Fig. 15-1 The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene

Fig. 15-1

The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene

Fig. 15-4a

EXPERIMENT

PGeneration

GenerationF1 All offspring

had red eyes

Fig. 15-4b

RESULTS

GenerationF2

Fig. 15-4c

EggsF1

CONCLUSION

Generation

Generation

P XX

w

Sperm

XY

+

+

++ +

EggsSperm

+

++ +

+

GenerationF2

w

w

w

ww

w

w

w

ww

w

w

w

ww

Fig. 15-7

(a) (b) (c)

XNXN XnY XNXn XNY XNXn XnY

YXnSpermYXNSpermYXnSperm

XNXnEggs XN

XN XNXn

XNY

XNY

Eggs XN

Xn

XNXN

XnXN

XNY

XnY

Eggs XN

Xn

XNXn

XnXn

XNY

XnY

The transmission of sex linked recessive genes

• Sex-linked genes follow specific patterns of inheritance

• For a recessive sex-linked trait to be expressed– A female needs two copies of the allele– A male needs only one copy of the allele

• Sex-linked recessive disorders are much more common in males than in females

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Partial Linkage

• Linkage is different from sex linkage• Linked genes tend to be inherited together

because they are located near each other on the same chromosome. Results from genes being closely linked on the same chromosome

• Linked genes in genetic experiments deviate from the results expected from Mendel’s law of independent assortment.

Dihybrid Testcross to

Detect Independent

Assort

YYRR X yyrr

YrRr X yyrr

YR Yr yRyr yr

yyRr

Yyrr

yyrr

YyRr

Eggs

Spermyr

YR

yr

Yr

yR

Phenotypic ratio1:1:1:1

Ratio of parental:Recombinant1:1

Dihybrid

Fig. 15-9-1

EXPERIMENTP Generation (homozygous)

Wild type(gray body,normal wings)

Double mutant(black body,vestigial wings)

b b vg vg b+ b+ vg+ vg+

Morgan 1912

Fig. 15-9-2




b b vg vg

b b vg vg

Double mutantTESTCROSS

b+ b+ vg+ vg+

F1 dihybrid(wild type)

b+ b vg+ vg

Fig. 15-9-3




b b vg vg

b b vg vg


b+ b+ vg+ vg+


b+ b vg+ vg

Testcrossoffspring Eggs b+ vg+ b vg b+ vg b vg+

Black-normal

Gray-vestigial

Black-vestigial

Wild type(gray-normal)

b vg

Sperm

b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg

Fig. 15-9-4


RESULTS



b b vg vg

b b vg vg


b+ b+ vg+ vg+


b+ b vg+ vg

Testcrossoffspring Eggs b+ vg+ b vg b+ vg b vg+

Black-normal

Gray-vestigial

Black-vestigial

Wild type(gray-normal)

b vg

Sperm

b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg

PREDICTED RATIOS

If genes are located on different chromosomes:

If genes are located on the same chromosome andparental alleles are always inherited together:

1

1

1

1

1 1

0 0

965 944 206 185

:

:

:

:

:

:

:

:

:

Fig. 15-10Testcrossparents

Replicationof chromo-somes

Gray body, normal wings(F1 dihybrid)

Black body, vestigial wings(double mutant)

Replicationof chromo-somes

b+ vg+

b+ vg+

b+ vg+

b vg

b vg

b vg

b vg

b vg

b vg

b vgb vg

b vg

b+ vg+

b+ vg

b vg+

b vg

Recombinantchromosomes

Meiosis I and II

Meiosis I

Meiosis II

b vg+b+ vgb vgb+ vg+Eggs

Testcrossoffspring

965Wild type

(gray-normal)

944Black-

vestigial

206Gray-

vestigial

185Black-

normal

b+ vg+

b vg b vg

b vg b+ vg

b vg b vg

b vg+

Sperm

b vg

Parental-type offspring Recombinant offspring

Recombinationfrequency =

391 recombinants2,300 total offspring

100 = 17%

20% recombination

A B

a b

With crossing overA B

a bParental

a B

A b

Recombinants

40%

40%

10%

10%

Testing for Assortment/Linkage

1. Generate a dihybrid2. Testcross the dihybrid3. Compare the % of parental to recombinants

A. If 50% parental:50% recombinant – Independent Assortment

B. If more parental than recombinant – partial linkageC. If only parental and no recombinant – complete linkage

• The discovery of linked genes and recombination due to crossing over led Alfred Strutevant to a method of constructing genetic maps

• He assumed the farther apart genes are , the higher the probability that a cross over will happen between them and therefore the higher the recombination frequency.

The closer the two genes are on a chromosome the fewer recombinants

Minimum = 0% recombinants

The further two genes are on a chromosome the more recombinants

Maximum = 50% recombinants

Linkage therefore can be used as a measure of genetic distance on chromosome

1 Map Unit = 1 % recombination

Gene Order on ChromosomeB – Vg 17 MUB – Cn 9 MUVg – Cn 9.5 MU

Partial Linkage – two genes are so close on the same chromosome that recombination occurs less than 50% of the time.

Complete Linkage – two genes on the same chromosome so close that recombination cannot separate them.

Independent Assortment – two genes on different chromosomes or two genes on the same chromosome but far enough apart that recombinant occurs 50% of the time.

Example ProblemIn Drosophila long wings is dominant to dumpy wings and round

eyes is dominant to star eyes. A dihybrid fly was generated by mating a long wing round eye fly with a dumpy wing star eye fly. This dihybrid fly was testcrossed and the following progeny were generated.

222 long wing round eye215 dumpy wing star eye33 long wing star eye30 dumpy wing round eye

a. Are these genes completely linked or partially linked?b. What is the genetic distance between these two genes?c. How would the results have differed if the genes

independently assorted?

Exception to chromosomal Inheritance (Organellar Genes)

• The inheritance of traits controlled by genes present in the chloroplasts or mitochondria– Depends solely on the maternal parent because

the zygote’s cytoplasm comes from the egg– Maternal Inheritance

Pedigree Symbols

Nuclear vs Organellar

Human GeneticsPedigree Analysis

Autosomal vs Sex Linked

Multifactorial Traits

• Heart disease• Personality• IQ

Alterations of chromosome number or structure cause some genetic disorders.

• So far we’ve seen that the phenotype can be affected by small scale changes involving individual genes

• Random mutations are the source of all new alleles, which can lead to a new phenotype.

Abnormal chromosome #: Aneuploidy

Human Aneuploids

• Trisomy 21• Sex chromosome

– XO – turner syndrome– XXY – klinefelters– XYY

Abnormal chromosome numbers

• Polyploidy: Common in plant• ~70 % of flowering plants, • Banana are triploid,• Wheat 6n• Strawberries 8n

Alterations in chromosome structure

• Meiosis errors and damaging agents such as radiation can cause breakage of the chromosome

• four types of structural damage

Chromosome Structure

reciprocal translocation between 9 and 22(Philadelphia Chromosome)

Disorders caused by structurally altered chromosomes

• Cri du chat – deletion in chromosome 5 • Chronic myogenous leukemia

Normal chromosome 9

Normal chromosome 22

Reciprocaltranslocation Translocated chromosome 9

Translocated chromosome 22

Fig. 15-8

X chromosomes

Early embryo:

Allele fororange fur

Allele forblack fur

Cell division andX chromosomeinactivationTwo cell

populationsin adult cat:

Active XActive X

Inactive X

Black fur Orange fur

Barr Body – inactive X visible in interphase nucleus

Genomic imprinting• Def: a parental effect on gene

expression• Identical alleles may have

different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm.

• Fragile X syndrome: higher prevalence of disorder and retardation in males

Documents

Fig. 15-1 The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene