Chromosome Number Variations

• Most diploid species have two haploid sets of chromosomes.
• Chromosomal mutations or aberrations include changes in chromosome number, gene deletion or duplication, and rearrangements.
• These mutations can cause phenotypic variations and can be lethal.

Variations in Chromosome Number

• Aneuploidy is when an organism gains or loses one or more chromosomes, but not a complete set.
• Euploidy involves complete haploid sets of chromosomes.
• Polyploidy happens when there are more than two sets of chromosomes.

• Chromosomal variation results from nondisjunction, where chromosomes or chromatids fail to separate during meiosis.
• Nondisjunction disrupts chromosome distribution into gametes.
• Fertilization involving abnormal gametes results in zygotes with three members (trisomy) or a single member (monosomy).

Monosomy and Trisomy

• Loss of a single chromosome can have severe phenotypic effects.
• Monosomy of the X chromosome causes Turner syndrome in humans.
• Monosomy of autosomes is not tolerated in humans and animals but is tolerated in plants.
• Death may occur due to lethals being unmasked or a single recessive gene that is insufficient.
• Trisomy is more viable than chromosome loss when smaller chromosomes are involved.
• Trisomy of autosomes has severe effects and can be lethal during development, but are viable for plants.
• Datura can have twelve primary trisomic conditions, each with its own capsule.
• Trisomic rice grows more slowly if the longer chromosome is involved.

Down Syndrome

• Trisomy 21 is the only human autosomal trisomy where individuals can survive past their first year with a life expectancy of around 50 years
• Down syndrome occurs in 1/800 live births.
• Approximately 250,000 people in the US have Down syndrome.
• Down syndrome includes 12-14 characteristics, and affected people express 6-8 on average.
• Characteristic features of Down Syndrome include:
  • Epicanthic fold in each eye
  • Flat face
  • Round head
  • Short stature with a protruding tongue
  • Short, broad hands with a unique pattern of fingerprints
• People with Down syndrome have mental and physical challenges.
• People with Down syndrome typically possess a mildly-to-moderately low IQ.
• People with Down syndrome speak slower than most.
• Children with Down syndrome are prone to heart defects and respiratory disease.
• The life expectancy for someone with Down syndrome is 50 years.
• Individuals with Down Syndrome often suffer from Alzheimer's Disease which occurs earlier than normal.
• Chromosome 21 contains genes sensitive to dosage and responsible for many phenotypes associated with Down syndrome.
• The Down syndrome critical region (DSCR) relates to the production of the Down syndrome phenotype.
• An extra copy of the DSCR1 (VEGF overexpression) can decrease the risk of some cancers which can reduce mortality by 10%

Origin of Down Syndrome

• 95% of cases are from sperm or egg carrying chromosome 21 due to nondisjunction during meiosis I, specifically from the ovum in 95% of trisomy cases.
• There is an increased incidence of Down syndrome along with maternal age.
• There is a higher probability of births with Down syndrome when childbearing women exceed 45.
• In females, all primary oocytes are formed by birth.
• Ovulation begins at puberty.
• Ova from women 30-40 years old are older since the process is arrested after ovulation unless fertilization occurs.
• Genetic counseling for women pregnant late in their reproductive years and diagnostic testing is recommended.
• Diagnostic testing uses fetal cells derived from amniotic fluid or placental chorion.
• NIPGD derives cells from maternal circulation.
• Fetal cells are cultured, and the karyotype is determined from cytogenetic analysis.
• Down syndrome is caused by nondisjunction of chromosome 21, and it is not inherited.
• Down syndrome could run in the families due to (familial Down syndrome).
• Translocation results from the bottom chromosome 21 part of chromosomal 14.
• Individuals with Down syndrome typically presents with an extra copy changes in the baby's development.

Other Human Aneuploidy

• Besides Down syndrome, only two human trisomies can survive to full term.
• Patau syndrome (trisomy 13)
• Edwards syndrome (trisomy 18)
• Both trisomies exhibit severe malformations as well as early death
• Figure 8-5 shows the abnormal karyotype

Polyploidy

• Polyploidy naming depends on sets of chromosomes.
  • Triploids have 3n chromosomes
  • Tetraploids have 4n chromosomes
  • Pentaploids have 5n chromosomes
• Polyploidy is uncommon in animal species, but common in lizards, amphibians, fish, and especially plants.
• Uneven number of homologs do not produce balanced gametes often.
• In humans, polyploidy occurs as triploidy with 69 chromosomes, and tetraploidy, with 92, but die shortly after birth.
• Polyploidy originates by the addition of chromosomes which is identical to the haploid of a species or a different species.

Autopolyploidy

• Autopolyploids have additional sets of chromosomes in tetraploids (AAAA)
• Autopolyploids arise if a gamete is fertilized by a haploid gamete causing nondisjunction of chromosomes.
• Two sperm may fertilize the ovum resulting in a zygote.
• Tetraploids crossed with diploids can give triploids.
• Since they have an even number tetraploids are more likely to be found in nature than autotriploids
• Tetraploids are more likely to produce balanced gametes.
• Tetraploids arise when chromosomes have replicated, and a cell enters interphase causing the chromosome number to duplicate.
• Experimentally done by adding heat or cold shock to cells or by adding colchicine to somatic cells undergoing mitosis.
• Autopolyploid flowers increase in size, increasing horticultural and commercial value.
• Potatoes, Winesap apples, commercial seedless bananas, and seedless watermelons are autopolyploids.
• Polyploidy increases gene expression around tenfold due to two genes encoding G1 cyclins
• Polyploid cell stays in G1 phase longer and grows to a larger size

Allotetraploid or Amphidiploid

• It results from hybridization between 2 species.
• Has four haploid genomes derived from separate species
• Sterile hybrids can have natural or induced chromosomal doubling to produce fertile amphidiploids.
• Amphidiploid plants can also be produced by somatic cell hybridization.

Endopolyploidy

• It is a condition where certain cells in a diploid organism are polyploid.
• The set of chromosomes replicates repeatedly without nuclear division.
• Vertebrate liver cell nuclei, as well as humans, often contain 4n, 8n, or 16n chromosome sets
• Although its role is unclear, the proliferation of chromosome copies often occurs in cells where high levels of certain gene products are required

Chromosome Composition & Arrangement Variation

• Chromosome structure is altered in two primary ways.
  • The total amount of genetic information can change.
  • The genetic material itself can remain the same while being rearranged.
• Those containing aberrations on chromosomes are heterozygous.

Deletion of a specific region

• A chromosome breaking in one or more regions as well loss is the missing piece, it is referred to as a deletion
• A deletion can occur at one end or in the interior of the chromosome.
• To achieve synapsis in individuals carrying long deletion, there has to be an unpaired area of the normal homolog
• Small deletions usually do not have an adverse effect and they can occur

Cri du Chat Syndrome

• Results from segmental deletion of a terminal area in the arm of chromosome 5.

• This condition is not inherited, It results from Irregular loss in genetic cell

• This can vary with length of depletion

• Symptoms include earie sound close to a Cats meow sounds alongside the malformations.

• people will often be mentally retarded.

• The missing portion of the chromosome holds nucleotides essential for preservation within the process for the gene that catalyzes.

Duplication of a Segment

• Duplications show up with the synapse chromosomes
• It can pass through a repelling error to the meiosis.

Cause of Duplication

• Gene redundancy
• phenotype variation
• Genetic diversity during evolution.
• Multiple copies of genes.
• This DNA is known as rDNA
• E. coli's contain haploid rDNA and seven copies.
• Drosophila contain over .03% and 130+ gene copies.

Drosophila Bar mutation

• Duplication lead to in Bar-Eyes flies that contain small and have a short slit look.
• 16A results in the shape for being responsible to triplicate from bar fly.

Inversions that change Linear gene sequence

• Linear arrangement rather than information lose, it will turn some degree turn up on inverted heterozygous.

Animals

• It needs for some meiotic error for errors too occur, creating an inviable
• Inversion requires 2 steps and segment inversion.
• An Inversion can rise during looping
• The paracentric inversion results from crossing over. This is an issue for both the paracentric and the pericentric for both can produce less effects.

The Inverted Segment

• Short or long also they contain the viability.
• Both paracentric and pericentric will cross over and 50% of gametes will be non effective .
• Viability will decrease significantly

Inversion Heterozygotes

• one Inverted crossing over occurs the inversion forms a loop to only be in the inversion loop
• loop formation and inverted during loop inversion happens.

Translocation alteration for Genomes

• the movement of segments into a new location of non homologous chromosomes
• It requires and unusual synapse during the meiosis period

Figure's Show Segregation Potential

• It leads normal balanced gamets
• This can lead to a massive hit from reproductive fitness

Familial Down syndrome.

• centric fusion happens, this involves breaks.

• It leads to loss of segments and acrocentric chromosomes

• Robertsonion is an example(figure 6-17)

• 1/21 is a 4/45 trans location, this will affect 45 chromosomes.

• Meiosis results for two copies for a chromo number, a number can translocate one. Fertilization will cause and individual with two with chromosome 21 exhibiting down syndrome