CHAPTER 9 Patterns of Inheritance

BIOLOGYCONCEPTS & CONNECTIONS

Fourth Edition

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor

From PowerPoint® Lectures for Biology: Concepts & Connections

CHAPTER 9 Patterns of Inheritance

Modules 9.11 – 9.23


• Mendel’s principles are valid for all sexually reproducing species

– However, often the genotype does not dictate the phenotype in the simple way his principles describe

VARIATIONS ON MENDEL’S PRINCIPLES

The relationship of genotype to phenotype is rarely simple


• When an offspring’s phenotype—such as flower color— is

in between the phenotypes of its parents, it exhibits incomplete dominance

1. Incomplete dominance results in intermediate phenotypes

P GENERATION

F1 GENERATION

F2 GENERATION

RedRR

Gametes R r

Whiterr

PinkRr

R r

R R

r r

1/21/2

1/2

1/21/2

1/2 SpermEggs

PinkRr

PinkrR

Whiterr

RedRR

Figure 9.12A


• Incomplete dominance in human hypercholesterolemia

Figure 9.12B

GENOTYPES:

HHHomozygous

for ability to makeLDL receptors

HhHeterozygous

hhHomozygous

for inability to makeLDL receptors

PHENOTYPES:

LDL

LDLreceptor

Cell

Normal Mild disease Severe disease


• In a population, multiple alleles often exist for a characteristic

– The three alleles for ABO blood type in humans is an example

– The alleles are iA…iB….and…i

2. Many genes have more than two alleles in the population


Figure 9.13

– 3. Codominance--The alleles for A and B blood types are codominant, and both are expressed in the phenotype

BloodGroup(Phenotype)

O

Genotypes

AntibodiesPresent inBlood

Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left

O A B AB

A

B

AB

ii

IA IA

orIA i

IB IB

orIB i

IA IB

Anti-AAnti-B

Anti-B

Anti-A


There are at least 29 different blood groups including the ABO blood group.


The Calico Cat-codominance Calico coloring is a mix of phaeomelanin based colors (red) and eumelanin based color (black, chocolate and cinnamon). Cats of this coloration are believed to bring good luck in the folklore of many cultures.[1]

The spotting gene causes white patches to cover the colored fur


Co-Dominance ---Both Alleles express themselves


4. A single gene may affect many phenotypic characteristics

• A single gene may affect phenotype in many ways

– This is called pleiotropy

– The allele for sickle-cell disease is an example


Individual homozygousfor sickle-cell allele

Sickle-cell (abnormal) hemoglobin

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

Sickle cells

Breakdown of red blood cells

Clumping of cells and clogging of

small blood vessels

Accumulation ofsickled cells in spleen

Physical weakness Anemia Heart

failurePain and

feverBrain

damageDamage to

other organsSpleen damage

Kidney failureRheumatism

Pneumonia and other infections

ParalysisImpaired mental

function

Figure 9.14


• This situation creates a continuum of phenotypes

– Example: skin color

5. A single characteristic may be influenced by many genes


Figure 9.16

P GENERATION

F1 GENERATION

F2 GENERATION

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

Eggs Sperm

Fra

cti

on

of

po

pu

lati

on

Skin pigmentation


• Genetic testing can be of value to those at risk of developing a genetic disorder or of passing it on to offspring

9.15 Connection: Genetic testing can detect disease-causing alleles

Figure 9.15B

Figure 9.15A

• Dr. David Satcher, former U.S. surgeon general, pioneered screening for sickle-cell disease

• Thallasemia, Tay Sachs


• Canavan Disease -- This condition is most common in people of Ashkenazi Jewish ancestry, with a carrier incidence of 1 in 40. Canavan disease is a central nervous system disease that is usually fatal in childhood, with a few people surviving to adulthood. This disease is the result of a substance that destroys the central nervous system over time. There is presently no effective treatment for Canavan disease.

• Fragile X Syndrome -- The Fragile X syndrome is not specific to a certain ethnic background. It is an inherited condition that can cause a range of intellectual and behavioral problems, from learning disabilities to mental retardation to autism. While Fragile X syndrome tends to be more severe in boys, it occurs in both males or females. It can be passed on to family members by individuals who have no signs of the syndrome. Review of your family history with a genetic counselor may help determine if Fragile X carrier testing is indicated.

• Sickle Cell Disease -- This condition is most common in persons of African-American, African, Mediterranean, Hispanic and South American ancestry, with the carrier risk ranging from 1/10 to 1/40, depending on your ethnic background. Sickle cell disease is caused by a variant hemoglobin that changes the shape of the red blood cells. This causes anemia, severe pain, a tendency toward infection, and other serious health problems. Frequent blood transfusions and infection preventing antibiotics are available treatment.

• Tay Sachs Disease -- People of both Ashkenazi Jewish and French Canadian ancestry have the greatest chance of being carriers of Tay Sachs disease, about 1/30 versus 1/250 in the general population. The disease results from a build up of certain substances in the brain, and is fatal in early childhood. There is presently no effective treatment for Tay Sachs disease.

• Thalassemia -- Individuals of Mediterranean, Southeast Asian and African ancestry have the greatest chance - 1 in 3 and 1 in 30, respectively -- of being carriers for thalassemia. In general, this group of blood disorders affects a person's ability to produce hemoglobins, the protein in our blood that carries oxygen and nutrients to all parts of the body. In severe cases, children with thalassemia may not survive. Others have anemia, bone growth problems and liver and spleen involvement. Blood transfusions may be needed for treatment.


Prenatal diagnosis for couples testing positive after genetic testing

• Chorionic villus sampling at 8 weeks of gestation.

• Amniocentesis at 12-18 weeks of gestation.

• Pre-implantation genetic testing and In vitro fertilization


• Genes are located on chromosomes

– Their behavior during meiosis accounts for inheritance patterns

THE CHROMOSOMAL BASIS OF INHERITANCE

9.17 Chromosome behavior accounts for Mendel’s principles


• The chromosomal basis of Mendel’s principles

Figure 9.17


Question?

• What is the phenotypic ratio between a dihybrid cross involving two heterozygotes?

• 9:3:3:1


• Certain genes are linked

– They tend to be inherited together because they reside close together on the same chromosome

9.18 Genes on the same chromosome tend to be inherited together

When would they not be inherited together?


Figure 9.18

If you cross PpLl x PpLl, what Phenotypic ratio do you expect?

When the organism was selved or self pollinated, most of the progeny looked like the parent but the 9:3:3:1 ratio was not realized!! Linked genes were then suspected.


• This produces gametes with recombinant chromosomes

• The fruit fly Drosophila melanogaster was used in the first experiments to demonstrate the effects of crossing over

9.19 Crossing over produces new combinations of alleles


A B

a b

Tetrad Crossing over

A B

a

ba

BA b

Gametes

Figure 9.19A, B

When does crossing over take place?


Question?

• What is the phenotypic ratio between a dihybrid cross involving two heterozygotes?

• 9:3:3:1


Figure 9.19C

When you cross a di-hybrid heterozygote with a homozygous recessive, you expect a ¼ distribution of each potential phenotype.

GgLl X ggll

When this particular cross was performed, this was not the case. Most of the organisms resembled the parents. A few had the independently assorted phenotypes.What had occurred is that genes for body color and wing type were on the same chromosome (linked) and in some gametes, crossing over did occur, but in most gamestes, G and L were linked as were g and l.


Crossing Over and LinkageIs crossing over more likely to occur between genes that are close together or farther apart?

Does crossing over comply with Mendel’s Law of Independent Assortment? Law of Segregation?


• Crossing over is more likely to occur between genes that are farther apart

– Recombination frequencies can be used to map the relative positions of genes on chromosomes.

– Determine the location of g and c and l on the chromosome. Recombination frequency between g and c is 9%, between c and l is 9.5%, and between g and l is 17%.

9.20 Geneticists use crossover data to map genes

g

Figure 9.20B

Chromosome

c l

17%

9% 9.5%


answer

• Answer: 3.6% recombination

• 3.6 map units

• 3.6 centiMorgans


• In corn C (colored) is dominant to c (colorless) and for the endosperm (part of seed where food is stored for embryo) Full (F) is dominant to shrunken (f). When an CcFf was test crossed, the results were as follows:

• Colored, full 4032

• colored, shrunken 149

• Colorless, full 152

• Colorless, shrunken 4035

• What do you expect if the genes are not linked? CcFf x ccff? 1:1:1:1

• We got 27:1:1:27


• The Chinese primrose-slate color (s) is recessive to blue (S), red stigma(r) is recessive to green stigma (R), and long style (l) is recessive to short style (L). All three genes are on the same chromosome. The F1 of a cross of true breeding strains, was test crossed and gave the following:

• Slate, green, short 27

• Slate, red, short 85

• Blue, red short 402

• Slate, red, long 977

• Slate, green, long 427

• Blue, green, long 95

• Blue, green, short 960

• Blue, red ,long 27

• Total 3000


Answer the following questions:

• What were the genotypes of the parents in the cross of the two true-breeding strains?

• Make a map of the genes, showing gene order and distance between them.

• Answer: hum??????????????????


• slate (s) red (r) long (l)

• Blue (S) Green (R) short (L)

• SsRrLl x ssrrll Expect 1:1:1:1 If not expect crossing over.

• To determine placement of genes on chromosomes:

– Inspect for highest frequencies for parental phenotypes. (NO crossing over)

– Inspect for other phenotypes to show single and double crossing over. (Lowest # is double crossing over)

– Determine position of genes on chromosome. The gene that has changed position relative to the other two is the central (observe double cross overs only).

– Designate regions I and II and calculate crossing over in those regions. Add all combinations in each region (both single cross over and double) to determine cross over frequency in that region.


• Blue, Green, Short and slate, red, long are parental phenotypes

• Blue, red, long and slate, Green, Sort are the double crossovers.

• Flower color (Blue vs slate)has changed position relative to stigma and style. “S: is in the middle. Rsl and RSL

• Region I ---Rsl and rSL + RsL and rSl = 427 + 402 + 27 + 27 = 883/3000 = 29.43%

• Region II ---RSl and rsL + RsL and rSl = 95 + 85 + 27 + 27 = 234/3000 = 7.8%

• Thus, R----------------S-----L

•


answer

• Between r and s the distance is 29.4 map units

• Between s and l is 7.8 map units.


• Alfred H. Sturtevant, seen here at a party with T. H. Morgan and his students, used recombination data from Morgan’s fruit fly crosses to map genes

Figure 9.20A


• A partial genetic map of a fruit fly chromosome

Figure 9.20C

Shortaristae

Blackbody(g)

Cinnabareyes(c)

Vestigialwings(l)

Browneyes

Long aristae(appendageson head)

Graybody(G)

Redeyes(C)

Normalwings(L)

Redeyes

Mutant phenotypes

Wild-type phenotypes


• A human male has one X chromosome and one Y chromosome

• A human female has two X chromosomes

• Whether a sperm cell has an X or Y chromosome determines the sex of the offspring

SEX CHROMOSOMES AND SEX-LINKED GENES

9.21 Chromosomes determine sex in many species


Figure 9.21A

X Y

Male

(male)

Parents’diploidcells

(female)

Sperm

Offspring(diploid)

Egg


• Other systems of sex determination exist in other animals and plants

Figure 9.21B-D

– The X-O system

– The Z-W system

– Chromosome number


• All genes on the sex chromosomes are said to be sex-linked

– In many organisms, the X chromosome carries many genes unrelated to sex

– Fruit fly eye color is a sex-linked characteristic

9.22 Sex-linked genes exhibit a unique pattern of inheritance

Figure 9.22A


– Their inheritance pattern reflects the fact that males have one X chromosome and females have two

Figure 9.22B-D

– These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait

Female Male Female Male Female Male

XrYXRXR

XRXr

XRY

XR Xr

Y

XRXr

XR

Xr XRXR

XR

Y

XRY

XrXR XRY

XrY

XRXr

XR

Xr

Xr

YXRXr

XrXr XRY

XrY

XrY

R = red-eye alleler = white-eye allele


• Most sex-linked human disorders are due to recessive alleles

– Examples: hemophilia, red-green color blindness

– These are mostly seen in males

– A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected

9.23 Connection: Sex-linked disorders affect mostly males

Figure 9.23A


• A high incidence of hemophilia has plagued the royal families of Europe

Figure 9.23B

QueenVictoria

Albert

Alice Louis

Alexandra CzarNicholas IIof Russia

Alexis

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CHAPTER 9 Patterns of Inheritance