Meiosis and Genetic Variation

Study Notes

The Basics of Meiosis

Animals and plants practice sexual reproduction, with parents passing chromosomes to their offspring.
Each child receives unique combinations of chromosomes from the parents, resulting in variation in the offspring.
Meiosis is the process of cell division that contributes to sexual reproduction and the resulting variation in the offspring.

Homologous Chromosomes

In humans, there are 23 pairs of homologous chromosomes, which are identical in size, shape, construction, and genes they contain.
The homologues within an organism may contain different versions, or alleles, of those genes.
Autosomes make up 22 pairs of homologous chromosomes, and the sex chromosomes comprise the last pair.
Males have an X and a smaller Y chromosome, while females have two equal X chromosomes.

The Human Life Cycle

The human life cycle involves two types of cell division: mitosis and meiosis.
Mitosis is involved in the growth of a child and repair of tissues.
Meiosis is a special form of cell division associated with sexual reproduction, producing gametes with unique combinations of chromosomes.
In males, meiosis is part of sperm production (spermatogenesis), while in females, it is part of egg production (oogenesis).
During fertilization, an egg and sperm unite to form a zygote, restoring the chromosomes to 46 total.

Overview of Meiosis

Meiosis involves two cellular divisions: meiosis I and meiosis II.
These two divisions produce four daughter cells, each with one chromosome of each pair.
During meiosis I, homologous chromosomes pair up in synapsis to form a tetrad, and later separate to provide one member of each homologous pair to each daughter cell.
During meiosis II, the sister chromatids (dyads) are separated, ensuring that gametes produced by an individual have unique combinations of homologous chromosomes.

Crossing-Over

Crossing-over occurs during meiosis I, where homologous chromosomes exchange genetic material, creating new combinations and increasing genetic variability of the gametes.

The Importance of Meiosis

Meiosis maintains the same number of chromosomes in each new generation.
Meiosis contributes new combinations of alleles to each new generation, increasing genetic variability.
Meiosis produces gametes with many different combinations of homologous chromosomes, resulting in a vast number of possible combinations (over 70 trillion).

Phases of Meiosis

The four stages of mitosis (prophase, metaphase, anaphase, and telophase) occur twice during meiosis, once in meiosis I and again in meiosis II.
During meiosis I, significant events occur during prophase I, such as synapsis and crossing-over.
During meiosis II, the events are similar to those of mitosis.

Abnormal Chromosome Inheritance

Nondisjunction, an error in meiosis, can lead to abnormal chromosome numbers in gametes.
Nondisjunction can occur in meiosis I or II, resulting in trisomy or monosomy.
Abnormal chromosome numbers can lead to disorders such as Down syndrome (trisomy 21) and Turner syndrome (monosomy XO).

Down Syndrome

Down syndrome is a trisomy of chromosome 21, resulting in recognizable characteristics, including short stature, eyelid folds, and mental retardation.
The probability of Down syndrome increases with maternal age, with a frequency of 1 in 800 births for women under 40 and 1 in 80 births for women over 40.

Abnormal Sex Chromosome Number

Nondisjunction can cause trisomy or monosomy of the sex chromosomes, typically with detrimental effects.
Turner syndrome females have monosomy XO, resulting in short stature, barrenness, and other characteristics.
Klinefelter syndrome males have XXY, with an extra X becoming a Barr body.

Description

Learn about the importance of meiosis in producing genetic variation through crossing-over and the formation of gametes. Understand the phases of meiosis and how it contributes to the creation of new combinations of alleles.