Meiosis and Genetic Variation

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.