Sexual differentiation and life cycles

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• Sexual differentiation and life cycles
• All sexually reproducing organisms alternate
  between short periods of sexual reproduction and
  prolonged periods of asexual reprodn. During
  their life cycle
• In higher forms of life the sexual
  differentiation is evident as phenotypic
  dimorphism in the males and females
• Individuals containing both male and female
  reproductive organs- in plants - monoecious
  hermaphroditic flowers
• only one type of reproductive organs- dioecious
  unisexual

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• Types of sexual differentiation
• Life cycle of green alga (Chlamydomonas)
  only infrequent periods of sexual reprodn most of
  the life cycle in haploid stage-asexually
  reproduce by mitosis- under unfavorable nutrient
  conditions haploid cells function as gametes
  and fertilize between m and m- (mating types)
  -diploid zygote withstand unfavorable conditions
  favorable conditions resume- meiosis occurs
  haploid vegetative cells produced. () and (-)
  cells differ chemically, but morphologically
  isogametes.

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• Life cycle of higher plants eg. Maize (Zea mays
  corn)
• Plants alternate between haploid gametophyte
  stage and diploid sporophyte stage. Meiosis links
  the two phases- monoecious seed plant (sporopytic
  stage) predominates the life cycle.
• Sex determination occurs differently in different
  tissues-
• In maize- stamens (tassel) diploid microspore
  mother cells - pollen pistil megaspore mother
  cells only 1 of the haploid cells survive-
  pollination occurs- double fertilization-diploid
  zygote nucleus and triploid endosperm nucleus
  develops into a corn kernal-with germination
  produces a new plant the sprophyte

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• The sex differentiation in a monoecious plant has
  to occur in cells which has the same genetic
  constitution. Therefore the products of specific
  genes has to play an important role in
  determining and differentiating the male and
  female tissues.
• Ex. The study of mutant genes hom. Recessive
  tassel seed gene interfere with tassel prodn and
  induce the formation of female plants- The
  recessive mutants silkless (sk) and barren stalk
  (ba) results in only functional male reproductive
  organisms

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• Sex chromosomes The genetic basis of sexual
  differentiation is often related to the
  chromosome composition in the two sexes, and also
  controlled by genes located both on sex
  chromosomes as well as in autosomes. Mostly
  animals and in some plants (Cannabias) the sex is
  determined by the sex chromosomes
• Sex determination based on X and Y chromosomes
• (a) Protenor mode the insect Protenor has
  12AXX (female)
• 
  12AXO (male)
• (b) Lygaeus mode the insect Lygaeus turicus has
  12A XX (female ) 12A XY (male)
• Y chromosome is called the heterochromosome
  small and lacks many genes homologous with X
  chromosome.

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• In the above types males produce different
  gametes heterogametic sex females
  homogametic sex
• In moths, butterflies, all birds, some fish etc.
  , female is the heterogametic sex ZZ/ZW
• Sex determination in humans In 1956 Hin Tijio
  and Albert Levan showed that humans have 46
  chromosomes consisting of 22 pairs of autosomes
  and one pair with different configuration in
  males and females. It was later shown that the Y
  chromosome determines the maleness.

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• The Y chromosome and male development It was
  earlier thought that the Y chrom. Was blank
  genetically. But current findings show that it
  has several important genes, present at both
  ends the pseudoautosomal regions which share
  homology with the X chrom., synapse and recombine
  during meiosis
• The remainder is named as NRY, (non recombining
  region of Y) but some portions of NRY contain
  genes that are absent in X and controls male
  sexual development . SRY or sex determining
  region of Y resides on the short arm of Y. It
  encodes the TDF (testis specific genes) Using
  molecular probes it has been shown that within
  the NRY region also contains house keeping genes

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• Sex ratio in humans- The proportion of male to
  female offspring
• The proportion of males to females conceived in a
  popn is the primary sex ratio, the secondary sex
  ratio reflects the proportion of each sex that is
  born. It has been found that the primary sex
  ratio in US Caucasians is 1.2-1.6., more males
  than females are conceived in human popns
• X chromosome and dosage compensation- does
  females have a potential to produce twice as gene
  products as males for all X linked genes? Or does
  a dosage compensation occurs?
• Keith Moore and Murray Barrs initial experiments
  showed that a darkly staining body in interphase
  nerve cells of female cats were present but were
  absent in male cats. This is named as Barr body

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• It was suggested by Mary Lyon and Liane Russel
  that one X chromosome is inactivated in all
  somatic cells of females and this happens
  randomly between the two X chromosomes and
  happens during the early embryonic development.
  So, the cells of females have only one
  functioning copy of each X-linked gene - the same
  as males.

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• This random inactivation is sometimes called as
  lyonization
• Lyonization was initially based on observations
  of female mice hets. For X linked coat color
  genes. The pigmentation of these het. Females was
  mottled, large patches expressing the color
  allele on one X other patches expressing the
  allele of the other X.
• Ex. Black and yellow-orange patches of female
  tortoiseshell and calico cats,

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• X-Chromosome Abnormalities
• As we saw above, people are sometimes found with
  abnormal numbers of X chromosomes.
• Examples
• Turner's syndrome females with but a single X
  chromosome. The phenotypic effect is mild because
  their cells have a single functioning X
  chromosome like those of XX females. Number of
  Barr bodies zero.
• XXX, XXXX, XXXXX karyotypes all females with
  mild phenotypic effects because in each cell all
  the extra X chromosomes are inactivated. Number
  of Barr bodies number of X chromosomes minus
  one.

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• Klinefelter's syndrome people with XXY or XXXY
  karyotypes are males (because of their Y
  chromosome). But again, the phenotypic effects of
  the extra X chromosomes are mild because, just as
  in females, the extra Xs are inactivated and
  converted into Barr bodies.

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• Variations in Chromosome number and structure
• Alterations in chromosome numbers, individual
  chromosomes
• In whole sets
• Monoploid number (n) The number of chromosomes
  in a basic set (haploid)
• Males of the social insects are monoploids,
  therefore sex cells are same as somatic cells,
  meiosis is not possible.
• In vitro culture of plant gametes can
  regenerate haploid plants (sterile in the haploid
  state, however can treat the regenerated plants
  with colchicine, which inhibits spindle formation
  during mitosis and effectively doubles the
  chromosome number in a cell after a round of
  mitosis. This technique is used for the
  production dihaploids, advantage, can produce
  homozygous diploids (dihaploids)

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• Euploids have a multiple of the basic set.
• Diploid (2n) Polyploid - Triploid
  (3n), tetraploid (4n), etc
• Odd numbers of chromosome sets are not usually
  maintained reliably from generation to
  generation, however
• Triploid- (3n) In nature they are formed by
  crossing a diploid with a tetraploid
• A basic chromosome B
  basic chromosome
• AA x BBBB
• ABB
• (n2n gametes to form 3n zygotes) Triploids are
  sterile eg. Bananas, seedles grapes and
  watermelon

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• In a polyploid, when the basic set is multiplied,
  it is an autopolyploid (AAA)
• When it is a combination of different basic
  chromosome sets it is an allopolyploid (AABB),
  hybridization between 2 species
• Bread wheat (Triticum aesitivum) hexaploid
  species which arose from the crossing of
  different diploid species (AABBDD)
• Tetraploid wheat x diploid rye Triticale
• 4n28 2n14
  6n42
• Triticale combines the desirable characters of
  wheat and rye
• The advantage in polyploids are they are larger
  than the diploid relatives, flowers and fruits of
  plants are increased in size
• In individual chromosomes
• Aneuploidy- Changes in the number of chromosomes
  that are not multiple the n number.

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• In individual chromosomes
• Aneuploidy- Changes in the number of chromosomes
  that are not multiple the n number.
• Variation in chromosome number occurs as a result
  of an error during meiosis. The failure of paired
  homologues to disjoin during segregation is named
  as nondisjunction one gametes lacks a
  chromosome (monosomy) and the other has an extra
  (trisomy)
• The direct result of this event is that gametes
  develop that have too few or too many
  chromosomes. If this occurs during meiosis I
  normal gametes are not developed, and if it
  occurs during meiosis II half of the gametes will
  be normal and the other half will be abnormal.

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• Nullisomics (2n-2) lethal in true diploids but
  not in some polyploids
• Monosomics (2n-1) all human autosome monosomics
  are aborted in utero usually a detrimental
  condition reduced fertility due to problems in
  gamete formation
• Turner syndrome 44 autosomes1X sterile
  females several abnormal physical characters
  about 15000 female births
• Trisomics (2n1) The 3 homologous chromosomes
  forms a trivalent and two copies segregate to one
  pole and the single to the other pole.

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• Human trisomic conditions
• XXY Klinefelter syndrome sterile males,
  mentally retarded, (1 1000 males)
• Down syndrome- Trisomic chromosome 21 (11500
  births), incidence increases with age of the
  mother.
• Trisomy 13 Patau syndrome Trisomy 18- Edwards
  syndrome both severe malformation and early
  lethality
• In plants trisomics produce more viable forms
  eg. Datura (2n24) 12 primary trisomics
  have been recovered each is different in oryza
  sativa (2n24) also 11 have been recovered
• Variation in chromosome structure
• This class of chromosomal abberations include
  structural changes that delete, add, or rearrange
  substantial portions of one or more chromosomes

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• Deletions- Any change which results in a loss of
  a segment of the chromosome.
• The chromosomal aberration detected by genetic
  evidence was in the notch mutant of Drosophila by
  Brigdes (1917).
• This is a sex linked trait that is lethal in
  males and homozygous females how it was
  identified was when a recessive gene was present
  on the homologous chromosome it was still
  observed to be dominant, i.e. white eye
• Muscular dystrophy is often associated with a
  deletion of the X-chromosome. It is found in
  1/3500 live male births
• Cri-du-chat syndrome- deletion in the short arm
  of human chromosome 5, infants with this syndrome
  exhibit anatomic malformations, cardiac
  complications and mental retardation
• Duplications Portions of the chromosomes are
  duplicated the duplications may be tandem,
  reverse tandem or terminal arrangements on the
  affected chromosomes
• .

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• Duplications can arise as a result of unequal
  crossing over between synapsed chromosomes during
  meiosis or multiple replication of a looped
  region of DNA by DNA polymerase.
• Duplications may or may not have phenotypic
  effects.
• Bar gene in Drosophila Bar is an incomplete
  dominant X linked gene effecting the shape and
  size of the eye.
• This mutation is due to a duplication of a small
  section of the chromosome
• Duplications are sometimes detected when an
  individual thought to be a homozygote for a
  recessive trait shows the dominant phenotype.
• Duplications may result in gene redundancy and
  also an important source of genetic variability
  during evolution
• The presence of gene families are a good example
  for gene duplications and of evolutionary
  significance

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• Inversions result when a segment of a chromosome
  is excised, inverted 180o, and reintegrated into
  the same chromosome.
• Two specific types of inversions are recognized,
  and they are classified as to whether or not the
  centromere is a portion of the inverted segment
• gene sequence A-B-C-D-E-F-G-H
• after inversion A-B-F-E-D-C-G-H fig.7.13
• Can be detected by mapping. The order of linked
  genes will be altered, map distance is greater in
  the inversion than in the normal
• Pericentric inversions include the centromere in
  the inversion,
• When the centromere is not a part of the
  rearranged chromosome segment, the inversion is
  named as paracentric.

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• Organisms with one inverted and the other non
  inverted homologs are called as inversion
  heterozygote, paring is possible if only they
  form a inversion loop. However the resulting
  gametes can receive aberrent chromatids and fail
  to develop normally.

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• Translocations Segments of a chromosome are
  exchanged between non homologous chromosomes
• Translocations can be detected by altered linkage
  analysis.
• In hets, the translocated chromosomes can pair
  with homologous regions of the normal
  chromosomes.
• Depending on the segregation patterns, both
  viable and non viable gametes can be formed.
• Translocations are associated with reduced
  fertility or semisterility.
• Translocations in humans only exist in the
  heterozygous forms and their offspring are often
  afflicted with genetic diseases.
• Robertsonian translocation- chromosomes breaks at
  the ends of two non homologous acrocentric
  chromosomes and the two larger segments fuse at
  the centromeric region. The result is a large
  choromosome with the translocation. This
  chromosome composition can lead to trisomy 21
  (Down syndrome) and is known as familial down
  syndrome (inherited ) 14/21 translocation fig
  7-16

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