concepts and connections - biology things - home · 12/14/2016 1 genetics essentials concepts and...

12/14/2016

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GENETICS ESSENTIALS

Concepts and ConnectionsTHIRD EDITION

Benjamin A. Pierce

CHAPTER 4

Extensions and Modifications of

Basic Principles

© 2014 W. H. Freeman and Company

Sex Is Determined by a Number of

Different Mechanisms

• There are several different mechanisms of sex

determination

• The X and Y chromosomes pair during meiosis,

even though they are not homologous (the

genes located on each are different)

Sex Determination

• Sexual reproduction: alternates between haploid

and diploid states

• Most organisms have two sexual phenotypes,

male and female

Chromosomal Sex-Determination Systems:

Sex Chromosomes and Non-Sex

Chromosomes (Autosomes)

• XX-XO system:

• XX – female

• XO – male

• grasshoppers

• XX-XY system:

• XX – female

• XY – male

• mammals

Chromosomal Sex-Determination

Systems

• ZZ-ZW system:

• ZZ – male

• ZW – female

• Birds, snakes, butterflies, some amphibians,

and fishes

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Genic Sex-Determining System

• No sex chromosomes, only the sex-determining

genes

• Found in some plants, fungi, protozoans, and

fish

Environmental Sex Determination

• Environmental factors

• Slipper shell: position in the stack

• Crepidula fornicata

• Temperature in reptiles

• Turtles: Warmer = more females

• Crocodiles: Warmer = more males

Sex Determination in Drosophila

melanogaster

• Genic balance system

• X : A ratio (X, number of X chromosomes;

A, number of haploid sets of autosomes)

Sex Determination in Humans

XX-XY

• SRY gene on the Y chromosome determines

maleness

• Turner syndrome: XO; 1/3000 female births

• Klinefelter syndrome: XXY, or XXXY, or

XXXXY, or XXYY; 1/1000 male births

• Poly-X females: 1/1000 female births

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4.2 Sex-Linked Characteristics Are

Determined by Genes on the Sex

Chromosomes

• X-Linked characteristics

– X-linked white eye in Drosophila

– X-linked color blindness in humans

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Concept Check 4

Hemophilia (reduced blood clotting) is an X-

linked recessive disease in humans. A woman

with hemophilia mates with a man who exhibits

normal blood clotting. What is the probability

that their child will have hemophilia?

All of their male offspring will have hemophilia, and

none of their female offspring will have hemophilia,

so the overall probability of hemophilia in their

offspring is ½.

Calico and Tortoiseshell cats

Orange is X-linked

X+ = black

Xo = orange

Random inactivation

Mean some cells produce

orange some cells produce black -

Tortoiseshell

Concept Check 5

How many Barr bodies will a male with XXXYY

chromosomes have in each of his cells? What

are those Barr bodies?

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Y-Linked Characteristics

• Only present in males

• All male offspring will exhibit the trait

• Y chromosome lost DNA over time

• Important for sex determination in SRY

Additional Factors at a Single Locus Can

Affect the Results of Genetic Crosses

• Genes at the same locus—two versions of the

same gene; each version of the same gene is

defined as allele.

• Types of dominance

– Complete dominance

– Incomplete dominance

– Codominance

• Penetrance: the percentage of individuals

having a particular genotype that express

the expected phenotype.

• Expressivity: the degree to which a

character is expressed.



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Lethal Alleles

If before birth genotypes do not appear among

offspring– Ratio skewed 2:1 • Multiple alleles: for a given locus, more

than two alleles are present within a group

of individuals.

• ABO blood group



Gene Interaction Takes Place When Genes

At Multiple Loci Determine a Single

Phenotype

Gene interaction: Effects of genes at one

locus depend on the presence of genes at

other loci.

– Gene interaction that produces novel

phenotypes

– Gene interaction with epistasis: one gene

masks the effect of another gene

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• Epistasis: one gene masks the effect of

another gene.

• Can look similar to dominance/recessive, but

not the same gene

‒ Recessive epistasis

‒ Dominant epistasis

‒ Duplicate recessive epistasis



Phenotype

Dominant epistasis in squash

BBEE X bbee

Black Lab Yellow Lab

Recessive Epistasis

BbEe

Black Lab

BbEe

Black LabX

9/16 B_E_ Black

3/16 bbE_ Brown

3/16 B_ee

1/16 bbeeYellow

ee is epistatic to B and b



Phenotype

Blue eye color hypothesis: Duplicate recessive epistasis.

Could two blue eyed parents have a brown eyed child?

Concept Check 8

A number of all-white cats are crossed, and they

produce the following types of progeny. 12/16

all-white, 3/16 black, and 1/16 gray. What is the

genotype of the black progeny?

a. Aa

b. Aa Bb

c. A_ B_

d. A_ bb

Sex Influences the Inheritance and

Expression of Genes in a Variety of Ways

Sex-influenced and sex-limited

characteristics

‒ Sex-influenced characteristics

‒ Sex-limited characteristics

Both sex-influenced and sex-limited characteristics are

encoded by autosomal genes whose expression is affected

by the sex of the individual who possesses the gene. Sex-

linked characteristics are encoded by genes on the sex

chromosomes.

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Cytoplasmic Inheritance

Some DNA is found in the mitochodria

(or in plants the chloroplasts).

Inheritance of mitochodria is almost

always just from the mother. Different

phenotypes exists mostly due to the

uneven distribution of the DNA (no

mieosis).

Position Effect

Offspring’s phenotype determined by

phenotype of branch where the pollen

germinated.

Maternal Effect

Offspring’s phenotype determined by

mother’s genotype.

• Sex-influenced and sex-limited characteristics

– Genetic maternal effect

– Genomic imprinting: differential expression of genetic

material depending on whether it is inherited from the

male or female parent

Epigenetics: phenomena due to alterations to DNA that

do not include changes in the base sequence; often

affect the way in which the DNA sequences are

expressed

Sex Influences the Inheritance and

Expression of Genes in a Variety of Ways

Genomic Imprinting

• Gene expressed only if

given by male parent

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The Expression of a Genotype May Be

Influenced By Environmental Effects

• Temperature-sensitive allele: an allele whose

product is functional only at certain

temperature.

The Inheritance of Continuous

Characteristics

• Discontinuous characteristics: relatively few

phenotypes

• Continuous characteristics: continuous

distribution of phenotypes; occurs when genes

at many loci interact

• Polygenic characteristics: characteristics

encoded by genes at many loci

• Pleiotropy: one gene affects multiple

characteristics

GENETICS ESSENTIALS

Concepts and ConnectionsTHIRD EDITION

Benjamin A. Pierce

CHAPTER 5

Linkage, Recombination, and

Eukaryotic Gene Mapping

© 2014 W. H. Freeman and Company

Linked Genes Do Not Assort

Independently

• Principle of segregation: alleles separate

during meiosis

• Independent assortment: alleles at one

locus sort independently from alleles at

another locus

• Recombination: alleles sort into new

combinations

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Linked Genes Segregate Together and

Crossing Over Produces Recombination

Between Them

• Complete linkage leads to nonrecombinant

gametes and nonrecombinant progeny

• Crossing over with linked genes leads to

recombinant gametes and recombinant progeny

Concept Check

For single crossovers, the frequency of recombinant gametes is half the frequency of crossing over because

a. a testcross between a homozygote and heterozygote produces ½ heterozygous and ½ homozygous progeny.

b. the frequency of recombination is always 50%.

c. each crossover takes place between only two of the four chromatids of a homologous pair.

d. crossovers occur in about 50% of meiosis.

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Between Them

• Calculating Recombination Frequency

– Recombination frequency = (No. recombinant

progeny/Total no. of progeny) x 100%

• Coupling and Repulsion Configuration of Linked Genes

– Coupling (cis configuration): wild-type alleles are found on one

chromosome; mutant alleles are found on the other chromosome

a+ b+

a b

a+ b

a b+

Coupling or cis Repulsion or trans

Testing for independent

assortment

• Our Friend Chi Square

• Gene Mapping with Recombination

Frequencies

– Genetics maps are determined by

recombinant frequency

– Map unit and centiMorgans

• Constructing a Genetic Map with Two-

Point Testcrosses?



Between Them

Genetic vs. Physical Maps

Two-point crosses – issues

1.Cannot distinguish between genes far apart on one chromosome &

on different chromosomes. Both may exhibit 50% recombination.

2. Testcross for two genes that are linked but not close to each other

will underestimate the true distance because of double

recombination

3. Double crossover will produce non-recombined phenotype

Genetic maps are based on rates of

recombination; physical maps are based on

physical distances.

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A Three-Point Testcross Can Be Used to

Map Three Linked Genes

• Constructing a Genetic Map with the

Three-Point Testcross

– Determining the gene order

– Determining the location of crossovers

Recombination Freq 50%

• On separate

chromosomes

• Or very far apart

Recombination Freq less than 50%

• In same linkage group

Basic idea

• % recombinants = distance

Genetic Mapping

Linkage groups

Start of worked problem:

a and b, 10% recomb. – 10m.u. apart – linkage group 1

a and c, 50% recomb. – assorted independently, c in another linkage group

a and d, 14% recomb. In linkage group 1,

b and d, 4% recomb, so b is closer to d than to a

Map so far is a__10___b__4_d

Three-point cross

Concept Check

Write the genotypes of all recombinant

and nonrecombinant progeny expected

from the following three-point cross:

Concept Check

A three-point testcross is carried out

between three linked genes. The

resulting nonrecombinant progeny are

s+r+c+ and s r c, and the double-

crossover progeny are s r c+ and s+r+c.

Which is the middle locus?

The c locus

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1. Determine gene order.

Based on double recombinants

Least number of progeny

2. Determine location of crossovers.

Identify number and location

of single crossovers.

Identify non-recombinant progeny.

3. Calculate recombination frequencies.

single recombinants + double recombinants

total number of offspring

Three-point cross

4. Coefficient of coincidence =

number of observed double crossovers

number of expected double crossovers

Number of expected double crossovers =

probability of single crossovers X probability of single crossovers X total number

between genes 1 & 2 between genes 2 & 3 progeny

5. Interference – the degree to which one crossover interferes with additional

crossovers.

Interference = 1 – coefficient of coincidence

When no double crossovers occur, interference is complete

(interference = 1). When it is between 0 and 1, one crossover

is interfering with additional crossovers. When it is 0, one

crossover does not interfere with additional crossovers. When

it is negative, a crossover increases the probability of another

crossover.

• Calculating the Recombination Frequencies

• Sum of all single and double

crossovers/Total progeny

• Interference and Coefficient of Coincidence

– Coefficient of coincidence = Number of observed

double crossovers/Number of expected double

crossovers

– Interference = 1n – Coefficient of coincidence

A Three-Point Testcross Can Be Used to

Map Three Linked Genes

P.127

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Mapping Techniques

• New Techniques – more closely look at

DNA variations

– Mapping with Molecular Markers

• RFLPs

• Genome-wide Association Studies

– Associations within populations

– Haplotype

CHAPTER 6

Chromosome Variation

Bananas have multiple

sets of chromosomes.

Autopolyploid: 3n

33 chromosomes

Chromosome Mutations Include

Rearrangements, Aneuploids, and

Polyploids

• Chromosome morphology (position of the centromere on the chromosome):

‒ Metacentric

‒ Submetacentric

‒ Acrocentric

‒ Telocentric

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• Types of Chromosome Mutations:

– Rearrangements:

• Aneuploidy, change in # of individual chromosome: 2n +1

• Polyploidy, addition of whole set: 3n

Chromosome Mutations Include

Rearrangements, Aneuploids, and

Polyploids

Chromosome Rearrangements Alter

Chromosome Structure

• Four types of chromosomal rearrangements:

– Duplication, Deletion, Inversion, Translocation

• Unbalanced gene dosage:

• Tandem Duplication – directly next to duplicated region

• Displaced Duplication – away from duplicated region

• Reverse Duplication – ABCDEFFEDCBA

Specific Duplications

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Bar phenotype

• Fewer eye facets

• Bar shape

• X-linked duplication

Unbalanced gene dosage• Developmental abnormalities

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• Deletions: loss of a chromosomal segment

• Large deletions easily detected; during pairing,

normal chromosome loops out

• Effects of deletions

‒ Imbalances in gene product

‒ Pseudodominance: ‒ Expression of a normally recessive gene in a heterozygous due to a

deletion on one chromosome

‒ Haploinsufficiency‒ When two copies are needed to produce wild type effect. A deletion

results in only one normal copy of the gene; Notch: Drosophila

needs two copies of gene to form on wings



• Inversion (depends on the involvement of the

centromere in the inversion):

‒ Paracentric inversion

‒ Pericentric inversion

Translocations

• Nonreciprocal translocation

• Reciprocal translocation

• Robertsonian translocation

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Aneuploidy Is an Increase or Decrease in

the Number of Individual Chromosomes

• Variations in copy number: aneuploidy and

polyploidy

• Causes of aneuploidy

– Deletion of centromere during mitosis and

meiosis

– Robertsonian translocation

– Nondisjunction during meiosis and mitosis

Types of Aneuploidy

• Nullisomy: loss of both members of a

homologous pair of chromosomes. 2n − 2

• Monosomy: loss of a single chromosome.

2n − 1

• Trisomy: gain of a single chromosome. 2n + 1

• Tetrasomy: gain of two homologous

chromosomes. 2n + 2

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Concept Check 5

A diploid organism has 2n = 36 chromosomes.

How many chromosomes will be found in a

trisomic member of this species?

2n + 1 = 36 + 1 = 37



• Effects of Aneuploidy

– In humans

• Sex-chromosome aneuploids

– Turner syndrome. XO

– Klinefelter syndrome. XXY



• Effects of Aneuploidy:

– In plants: mutants could actually be trisomics

– In humans

• Autosomal aneuploids:

– Trisomy 21: Down syndrome

» Primary Down syndrome, 75% random

nondisjunction in egg formation

» Familial Down syndrome, Robertsonian

translocation between chromosomes 14 and

21



• Effects of Aneuploidy:

– In humans

• Autosomal aneuploids

– Trisomy 18: Edward syndrome, 1/8000 live

births

– Trisomy 13: Patau syndrome, 1/15,000 live

births

– Trisomy 8: 1/25,000 ~ 1/50,000 live births

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• Effects of Aneuploidy:– In humans

• Autosomal aneuploids

– Aneuploidy and maternal age:

– Possible interpretation of this connection?

• Uniparental disomy: both chromosomes are inherited from

the same parent.

– Mosaicism and nondisjunction in mitotic division

Polyploidy is the Presence of More

Than Two Sets of Chromosomes

• Autopolyploidy

– From single species

• Allopolyploidy

– From two species

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Allopolyploid Allopolyploid

Allopolyploid Allopolyploid

• The significance of polyploidy

– Increase in cell size

– Larger plant attributes

– Evolution: may give rise to new species

Polyploidy is the Presence of More

Than Two Sets of Chromosomes

Polyploidy and Sterility

No seeds - Bananas & Watermelon

Wheat is hexploid

• Originally 3 species

Allopolyploidy

between species

Autopolyploidy same

species

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Concept Check

Species A has 2n = 16 chromosomes and species

B has 2n = 14. How many chromosomes would be

found in an allotriploid of these two species?

a. 21 or 24

b. 42 or 48

c. 22 or 23

d. 45

Symbols

• 2n = diploid

• 3n, 4n = polyploid

• 2n + 1 = trisomy (type of aneuploidy),+2

tetrasomy

• 2n – 1 = monosomy

– (X_ unk or XO indicates no second sex

chromosome

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