Revision:Meiosis

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State that meiosis is a reduction division in terms of diploid and haploid numbers of chromosomes

Outline the process of meiosis including pairing of chromosomes followed by two divisions which result in four haploid cells

In meiosis, the number of chromosomes is halved, the daughter cells receiving only one of each type of chromosome instead of two. It occurs in the formation of gametes (sex cells). In the diploid parent cell, chromosomes occur in homologous pairs (the same size). The parent splits in two (1^stmeiotic division), and the homologous chromosomes of each pair line up together and then get separated from one another and go into different cells. The products then divide again (2^ndmeiotic division, and the chromatids are separated and a total of 4 daughter cells result

Explain how the movement of chromosomes during meiosis can give rise to genetic variety in the resulting haploid cells

Chromosomes are inherited as a group; that is, during cell division they act and move as a unit rather than independently. The existence of linkage groups is the reason some traits do not comply with Mendel's law of independent assortment (recombination of genes and the traits they control); i.e., the principle applies only if genes are located on different chromosomes. Variation in the gene composition of a chromosome can occur when a chromosome breaks, and the sections join with the partner chromosome if it has broken in the same places. This exchange of genes between chromosomes, called crossing over, usually occurs during meiosis, when the total number of chromosomes is halved.

Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to down's syndrome

• Congenital disorder caused by an extra chromosome on the chromosome 21 pair, thus giving the person a total of 47 chromosomes rather than the normal 46. Persons born with Down syndrome are characterized by several of the following: broad, flat face; short neck; up-slanted eyes, sometimes with an inner epicanthal fold; low-set ears; small nose and enlarged tongue and lips; sloping underchin; poor muscle tone; mental retardation; heart or kidney malformations or both; and abnormal dermal ridge patterns on fingers, palms, and soles. The mental retardation seen in persons with Down syndrome is usually moderate, though in some it may be mild or severe. Congenital heart disease is found in about 40 percent of people with Down syndrome.
• Most persons with Down syndrome have an extra (third) chromosome -a condition known as trisomy--associated with the chromosome 21 pair. Almost all individuals with Down syndrome have this trisomy, but a small number (perhaps 4 percent) have an abnormality called translocation, in which the extra

chromosome in the 21 pair breaks off and attaches itself to another chromosome. The cause of the chromosomal abnormalities in Down syndrome remains unknown.

State Mendel's law of segregation

The characteristics of an organism are determined by internal factors that occur in pairs. Only one of a pair of such factors can be represented in a single gamete.

Explain the relationship between Mendel's law of segregation and meiosis

Meiosis is the process that separates allele pairs to create the gametes (sex cells; sperm, egg) that later fuse together during fertilisation.

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