Meiosis topics My osis is the best osis

Segregation & Independent Assortment By crossing purple and white pea plants, Mendel found the offspring were purple rather than mixed, indicating one color was dominant over the other. Mendel’s Law of Segregation states individuals possess two alleles and a parent passes only one allele to his/her offspring. Mendel’s Law of Independent Assortment states the inheritance of one pair of factors ( genes ) is independent of the inheritance of the other pair. If the two alleles are identical, the individual is called homozygous for the trait; if the two alleles are different, the individual is called heterozygous. Mendel cross-bred dihybrids and found that traits were inherited independently of each other.

Crossing over and non-disjunction Crossing over: A process that takes place in prophase I. Non-sister chromatids exchange bits and pieces of chromosome. Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis. Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy).

Nondisjunction

Down syndrome (trisomy 21) Karyotype of trisomy 21 (Down syndrome) Note that chromosome 21 is present in 3 copies, while all other chromosomes show the normal diploid state with 2 copies. Most cases of trisomy of chromosome 21 are caused by a nondisjunction event during meiosis I. Down syndrome, a trisomy of chromosome 21, is the most common anomaly of chromosome number in humans. The majority of cases results from nondisjunction during maternal meiosis I. Trisomy occurs in at least 0.3% of newborns and in nearly 25% of spontaneous abortions. It is the leading cause of pregnancy wastage and is the most common known cause of mental retardation. It is well documented that advanced maternal age is associated with greater risk of meiotic nondisjunction leading to Down syndrome. This may be associated with the prolonged meiotic arrest of human oocytes potentially lasting for more than four decades.

Turner syndrome (X monosomy) (45, X0) Karyotype of X monosomy (Turner syndrome) This condition is characterized by the presence of only one X chromosome and no Y chromosome (see bottom right corner). Complete loss of an entire X chromosome accounts for about half the cases of Turner syndrome. The importance of both X chromosomes during embryonic development is underscored by the observation that the overwhelming majority (>99%) of fetuses with only one X chromosome (karyotype 45, X0) are spontaneously aborted.

Turner syndrome Short stature Lymphedema (swelling) of the hands and feet of a newborn Broad chest (shield chest) and widely spaced nipples Low posterior hairline Low-set ears Reproductive sterility Rudimentary ovaries gonadal streak (underdeveloped gonadal structures that later become fibrotic) Amenorrhoea, the absence of a menstrual period Increased weight, obesity Shortened metacarpal IV Small fingernails Characteristic facial features Webbed neck from cystic hygroma in infancy Aortic valve stenosis Coarctation of the aorta Bicuspid aortic valve (most common cardiac problem) Horseshoe kidney Visual impairments – sclera, cornea, glaucoma, etc. Ear infections and hearing loss High waist-to-hip ratio (the hips are not much bigger than the waist) Attention deficit hyperactivity disorder (problems with concentration, memory, attention with hyperactivity seen mostly in childhood and adolescence) Nonverbal learning disability (problems with maths, social skills, and spatial relations)

Mitosis vs meiosis