People beleived that the nucleus and its contents disappeared between cell divisions and re-formed again when it was time for the cell to divide again.

What actually happens is that the nucleus and its contents continue to exist during interphase.

Chromosomes and genes are both present in pairs in diploid cells.

Both homologous chromosomes separate and alleles separate.

Fertilization restores the paired condition for both chromosomes and genes.

He bred fruit flies (prolific breeders and generation time =2weeks).

Discovered that they have three pairs of autosomes and one pair of sex chromosomes.

crossed white-eyed male with a red-eyed female

->F1 offspring had red eyes = dominant

crossed F1 offspring

->F2 offspring = 3:1 phenotypic ratio

White-eyed trait only in males.

Red eyes in females and half in males.

Fly's eye color linked to its sex.

Conclusion: White-eyed mutation is on the X chromosome only.

Sum up the significance of Morgan's experiment.

1.) Mutations couldn't be produced

He tried exposing cultures of flies to various toxins, temperatures, and radiation

2.) F1 generation had red eyes (dominance)

When crossing white-eyed male with red-eyed female

3.) F2 generation had 3:1 phenotypic ratio

Crosses between F1 generation

2.) Determining sex-linked genes.

(Genes that are carried on a sex chromosome.)

White-eyed trait appeared only in males.

Red eyes in females and half in males.

Conclusion: white-eyed mutation is only on X chromosome.

Knowing that the "red-eye" allele is dominant, females with XX could either be homozygous or hetrozygous for the red-eye trait.

Males with XY could be red-eyed with the red-eyed allele or white-eyed with the white-eyed allele.

The normal character phenotype.

Alternative phenotype.

(Genes that are on the same chromosome tend to travel together.)

1.) He observed this linkage and deviations when following the inheritance of characters for body color and wing size.

wild type body color = gray (b+)

mutant = black (b)

wild-type wing size = normal (vg+)

mutant = vestigial wings (vg)

2.) Crossed heterozygous females wild type (b+b/vg+vg) with homozygous recessive males black-vestigial (bb/vgvg)

b+b/vg+vg X bb/vgvg

-> supposed to be 4 phenotypes in a 1:1:1:1 ratio

(wild type, black-vestigial, gray-vestigial, black-normal = all 575)

3.) more wild-type (gray-normal) and double-mutant (black-vestigial) flies (like parents)

How do genes become unlinked...? (next)

genetic recombination

How does this happen?

production of offspring with new combinations of traits inherited from two parents

1.) genes aren't on same chromosome
->Law of independent assortment

-> should get 4 different phenotypes in F2 generation (2 same as parents, 2 are "recombinants" = different)

ex. YyRr and yyrr -> YyRr yyrr Yyrr yyRr

-> orientation of tetrad with one character has no bearing on orientation of tetrad with other character

2.) genes are on the same chromosome

->crossing over takes place between loci located on homologous chromosomes

-> genes move together through meiosis and fertilization

-> should not expect linked genes to recombine into assortments of alleles not found in parents -> COMPLETE LINKAGE

Morgan's testcross

Connection with independent assortment or complete linkage?

Conclusion?

Independent assortment

 both parental phenotypes and 2 different recombinant phenotypes

Complete linkage

->1:1:0:0 ratio with only parental phenotypes among offspring

Only most of offspring had parental phenotypes

->incomplete linkage (17% = recombinants)

Conclusion: some mechanism exchanged segments between homologous chromosomes

Recombinant genes

What is the mechanism that resulted in incomplete linkage?

Crossing over during prophase I

-> production of more types of gametes than predicted by Mendel

Alfred Sturtevant

->used crossing over of linked genes for chromosome map (ordered list of genetic loci along a particular chromosome)

Recombinant genes

What is the use of the chromosome map?

Hypothesis: frequency of recombinant offspring = function of distances between genes on a chromosome

Farther apart of 2 genes are

-> more points between them where crossing over can occur

->higher the probability that a cross over will occur between them

-> higher frequency of recombination than genes that are closer together

Recombinant genes

1. What is the linkage map?

2. Why are recombination frequencies not additive?

3. Why is this an imperfect picture of a chromosome?

1. Map of the relative position of genes along chromosomes using recombination frequencies from test crosses

ex. map of relative position of 3 fruit fly genes (b, vg, cn [eyecolour])

cn-b = 9%

cn -vg = 9.5%

b-vg = 17%

cn must be between the other 2

2. multiple crossing over events

-(second crossing over can reverse earlier one - cancelling out the first, reducing observed number of recombinant offspring)

-genes farther apart (b-vg) are more likely to experience multiple crossing over events

3. frequency of crossing over is not uniform over the length of a chromosome

- map units indicate relative distance and order, not precise locations of genes

-most recent techniques show absolute distances between gene loci in DNA nucleotides

Sex chromosomes

1. What are they?

2. What systems are used for the same function?

3. How does the X-Y system behave during meiosis?

1. determine the sex of an individual

XX = usually female

XY = usually male

2. Besides X-Y system, also....

X-0 system, Z-W system, and haploid and diploid system

3. X and  Y chromosomes behave as homologous chromosomes during meiosis

- partially homologous and rarely undergo crossing over

meiosis : each gamete gets one

ovum gets X, sperm gets either X or Y (50:50)

- 50:50 for either sex

First signs of sex = 2 months old

Sex chromosomes

1. Describe the SRY gene

2. What other roles do sex chromosomes have?

1. sex determining region of the Y chromosome

-genetic embryonic gonads are modified into testes

-triggers sequence of chemical events that will give developing foetus appropriate male phenotypic traits

2. genes for characters unrelated to sex

if a sex-linked trait is due to a recessive allele, a female will have this phenotype only if she's homozygous for the trait.

because males have only one X chromosome any male receiving the recessive allele from mother will express trait
-chance of a female inheriting a double dose of the mutant allele is much less than the chance of a male inheriting a single dose

->>males more likely to inherit sex-linked recessive disorders

Sex-linked diseases

1. Duchenne muscular dystrophy

2. What happens to one of the X chromosomes of females? What is its result?

3. What is a Barr body?

4. ^ What does this mean?

1. due to mutation in X-linked gene for a key muscle protein called dystrophin

muscle weakening, loss of coordination

affects at 3-7 years of age

rarely live past early 20s

2. inactivated = males and females have same effective dose of X chromosome genes

3. an X chromosome per cell condensed into a compact object during embryonic development

-> this inactivates most of its genes

-> Barr body is reactivated in ovarian cells that produce ova

4. Females consist of a mosaic of cells - some with an active paternal X, others with an active maternal X

After Barr body formation in a given cell, all cells descended from that cell have the same inactive X

-if a female is heterozygous for a sex-linked trait, half her cells will express one allele and other half the other allele

ex. heterozygous for X-linked mutation preventing development of sweat glands

heterozygous will have patches of normal skin and skin patches lacking sweat glands

ex. orange and black pattern on tortoise shell cat is due to patches of cells expressing orange alelle while others have non-orange allele

Sex-linked traits

Nondisjunction?

1. How can this happen?

Problems with meiotic spindle causes errors in gamete prdouction

1.homologous chromosome pairs may not separate properly during meiosis I

-> half gametes have no copy of affected chromosome

->other half get 2 of each

OR

2. sister chromatids do not separate during meiosis II

-> 2 gametes will get normal single copy

-> one will get 2, last will get nothing

Sex-linked diseases

1. Aneuploidy

2. Poplypoidy

1. abnormal chromosome number, resulting from fertilization of a normal gamete with one suffering nondisjunction

-trisomic cells = 3 copies of a particular chromosome type and have 2n+1 total chromsomes

-monosomic cells = 1 copy of a particular chromosome type and have 2n-1 chromosomes

->>leads to distinct phenotype

2. results in more than two complete sets of chromosomes

may occur when a normal gamete fertilizes another gamete in which there has been a nondisjunction of all its chromosomes

- ^resulting zygote = triploid

- if 2n zygote failed to divide after replicating its chromosomes = tetraploid embryo would result

-more comman in plants than animals

4 different mutations in chromosome structure

deletion

when chromosome fragment is lost during cell division

chromosome will be missing certain genes

most commonly occurs during meiosis (homologous chromatids may break and rejoin at incorrect places, such that one chromatid will lose more genes than it receives)

-> one has deletion, other has duplication

4 mutations in chromosome structure

duplication

when fragment becomes attached as an extra segment to a sister chromatid

most commonly occurs during meiosis

(homologous chromatids break and rejoin at incorrect places, such that one chromatid will lose more genes than it receives)

->one chromosome has deletion, other has duplication

4 mutations in chromosome structure

inversions

when chromosome fragment reattaches to the original chromosome but in the reverse orientation

harmful but not fatal

->can alter phenotype because gene's expression is influenced by location in relation to other genes

4 mutations in chromosome structure

translocation

chromosomal fragment joins a non-homologous chromosome

some are reciprocal, some not

can alter phenotype b.c gene's express can be influenced by location in relation to other genes

those with Down's syndrome have aneuploidy, with 3 copies of chromosome 21

affects 1 in 700 children

strongly correlated with maternal age - rises steeply after 35

Klinefelter's syndrome (XXY)

Turner's syndrome (XO)

Non-disjunction in sex chromosomes is less often lethal

Klinefelter's syndrome - 1 in 2000 males

-sterile, other traits

Turner's syndrome affects 1 in 5000 females

-short, sterile

-having single X chromosome can be lethal to embryo

Structural alternations of chromosomes ->human disorders

there are numerous deletions which, even in a heterozygous state, cause severe physical and mental problems

What is cri du chat?

syndrome that results from specific deletion in chromosome 5.

-mentally retarded, small head, unusual facial features, cry like mewing of distressed cat

-fatal in infancy or early childhood

Chromosomal translocations between nonhomologous chromosome are associated with human disorders.

Chromosomal translocations have been implicated in certain cancers, including chronic myelogenous leukemia (CML). When does it occur?

small circles of DNA in mitochondria and chloroplasts, both of which reproduce themselves

->not distributed to offspring by meiosis

Karl Correns in 1909 observed in plants that do not display Mendelian inheritance

1.) leaf coloration of the offspring was determined only by the maternal parent

coloration patterns due to genes in the platids, inherited only via the ovum not the pollen

2.) mitochondria and plastids are inherited only through the ovum

->all mitochondrial genes in mammals demonstrate maternal inheritance

(zygote inherits mitochondria only from ovum b.c sperm is smaller)

How would maternal inheritance lead to inherited human disorders?

What is affected? What are the defects? What are the mitochondrial mutations?

some human disorders are produced by mutations to mitochondrial DNA

Defects:

in the electron transport chain, or ATP synthase

Affects:

ATP supply - tissues that require high energy supplies may supper energy deprivation (nervous system and muscles)

->>diabetes, heart disease, diseases of aging