Chapter 7 Biology 1010 - Cellular Basis of Inheritance

Homologous Pairs

* Chromosomes are matched in homologous pairs
* In humans, somatic cells have
* 23 pairs of homologous chromosomes
* One from each parent
* The human sex chromosomes X and Y differ in size and genetic composition
* Other 22 pairs of chromosomes are autosomes w/ same size + genetic composition.

Homologous Chromosomes

* They are matched in
* Length
* Centrome position
* Gene locations
* A locus (plural, loci) is the position of a gene
* Different versions of a gene may be found at the same locus on maternal and paternal chromosomes

Gametes

* They have a single set of chromosomes
* Humans + many animals and plants are diploid
* Two sets of chromosomes
* One from each parent
* Meiosis is a process that convert diploid nuclei to haploid nuclei
* Diploid cells have two homologous sets of chromosomes
* Haploid cells have one set of chromosomes
* Meiosis occurs in the sex organs, producing **gametes** – sperm and eggs
* Fertilization is union of sperm and egg
* Zygote has a diploid chromosome number, one set from each parent

Meiosis

* Meiosis reduces chromosome number from diploid to haploid
* Two haploid gametes combine in fertilization to restore the diploid in the zygote
* 2 cell divisions and four daughter cells after meiosis

Meiosis 1 - Prophase 1

* Events occurring in the nucleus
* Chromosomes coil + become compact
* Homologous chromosomes come together as pairs by synapsis
* Each pair, with four chromatids, is called a tetrad
* Nonsister chromatids exchange genetic material by **crossing over**

Crossing-Over

* Crossing-over occurs when the tetrads synapse during Meiosis 1
* Crossing-over recombines the alleles of the homologous chromosomes, creating new combinations + increasing genetic variability of the gametes

Mitosis VS Meiosis

* Mitosis and Meiosis both -
* Being w/ diploid parent cells
* Have chromosomes duplicated during the previous interphase
* However, the end products differ -
* Mitosis produces two genetically identical diploid somatic daughter cells
* Meiosis produces four genetically unique haploid gametes

Karyotype

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* Is an ordered display of magnified images of an individual’s chromosomes arranged in pairs
* Are often produced from dividing cells arrested at metaphase of mitosis (chromosomes already duplicated)
* Allow for observation of -
* Homologous chromosome pairs
* Chromosome number
* Chromosome structure
* Chromosomal disorder (addition or deletion of an entire chromosome)
* Different from genetic disorder because genetic disorder refers to 1 or two genes)

Down Syndrome

* 3 chromosomes of 21 instead of 2
* Trisomy 21, called Down syndrome, produces a characteristic set of symptoms, which include:
* Mental disabilities
* Characteristic facial features
* Short stature
* Heart defects
* Susceptibility to respirator infections, leukemia, and Alzheimer’s disease
* Shortened life span
* The incidence increases w/ age of mother

Accidents during Meiosis

* Can alter chromosome number
* **Nondisjunction** is the failure of chromosomes/chromatids to separate normally during Meiosis. Can happen during -
* Meiosis 1 -
* If both membranes of homologous pair go to one pole
* Meiosis 2 -
* If both sister chromatids go to one pole
* Fertilization after nondisjunction yields zygotes w/ altered numbers of chromosomes

Nondisjunction in Sex Chromosomes

* Turner Syndrome -
* Missing an X
* XO
* Trisomy X -
* Three X’s
* XXX
* Klinefelter’s Syndrome -
* Extra copies of X’s
* XXY, XXXY, XXXXY
* Jacob’s Syndrome -
* Two Y’s instead of one
* XYY
* All survivable