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Cell Division and Genetics

Mitosis and Meiosis Similarities

  • Both are types of cell division.

  • They have the same main stages (Prophase, Metaphase, Anaphase, Telophase).

Mitosis and Meiosis Differences

  • Number of Divisions: Mitosis divides once, while meiosis divides twice.

  • Chromosome Separation: In mitosis, sister chromatids split up. In meiosis I, homologous pairs of chromosomes split up.

  • End Result: Mitosis results in two cells.

    • It was said meiosis is "two", but it implies that it results in four cells after the second division.

  • Ploidy: Mitosis results in diploid cells.

    • Diploid cells have two copies of every chromosome.

    • Meiosis results in haploid cells.

      • Haploid cells have one copy of every chromosome.

  • Crossing Over: During prophase I of meiosis, non-sister homologous chromatids exchange equivalent portions of DNA.

Aneuploidy

  • Aneuploidy is a condition where there are too many or too few of a particular chromosome(s).

    • It refers to the state of chromosomes.

  • It is different from polyploidy.

Polyploidy

  • Polyploidy refers to having more than two copies of every chromosome in the set.

    • Triploidy (3 copies of each chromosome)

    • Octoploidy (8 copies of each chromosome)

      • Strawberries are octoploid.

Nondisjunction

  • Nondisjunction is when chromosomes don't separate properly during meiosis I or meiosis II.

  • It results in gametes with either one too many or one too few chromosomes.

Chromosome Number in Somatic Cells

  • Typically, a somatic cell has 46 chromosomes, which is 23 pairs.

  • 22 pairs are autosomes (non-sex chromosomes).

  • One pair is sex chromosomes.

Aneuploidy Conditions Compatible with Life

  • Most autosomal aneuploidies (one too few or one too many) are not compatible with life and result in non-detectable pregnancies.

  • However, some chromosomal abnormalities do result in potentially living offspring.

  • Polyploidy is not observed in humans but is observed in other organisms like lizards.

Trisomy

  • Having three copies of one chromosome.

Monosomy

  • Having one copy of one chromosome.

Trisomy 21 (Down Syndrome)

  • It is the most survivable trisomy and relatively common, especially in women after 30 (1 in 100 chance).

  • Common features:

    • Palmar crease (single crease on the palm)

    • Wider space between the big toe and second toe.

    • Facial features: epicanthic folds at the corner of the eye, flattened facial features, and lower set ears.

  • Health Issues:

    • Speech impediments (due to smaller jaw and normal tongue size).

    • Heart conditions.

    • Respiratory conditions.

    • Prone to being overweight.

  • Lifespan:

    • Average lifespan is about 60 years.

    • Increased risk of Alzheimer's disease.

Trisomy 13 (Patau Syndrome)

  • Results in severe malformations/defects.

    • Cleft lip.

    • Fused eyes (cyclops phenomenon).

    • Polydactyly (extra fingers/toes).

  • Most individuals do not make it to term, and those that are born typically die within the first year of life.

  • Some have survived into their teenage years.

Trisomy 18 (Edwards Syndrome)

  • Rarely compatible with life.

  • Features:

    • Clenched hands.

    • Rounded feet.

    • Microcephaly (smaller than average brain cavity).

  • Babies born with this condition typically die within a week after birth.

  • Second most survivable trisomy next to trisomy 21, but survival is very limited.

Trisomy X

  • Often has almost no effect.

  • Females inactivate one of their X chromosomes early in cell division, so having an extra X has little impact.

Monosomic X (Turner Syndrome)

  • Individuals with one X chromosome are female but do not develop fully.

  • They remain infertile and have characteristic features:

    • Webbed neck.

    • Smaller stature.

    • Broad chest.

XXY (Klinefelter Syndrome)

  • Males with an extra X chromosome.

  • Can result in:

    • Elongated face.

    • Weakened cardiovascular tissue (aortic rupture is a common cause of death).

    • Reduced fertility.

    • Androgynous development characteristics.

    • Learning disorders.

Environmental Factors and Genetic Conditions:

  • Chemical exposure during pregnancy can have negative impacts on development.

    • Thalidomide was mentioned and caused birth defects.

Genetics

  • Definition: The study of genes and the mechanisms of heredity (how traits are inherited from previous generations).

  • Gregor Mendel: Austrian monk, father of genetics, studied pea plants to understand inheritance.

  • Mendelian Trait: A trait with an easily predictable pattern of inheritance.

Purebred

  • When crossed with an individual like itself, another purebred will produce offspring identical to itself

Mendel's Experiments

  • Manipulated breeding of pea plants (purebreds) by cross-pollination to study inheritance patterns.

  • Observed traits like flower color, flower position, seed color, seed shape, pod shape, and plant height.

  • First Generation Results:
    Pure bred plants were crossed with one another:

    • Purple flowers, axial flowers, yellow pods that were round and inflated.

    • White flowering, terminal flowers that had green seeds that were wrinkled in a constricted pod.

  • Consistently, only one form of the trait would appear in the first generation of offspring (F1 generation), regardless of the parent it came from.

  • Second Generation Results:
    Treated the F1 generation like the parents, and pollinated them again to create the F2 generation:

    • The other trait will return and appear in all of the subsequent generations!

Mendelian Genetics

  • Dominant Traits: Mask the expression of recessive traits when present.

Genetic Terms and Definitions

  • Characters: Heritable features that vary among individuals.

  • Traits: Different forms of a character.

  • Gene: The genetic underpinning for an expressed heritable trait; a region on a chromosome.

  • Genome: Genes at the same mailing address. All within a species, these genes are located at the exact same address.

  • Locus: The specific location or address of a gene on a chromosome.

  • Allele: A version of a gene; variations in the nucleotide sequence that result in different expressions of a gene.

Chromosomes

  • Heritable components are actual physical elements.

  • Passed onto offspring.

Alleles and Genes

  • Variations and versions pass on to offspring.

  • An example would be a gene for a plant to have a particular flower color and therefore a gene will code for white flowers, one for purple.

Homozygous vs Heterozygous Genotype

If mom and dad gave you the same version of that gene:

  • We call that a homozygous genotype.

If the version of that genes, if the alleles of the genes are different:

  • We call that list of the versions of the gene you have heterozygous.

Genotype and Phenotype

  • Phenotype: What is expressed of your genes. Observable characteristics.

  • Genotype: list of alleles you have. Only have two versions of every gene, two options.

  • Dominant:
    Determines the expression of the genes when they're different. We consider purple over white, since its the color that has the pigment, whereas, the absence of it, the white will never show up.

Mendelian Traits

  • Relationship between genes is clear. One is clearly dominant and one is recessive. There are no intermediates or gray area.

Inherited Diseases

  • Most of our genetically inherited diseases are recessive traits. If you carry it but do not express, we call you a carrier.

  • Examples, albinism, sickle cell anemia, cystic fibrosis, taste test, PKU

    • If both the the parents are carriers, one in four of the children will actually express that condition.

Law of Segregation

  • Alleles of genes, or homologous pairs, separate during the formation of gametes. So you only pass on one version that you have to your offering.

Law of the Independent Assortment.

  • We see homologous pairs during miosis one are separating.

  • Lining up at the metaphase plate is totally random so how they line up has no impact on subsequent pairs and their lining up.

  • Independent assortment just means there's fifty fifty chance you got two copies of every chromosome and the version of it that is in an egg. Or sperm is it 5050% chance that you get one or the other and then you would use this to figure this out.

Tool called Punnett Square

Uses letters to label each gene and allele. Dominant uses capital letters, while recessive uses lower case letters.