mendel called a “heritable factor” is what we now call a gene

Mendel’s Model

• Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring

• Four related concepts make up this model

• These concepts can be related to what we now know about genes and chromosomes

Allele for purple flowers

Homologouspair ofchromosomes

Allele for white flowers

Locus for flower-color gene

• The second concept is that for each character an organism inherits two alleles, one from each parent

• Mendel made this deduction without knowing about the role of chromosomes

• The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation

• Alternatively, the two alleles at a locus may differ, as in the F1 hybrids

• The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance

• In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant

• The fourth concept, now known as the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

• Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism

• This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

• Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses

• The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup

• A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

Appearance:

P Generation

Genetic makeup:

Gametes

F1 Generation

Appearance:Genetic makeup:

Gametes:

F2 Generation

Purple flowersPp

P p1 21 2

P p

F1 sperm

F1 eggsPP Pp

Pp pp

P

p

3 : 1

Purpleflowers

PP

Whiteflowers

pp

P p

The Testcross

• How can we tell the genotype of an individual with the dominant phenotype?

• Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous

• The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual

• If any offspring display the recessive phenotype, the mystery parent must be heterozygous

Dominant phenotype,unknown genotype:

PP or Pp?

If PP,then all offspring

purple:

p p

P

P

Pp Pp

Pp Pp

If Pp,then 1

2 offspring purpleand 1

2 offspring white:

p p

P

Ppp pp

Pp Pp

Recessive phenotype,known genotype:

pp

P Generation

F1 Generation

YYRR

Gametes YR yr

yyrr

YyRr

Hypothesis ofdependentassortment

Hypothesis of independent assortment

SpermEggs

YR

Yr

yrYR

YR

yr

Eggs

YYRR YyRr

YyRr yyrr yR

yrPhenotypic ratio 3:1

F2 Generation(predictedoffspring)

YYRR YYRr YyRR YyRr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr Yyrr yyRr yyrr

Phenotypic ratio 9:3:3:1

YR Yr yR yr

Sperm

12

14

14

14

14

1 43

4

12

12

12

14

916

316

316

316

14

14

14

• Using a dihybrid cross, Mendel developed the law of independent assortment

• The law of independent assortment states that each pair of alleles segregates independently of other pairs of alleles during gamete formation

• Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes

• Genes located near each other on the same chromosome tend to be inherited together

The Law of Independent Assortment

The laws of probability govern Mendelian inheritance

• Mendel’s laws of segregation and independent assortment reflect the rules of probability

• When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss

• In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles

The Multiplication and Addition Rules Applied to Monohybrid Crosses

• The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities

• Probability in an F1 monohybrid cross can be determined using the multiplication rule

• Segregation in a heterozygous plant is like flipping a coin: Each gamete has a 1/2 chance of carrying the dominant allele and a 1/2 chance of carrying the recessive allele

Segregation ofalleles into eggs

Rr

Sperm

R r

R

rR

R

R

r

Eggs

r

R

r

r

Segregation ofalleles into sperm

Rr

12

12

14

14

14

14

12

12

Extending Mendelian Genetics for a Single Gene

• Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:

– When alleles are not completely dominant or recessive

– When a gene has more than two alleles

– When a gene produces multiple phenotypes

The Spectrum of Dominance

• Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical

• In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

• In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

RedCRCR

Gametes

P Generation

CR CW

WhiteCWCW

PinkCRCW

CRGametes CW

F1 Generation

F2 Generation Eggs

CR CW

CR

CRCR CRCW

CRCW CWCW

CW

Sperm

12

12

12

12

12

12

Frequency of Dominant Alleles

• Dominant alleles are not necessarily more common in populations than recessive alleles

• For example, one baby out of 400 in the United States is born with extra fingers or toes

• The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage

• In this example, the recessive allele is far more prevalent than the dominant allele in the population

Multiple Alleles

• Most genes exist in populations in more than two allelic forms

• For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.

• The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

Pleiotropy

• Most genes have multiple phenotypic effects, a property called pleiotropy

• For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

Epistasis

• In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus

• For example, in mice and many other mammals, coat color depends on two genes

• One gene determines the pigment color (with alleles B for black and b for brown)

• The other gene (with alleles C for pigment color and c for no pigment color ) determines whether the pigment will be deposited in the hair

Sperm

BC bC Bc bc

BbCcBBCcBbCCBBCC

BbCC bbCC BbCc bbCc

BbccBBccBbCcBBCc

BbCc bbCc Bbcc bbcc

BC

bC

Bc

bc

BbCc BbCc

14

14

14

14

14

14

14

14

916

316

416

Polygenic Inheritance

• Quantitative characters are those that vary in the population along a continuum

• Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype

• Skin color in humans is an example of polygenic inheritance

aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC

AaBbCcAaBbCc

20/64

15/64

6/64

1/64

Fra

ctio

n o

f p

rog

eny

Nature and Nurture: The Environmental Impact on Phenotype

• Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype

• The norm of reaction is the phenotypic range of a genotype influenced by the environment

• For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity

Integrating a Mendelian View of Heredity and Variation

• An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior

• An organism’s phenotype reflects its overall genotype and unique environmental history

Pedigree Analysis

• A pedigree is a family tree that describes the interrelationships of parents and children across generations

• Inheritance patterns of particular traits can be traced and described using pedigrees

Ww ww ww Ww

Ww wwWwwwwwWw

WW wwor

Ww

No widow’s peak

Thirdgeneration

(two sisters)

Widow’s peak

Second generation(parents plus aunts

and uncles)

First generation(grandparents)

Dominant trait (widow’s peak)

First generation(grandparents) Ff Ff

FF or Ff ff ff Ff Ff

ff

ff

Ff

Second generation(parents plus aunts

and uncles)

Thirdgeneration

(two sisters)

Attached earlobe Free earlobe

ff FForFf

Recessive trait (attached earlobe)

Recessively Inherited Disorders

• Many genetic disorders are inherited in a recessive manner

• Recessively inherited disorders show up only in individuals homozygous for the allele

• Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal

Cystic Fibrosis

• Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent

• The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes

• Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

Sickle-Cell Disease

• Sickle-cell disease affects one out of 400 African-Americans

• The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells

• Symptoms include physical weakness, pain, organ damage, and even paralysis

Dominantly Inherited Disorders

• Some human disorders are due to dominant alleles

• One example is achondroplasia, a form of dwarfism that is lethal when homozygous for the dominant allele

• Huntington’s disease is a degenerative disease of the nervous system

• The disease has no obvious phenotypic effects until about 35 to 40 years of age

Multifactorial Disorders

• Many diseases, such as heart disease and cancer, have both genetic and environment components

• Little is understood about the genetic contribution to most multifactorial diseases

Counseling Based on Mendelian Genetics and Probability Rules

• Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders

Fetal Testing

• In amniocentesis, the liquid that bathes the fetus is removed and tested

• In chorionic villus sampling (CVS), a sample of the placenta is removed and tested

• Other techniques, ultrasound and fetoscopy, allow fetal health to be assessed visually in utero

Amniocentesis

Amnioticfluidwithdrawn

Fetus

A sample ofamniotic fluid canbe taken starting atthe 14th to 16thweek of pregnancy.

Centrifugation

Placenta Uterus Cervix

Fluid

Fetalcells

Biochemical tests can beperformed immediately onthe amniotic fluid or lateron the cultured cells. Biochemical

tests

Severalweeks

Karyotyping

Fetal cells must be culturedfor several weeks to obtainsufficient numbers forkaryotyping.

Chorionic villus sampling (CVS)

Placenta Chorionic villi

Fetus

Suction tubeinserted throughcervix

Fetalcells

Biochemicaltests Karyotyping and biochemical

tests can be performed onthe fetal cells immediately,providing results within a dayor so.

Severalhours

A sample of chorionic villustissue can be taken as earlyas the 8th to 10th week ofpregnancy.

Karyotyping

Newborn Screening

• Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States

X-linked red-green colorblindness

Dosage compensation in female mammals

Multiple alleles in rabbits

Geneinteraction

Epistasis in Labrador retrievers

It is caused by a dysfunctional enzyme that fails to break down specific brain lipids.

The symptoms begin with seizures, blindness, and degeneration of motor and mental performance a few months after birth. Inevitably, the child dies after a few years.

Among Ashkenazic Jews (those from central Europe), this disease occurs in one of 3,600 births, about 100 times greater than the incidence among non-Jews or Mediterranean (Sephardic) Jews.

Tay-Sachs disease

Humans with Tay-Sachs disease lack a functioning enzyme to metabolize certain lipids. These lipids accumulate in the brain, harming brain cells, and ultimately leading to death.

Children with two Tay-Sachs alleles (homozygotes) have the disease.

Both heterozygotes with one working allele and homozygotes with two working alleles are healthy and normal at the organismal level.

The activity level of the lipid-metabolizing enzyme is reduced in heterozygotes. At the biochemical level, the alleles show incomplete dominance.

Heterozygous individuals produce equal numbers of normal and dysfunctional enzyme molecules. At the molecular level, the Tay-Sachs and functional alleles are codominant.

Huntington’s disease

Degenerative disease of the nervous system. The dominant lethal allele has no obvious phenotypic effect until an individual is about 35 to 45 years old.

The deterioration of the nervous system is irreversible and inevitably fatal.

Any child born to a parent who has the allele for Huntington’s disease has a 50% chance of inheriting the disease and the disorder.

Achondroplasia

Heterozygous individuals have the dwarf phenotype.

Those who are not achondroplastic dwarfs, 99.99% of the population, are homozygous recessive for this trait.

This provides another example of a trait for which the recessive allele is far more prevalent than the dominant allele.

Sickle-cell disease

Sickle-cell disease is caused by the substitution of a single amino acid in hemoglobin.

When oxygen levels in the blood of an affected individual are low, sickle-cell hemoglobin aggregate into long rods that deform red blood cells into a sickle shape.

This sickling creates a cascade of symptoms, demonstrating the pleiotropic effects of this allele, as sickled cells clump and clog capillaries throughout the body.

Carriers are said to have sickle-cell trait.

These individuals are usually healthy, although some suffer some symptoms of sickle-cell disease under blood oxygen stress.

Homozygous normal individuals die of malaria and homozygous recessive individuals die of sickle-cell disease, while carriers are relatively free of both.

Animations and Videos

• Mendel's Experiments

• Bozeman - Mendelian Genetics

• The Punnett Square

• Bozeman - Genotypes and Phenotypes

• Bozeman - Genotype Expression

• Blood Types Animated

• Blood Typing Animation

• Bozeman - Blood Types


• Bozeman - Punnett Squares

• Bozeman - Linked Genes

• Bozeman - Genetics Review

• Bozeman - Mechanisms that Increase Genetic Variation

• Bozeman - Advanced Genetics

• Bozeman - Chi-squared Test

• Bozeman – Epigenetics

• Bozeman - Genetics Review


• Chapter Quiz Questions – 1

• Chapter Quiz Questions – 2

• Bozeman - Review – Genetics

• Genetic Problems with Solutions

Pea plants were particularly well suited for use in Mendel’s breeding experiments for all of the following reasons except that

• peas show easily observed variations in a number of characters, such as pea shape and flower color.

• it is possible to control matings between different pea plants.

• it is possible to obtain large numbers of progeny from any given cross.

• peas have an unusually long generation time.

• many of the observable characters that vary in pea plants are controlled by single genes.

Pea plants were particularly well suited for use in Mendel’s breeding experiments for all of the following reasons except that

• peas show easily observed variations in a number of characters, such as pea shape and flower color.

• it is possible to control matings between different pea plants.

• it is possible to obtain large numbers of progeny from any given cross.

• peas have an unusually long generation time.

• many of the observable characters that vary in pea plants are controlled by single genes.

A pea plant is heterozygous at the independent loci for flower color (white versus purple) and seed color (yellow versus green). What types of gametes can it produce?

• two gamete types: white/white and purple/purple

• two gamete types: white/yellow and purple/green

• four gamete types: white/yellow, white/green, purple/yellow, and purple/green

• four gamete types: white/purple, yellow/green, white/white, and purple/purple

• one gamete type: white/purple/yellow/green

A pea plant is heterozygous at the independent loci for flower color (white versus purple) and seed color (yellow versus green). What types of gametes can it produce?

• two gamete types: white/white and purple/purple

• two gamete types: white/yellow and purple/green

• four gamete types: white/yellow, white/green, purple/yellow, and purple/green

• four gamete types: white/purple, yellow/green, white/white, and purple/purple

• one gamete type: white/purple/yellow/green

A cross between homozygous purple-flowered and homozygous white-flowered pea plants results in offspring with purple flowers. This demonstrates

• the blending model of genetics.

• true breeding.

• dominance.

• a dihybrid cross.

• the mistakes made by Mendel.

A cross between homozygous purple-flowered and homozygous white-flowered pea plants results in offspring with purple flowers. This demonstrates

• the blending model of genetics.

• true breeding.

• dominance.

• a dihybrid cross.

• the mistakes made by Mendel.

Imagine a genetic counselor working with a couple who have just had a child who is suffering from Tay-Sachs disease. Neither parent has Tay-Sachs, nor does anyone in their families. Which of the following statements should this counselor make to this couple?

• “Because no one in either of your families has Tay-Sachs, you are not likely to have another baby with Tay-Sachs. You can safely have another child.”

• “Because you have had one child with Tay-Sachs, you must each carry the allele. Any child you have has a 50% chance of having the disease.”


• “Because you have had one child with Tay-Sachs, you must both carry the allele. However, since the chance of having an affected child is 25%, you may safely have thee more children without worrying about having another child with Tay-Sachs.”

• “You must both be tested to see who is a carrier of the Tay-Sachs allele.”

Imagine a genetic counselor working with a couple who have just had a child who is suffering from Tay-Sachs disease. Neither parent has Tay-Sachs, nor does anyone in their families. Which of the following statements should this counselor make to this couple?

• “Because no one in either of your families has Tay-Sachs, you are not likely to have another baby with Tay-Sachs. You can safely have another child.”



• “Because you have had one child with Tay-Sachs, you must both carry the allele. However, since the chance of having an affected child is 25%, you may safely have thee more children without worrying about having another child with Tay-Sachs.”

• “You must both be tested to see who is a carrier of the Tay-Sachs allele.”

Imagine a locus with four different alleles for fur color in an animal. The alleles are named Da, Db, Dc, and Dd. If you crossed two heterozygotes, DaDb and DcDd, what genotype proportions would you expect in the offspring?

• 25% DaDc, 25% DaDd, 25% DbDc, 25% DbDd

• 50% DaDb, 50% DcDd

• 25% DaDa, 25% DbDb, 25% DcDc, 25% DdDdDcDd

• 50% DaDc, 50% DbDd

• 25% DaDb, 25% DcDd, 25% DcDc, 25% DdDd

Imagine a locus with four different alleles for fur color in an animal. The alleles are named Da, Db, Dc, and Dd. If you crossed two heterozygotes, DaDb and DcDd, what genotype proportions would you expect in the offspring?

• 25% DaDc, 25% DaDd, 25% DbDc, 25% DbDd

• 50% DaDb, 50% DcDd

• 25% DaDa, 25% DbDb, 25% DcDc, 25% DdDdDcDd

• 50% DaDc, 50% DbDd

• 25% DaDb, 25% DcDd, 25% DcDc, 25% DdDd

John, age 47, has just been diagnosed with Huntington’s disease, which is caused by a dominant allele. His daughter, age 25, now has a 2-year-old son. No one else in the family has the disease. What is the probability that the daughter will contract the disease?

• 0%

• 25%

• 50%

• 75%

• 100%

• 0%

• 25%

• 50%

• 75%

• 100%

John, age 47, has just been diagnosed with Huntington’s disease, which is caused by a dominant allele. His daughter, age 25, now has a 2-year-old son. No one else in the family has the disease. What is the probability that the daughter will contract the disease?

An individual with the genotype AaBbEeHH is crossed with an individual who is aaBbEehh. What is the likelihood of having offspring with the genotype AabbEEHh?

• 1/8

• 1/16

• 1/32

• 1/64

• That genotype would be impossible.

An individual with the genotype AaBbEeHH is crossed with an individual who is aaBbEehh. What is the likelihood of having offspring with the genotype AabbEEHh?

• 1/8

• 1/16

• 1/32

• 1/64

• That genotype would be impossible.

ABO blood type in humans exhibits codominance and multiple alleles. What is the likelihood of a type A father and a type A mother having a type O child?

• It is impossible.

• 25% if both parents are heterozygous

• 50% if both parent are heterozygous

• 25% if only the father is heterozygous

• 25% if only the mother is heterozygous

ABO blood type in humans exhibits codominance and multiple alleles. What is the likelihood of a type A father and a type A mother having a type O child?

• It is impossible.

• 25% if both parents are heterozygous

• 50% if both parent are heterozygous

• 25% if only the father is heterozygous

• 25% if only the mother is heterozygous

If roan cattle (incomplete dominance) are allowed to breed, what ratio of phenotypes is expected in the offspring?

• 1:1 red:white

• all roan

• 1:2:1 red:roan:white

• 3:1 red:white


If roan cattle (incomplete dominance) are allowed to breed, what ratio of phenotypes is expected in the offspring?

• 1:1 red:white

• all roan


• 3:1 red:white


Imagine that the last step in a biochemical pathway to the red skin pigment of an apple is catalyzed by enzyme X, which changes compound C to compound D. If an effective enzyme is present, compound D is formed and the apple skin is red. However, if the enzyme is not effective, only compound C is present and the skin is yellow. What can you accurately say about a heterozygote with one allele for an effective enzyme X and one allele for an ineffective enzyme X?

• The phenotype will probably be yellow but cannot be red.

• The phenotype will probably be red but cannot be yellow.

• The phenotype will be a yellowish red.

• The phenotype will be either yellow or red.

• The phenotype will be either yellowish red or red.

Imagine that the last step in a biochemical pathway to the red skin pigment of an apple is catalyzed by enzyme X, which changes compound C to compound D. If an effective enzyme is present, compound D is formed and the apple skin is red. However, if the enzyme is not effective, only compound C is present and the skin is yellow. What can you accurately say about a heterozygote with one allele for an effective enzyme X and one allele for an ineffective enzyme X?

• The phenotype will probably be yellow but cannot be red.

• The phenotype will probably be red but cannot be yellow.

• The phenotype will be a yellowish red.

• The phenotype will be either yellow or red.

• The phenotype will be either yellowish red or red.

Examine this genetic pedigree. What mode of inheritance does this trait follow?

• autosomal dominant

• autosomal recessive

• sex-linked recessive

• mitochondrial





• mitochondrial





• mitochondrial





• mitochondrial

The following offspring were observed from many crossings of the same pea plants. What genotypes were the parents?

465 purple axial flowers 152 purple terminal flowers140 white axial flowers 53 white terminal flowers

• PpAa x PpAA

• PpAa x ppAA

• PPAA x ppaa

• PpAa x PpAa

• PPaa x ppAA

The following offspring were observed from many crossings of the same pea plants. What genotypes were the parents?

465 purple axial flowers 152 purple terminal flowers140 white axial flowers 53 white terminal flowers

• PpAa x PpAA

• PpAa x ppAA

• PPAA x ppaa

• PpAa x PpAa

• PPaa x ppAA

Documents

In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are the P generation