Chromosomes and Inheritance. Thomas Hunt Morgan The common fruit fly – Drosophila melanogaster

Chromosomes and Inheritance

ThomasHuntMorgan

The common fruit fly – Drosophila melanogaster

Gene Linkage

• Each chromosome has hundreds or thousands of genes.

• Genes located on the same chromosome, linked genes, tend to be inherited together because the chromosome is passed along as a unit.

• Results of crosses with linked genes deviate from those expected according to independent assortment.

P Generation (homozygous)

Wild type(gray body, normal wings)

b b vg vg b b vg vg

Double mutant(black body,vestigial wings)

EXPERIMENT



F1 dihybrid(wild type)

TESTCROSS

b b vg vg

b b vg vg

b b vg vg

b b vg vg


Double mutant

EXPERIMENT




Testcrossoffspring

TESTCROSS

b b vg vg

b b vg vg

b b vg vg

b b vg vg


Double mutant

Eggs

Sperm

EXPERIMENT

Wild type(gray-normal)

Black-vestigial

Gray-vestigial

Black-normal

b vg b vg b vg b vg

b b vg vg b b vg vg b b vg vg b b vg vg

b vg




Testcrossoffspring

TESTCROSS

b b vg vg

b b vg vg

b b vg vg

b b vg vg


Double mutant

Eggs

Sperm

EXPERIMENT

RESULTS

PREDICTED RATIOS

Wild type(gray-normal)

Black-vestigial

Gray-vestigial

Black-normal

b vg b vg b vg b vg

b b vg vg b b vg vg b b vg vg b b vg vg

965 944 206 185

1

1

1

1

1

0

1

0

If genes are located on different chromosomes:

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

:

:

:

:

:

:

:

:

:

b vg

Genetic Recombination

• The production of offspring with new combinations of traits inherited from two parents is genetic recombination.

• Genetic recombination can result from independent assortment of genes located on nonhomologous chromosomes or from crossing over of genes located on homologous chromosomes.

Most offspring

F1 dihybrid femaleand homozygousrecessive malein testcross

or

b+ vg+

b vg

b+ vg+

b vg

b vg

b vg

b vg

b vg

Mendel and recombination

• Mendel’s dihybrid cross experiments produced some offspring that had a combination of traits that did not match either parent in the P generation.

– If the P generation consists of a yellow-round parent (YYRR) crossed with a green-wrinkled seed parent (yyrr), all F1 plants have yellow-round seeds (YyRr).

– A cross between an F1 plant and a homozygous recessive plant (a test-cross) produces four phenotypes.

– Half are parental types, with phenotypes that match the original P parents, either with yellow-round seeds or green-wrinkled seeds.

– Half are recombinants, new combination of parental traits, with yellow-wrinkled or green-round seeds.

• A 50% frequency of recombination is observed for any two genes located on different (nonhomologous) chromosomes.

Gametes from green-wrinkled homozygousrecessive parent (yyrr)

Gametes from yellow-rounddihybrid parent (YyRr)

Recombinant offspring

Parental-type

offspring

YR yr Yr yR

yr

YyRr yyrr Yyrr yyRr

Testcrossparents

Replicationof chromosomes

Gray body, normal wings(F1 dihybrid)

Black body, vestigial wings(double mutant)


Meiosis I

Meiosis II

Meiosis I and II

Recombinantchromosomes

Eggs

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

bvg b vgb vg b vg

Testcrossoffspring

965Wild type

(gray-normal)

944Black-

vestigial

206Gray-

vestigial

185Black-normal

Sperm

Parental-type offspring Recombinant offspring

Recombinationfrequency

391 recombinants2,300 total offspring

100 17%

b vg b vgb vgb vg

b vg b vg b vg b vg

b vg

Testcrossparents


Gray body, normal wings(F1 dihybrid)

Black body, vestigial wings(double mutant)


Meiosis I

Meiosis II

Meiosis I and II


Eggs

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vgb vg

b vg

bvg b vgb vg

b vg

Spermb vg

Testcrossoffspring

965Wild type

(gray-normal)

944Black-

vestigial

206Gray-

vestigial

185Black-normal

Sperm

Parental-type offspring Recombinant offspring

Recombinationfrequency

391 recombinants2,300 total offspring

100 17%

b vg b vgb vgb vg

b vg b vg b vg b vg

b vg

Eggs


bvg b vg b vg b vg

Alfred Sturtevant

Linkage Maps

• A linkage map is an ordered list of the genetic loci along a particular chromosome.

• Sturtevant hypothesized that the frequency of recombinant offspring reflected the distances between genes on a chromosome.

• The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore a higher recombination frequency. – The greater the distance between two genes, the more

points between them where crossing over can occur.• Sturtevant used recombination frequencies from fruit fly

crosses to map the relative position of genes along chromosomes, a linkage map.

Chromosome

Recombinationfrequencies

9% 9.5%

17%

b cn vg

RESULTS

Mutant phenotypes

Shortaristae

Blackbody

Cinnabareyes

Vestigialwings

Browneyes

Long aristae(appendageson head)

Gray body

Red eyes

Normalwings

Redeyes

Wild-type phenotypes

104.567.057.548.50

Linkagedistances onfruit flychromosome 2

More on Linkage Maps

• A linkage map provides an imperfect picture of a chromosome.

• Map units indicate relative distance and order, not precise locations of genes.– The frequency of crossing over is not actually uniform over

the length of a chromosome.• Combined with other methods like chromosomal banding,

geneticists can develop cytological maps.– These indicated the positions of genes with respect to

chromosomal features.• More recent techniques show the absolute distances between

gene loci in DNA nucleotides.

Errors and Exceptions in Chromosomal Inheritance

1. Alterations of chromosome number or structure cause some genetic disorders

2. The phenotypic effects of some mammalian genes depend on whether they are inherited from the mother or the father (imprinting)

3. Extranuclear genes exhibit a non-Mendelian pattern of inheritance

More Errors and Exceptions in Chromosomal Inheritance

• Sex-linked traits are not the only notable deviation from the inheritance patterns observed by Mendel.

• Gene mutations are not the only kind of changes to the genome that can affect phenotype.

• Major chromosomal aberrations and their consequences produce exceptions to standard chromosome theory.

Nondisjunction • Nondisjunction occurs when problems with the meiotic spindle cause

errors in daughter cells.– This may occur if tetrad chromosomes do not separate properly

during meiosis I.

– Alternatively, sister chromatids may fail to separate during meiosis II.

• As a consequence of nondisjunction, some gametes receive two of the same type of chromosome and another gamete receives no copy.

• Offspring resulting from fertilization of a normal gamete with one after nondisjunction will have an abnormal chromosome number or aneuploidy.– Trisomic cells have three copies of a particular chromosome type

and have 2n + 1 total chromosomes.– Monosomic cells have only one copy of a particular chromosome

type and have 2n - 1 chromosomes.

Meiosis I

Nondisjunction

Meiosis I

Meiosis II

Nondisjunction

Non-disjunction

Meiosis I

Meiosis II

Nondisjunction

Non-disjunction

Gametes

Number of chromosomes

Nondisjunction of homo-logous chromosomes inmeiosis I

(a) Nondisjunction of sisterchromatids in meiosis II

(b)

n 1 n 1n 1 n 1 n 1n 1 n n

Aneuploidy• If the organism survives, aneuploidy typically leads to a

distinct phenotype.• Aneuploidy can also occur during failures of the mitotic

spindle.• If aneuploidy happens early in development, this condition

will be passed along by mitosis to a large number of cells.• This is likely to have a substantial effect on the organism.• Several serious human disorders are due to alterations of

chromosome number and structure.• Although the frequency of aneuploid zygotes may be quite

high in humans, most of these alterations are so disastrous that the embryos are spontaneously aborted long before birth.– These developmental problems result from an imbalance

among gene products.• Certain aneuploid conditions upset the balance less, leading to

survival to birth and beyond.

Down’s syndrome or Trisomy 21

Aneuploidy and Sex Chromosomes

• Nondisjunction of sex chromosomes produces a variety of aneuploid conditions in humans.

• Unlike autosomes, this aneuploidy upsets the genetic balance less severely.

• This may be because the Y chromosome contains relatively few genes.

• Also, extra copies of the X chromosome become inactivated as Barr bodies in somatic cells.

Aneuploidy and Sex Chromosomes

• Klinefelter’s syndrome, an XXY male, occurs once in every 1000 live births.– These individuals have male sex organs, but are sterile.– There may be feminine characteristics– Their intelligence is normal.

• Males with an extra Y chromosome (XYY) tend to somewhat taller than average. Occurs in about 1 in every 1000 live births.

• Trisomy X (XXX), which occurs once in every 1000 live births, produces healthy females. May have learning disabilities.

• Monosomy X or Turner’s syndrome (X0), which occurs once in every 2500 births, produces phenotypic, but immature females. Most have normal intelligence.

Polyploidy

• Organisms with more than two complete sets of chromosomes, have undergone polypoidy.

• This may occur when a normal gamete fertilizes another gamete in which there has been nondisjunction of all its chromosomes.– The resulting zygote would be triploid (3n).

• Alternatively, if a 2n zygote failed to divide after replicating its chromosomes, a tetraploid (4n) embryo would result from subsequent successful cycles of mitosis.

• Polyploidy is common among plants and much less common among animals.

• The spontaneous origin of polyploid individuals plays an important role in the evolution of plants – in fact as many as 60% of all flowering plant species may have arisen via hybridization which is frequently followed by polyploidy

Hybridizationfollowed byPolyploidy

The Red Viscacha Rat from Argentina – A tetraploid mammal?

Mammalian Chromosome Variation

A. Indian muntjac deer, 2n=6

B. Red Viscacha Rat, 4n=102

C. Siberian roe deer, 2n=70 + up to 14 B’s (accessory chromosomes)

D. Transcaucasian vole, 2n=17, both sexes XO

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Chromosomes and Inheritance. Thomas Hunt Morgan The common fruit fly – Drosophila melanogaster