Introduction to Meiosis Sexual reproduction combines gametes from two parents. Gametes are reproductive cells, such as sperm and egg. As gametes are produced, the number of chromosomes must be reduced by half. Why? The zygote must contain genetic information from the mother and from the father, so the gametes must contain half of the chromosomes found in normal body cells. When two gametes come together at fertilization, the normal amount of chromosomes results. Gametes are produced by a special type of cell division known as meiosis. Meiosis contains two rounds of cell division without DNA replication in between. This process reduces the number of chromosomes by half.
Human cells have 23 pairs of chromosomes, and each chromosome within a pair is called a homologous chromosome. For each of the 23 chromosome pairs, you received one chromosome from your father and one chromosome from your mother. Alleles are alternate forms of genes found on chromosomes. Homologous chromosomes have the same genes, though they may have different alleles. So, though homologous chromosomes are very similar, they are not identical. The homologous chromosomes are separated when gametes are formed. Therefore, gametes have only 23 chromosomes, not 23 pairs.
Haploid vs. Diploid A cell with two sets of chromosomes is diploid, referred to as 2n, where n is the number of sets of chromosomes. Most of the cells in a human body are diploid. A cell with one set of chromosomes, such as a gamete, is haploid, referred to as n. Sex cells are haploid. When a haploid sperm (n) and a haploid egg (n) combine, a diploid zygote will be formed (2n). In short, when a diploid zygote is formed, half of the DNA comes from each parent. Overview of Meiosis Before meiosis begins, DNA replication occurs, so each chromosome contains two sister chromatids that are identical to the original chromosome. Meiosis (Figurebelow) is divided into two divisions: Meiosis I and Meiosis II. Each division can be divided into the same phases: prophase, metaphase, anaphase, and telophase. Cytokinesis follows telophase each time. Between the two cell divisions, DNA replication does not occur. Through this process, one diploid cell will divide into four haploid cells.
Meiosis I During meiosis I, the pairs of homologous chromosomes are separated from each other. This requires that they line up in their homologous paris during metaphase I. The steps are outlined below:
Prophase I: The homologous chromosomes line up together. Also during prophase I, the spindle forms, the chromosomes condense as they coil up tightly, and the nuclear envelope disappears.
Metaphase I: The homologous chromosomes line up in their pairs in the middle of the cell. Chromosomes from the mother or from the father can each attach to either side of the spindle. Their attachment is random, so all of the chromosomes from the mother or father do not end up in the same gamete. The gamete will contain some chromosomes from the mother and some chromosomes from the father.
Anaphase I: The homologous chromosomes are separated as the spindle shortens, and begin to move to opposite sides (opposite poles) of the cell.
Telophase I: The spindle fibers dissolves, but a new nuclear envelope does not need to form. This is because, after cytokinesis, the nucleus will immediately begin to divide again. No DNA replication occurs between meiosis I and meiosis II because the chromosomes are already duplicated. After cytokinesis, two haploid cells result, each with chromosomes made of sister chromatids.
Since the separation of chromosomes into gametes is random during meiosis I, this process results in different combinations of chromosomes (and alleles) in each gamete. With 23 pairs of chromosomes, there is a possibility of over 8 million different combinations of chromosomes (2^23) in a human gamete.
Meiosis II During meiosis II, the sister chromatids are separated and the gametes are generated. This cell division is similar to that of mitosis, but results in four genetically unique haploid cells. The steps are outlined below:
Prophase II: The chromosomes condense.
Metaphase II: The chromosomes line up one on top of each other along the middle of the cell, similar to how they line up in mitosis. The spindle is attached to the centromere of each chromosome.
Anaphase II: The sister chromatids separate as the spindle shortens and move to opposite ends of the cell.
Telophase II: A nuclear envelope forms around the chromosomes in all four cells. This is followed by cytokinesis.
After cytokinesis, each cell has divided again. Therefore, meiosis results in four haploid genetically unique daughter cells, each with half the DNA of the parent cell. In human cells, the parent cell has 46 chromosomes (23 pairs), so the cells produced by meiosis have 23 chromosomes. These cells will become gametes.
Mendel and Molecular Genetics Mendel was perhaps lucky in that the characteristics he chose to study in the pea plants had a relatively simple pattern of inheritance. These characteristics were determined by one gene for which there were exactly two alleles. One of these alleles was dominant and the other recessive.
In many instances, the relationship between genes and inheritance is more complex than that which Mendel found. However, geneticists have since found that Mendel’s findings can be applied to many organisms. For example, there are clear patterns of Mendelian inheritance in humans. These include the inheritance of normal characteristics and characteristics that occur less often. Easily observable Mendelian traits in humans include free ear lobes (in most people the ear lobes hang free (dominant), whereas the attached earlobe is recessive), hitchhiker's thumb (a straight thumb is dominant, while a bent thumb is recessive), widow's peak (a hairline with a distinct point in the middle of the forehead is dominant, while a straight hairline is recessive), dimpled chin (a cleft in the chin is dominant, whereas the absence of a cleft is recessive), and mid-digital hair (hair on any middle segments of the fingers is dominant). Of course, many severe human phenotypes are inherited in a Mendelian fashion including Phenylketonuria (PKU), cystic fibrosis, Huntington's disease, hypercholesterolemia, and sickle-cell anemia. These are termed genetic disorders and will be discussed in additional concepts.
Dominant and Recessive Alleles Mendel used letters to represent dominant and recessive factors. Likewise, geneticists now use letters to represent alleles. Capital letters refer to dominant alleles, and lowercase letters refer to recessive alleles. For example, the dominant allele for the trait of green pod color is indicated by G. The recessive trait of yellow pod color is indicated by g. A true-breeding plant for green pod color would have identical alleles GG in all its somatic cells. Likewise, a true-breeding plant for yellow pod color would have identical alleles gg in all of its somatic cells. During gamete formation, each gamete receives one copy of an allele. When fertilization occurs between these plants, the offspring receives two copies of the allele, one from each parent. In this case, all of the offspring would have two different alleles, Gg, one from each of its parents. An organism that has an identical pair of alleles for a trait is called homozygous. The true-breeding parents GG and gg are homozygous for the pod color gene. Organisms that have two different alleles for a gene are called heterozygous (Gg). The offspring of the cross between the GG (homozygous dominant) and gg (homozygous recessive) plants are all heterozygous for the pod color gene. A homozygous individual is known as a homozygote, and a heterozygous individual is known as a heterozygote. Due to dominance and recessiveness of alleles, an organism’s traits do not always reveal its genetics. Therefore, geneticists distinguish between an organism’s genetic makeup, called its genotype, and its physical traits, called its phenotype. For example, the GG parent and the Gg offspring have the same phenotype (green pods) but different genotypes. An organism's genotype results in an organism's phenotype. For example, if your dog has black hair, you cannot easily tell its genotype (that would take some scientific analysis), but you can easily tell its phenotype.
Pedigree Analysis A pedigree is a chart that shows the inheritance of a trait over several generations. A pedigree is commonly created for families, and it outlines the inheritance patterns of genetic disorders and traits. A pedigree can help predict the probability that offspring will inherit a genetic disorder. Pictured below is a pedigree displaying recessive inheritance of a disorder through three generations (Figurebelow). From studying a pedigree, scientists can determine the following:
If the trait is sex-linked (on the X or Y chromosome) or autosomal (on a chromosome that does not determine sex).
If the trait is inherited in a dominant or recessive fashion.
Sometimes pedigrees can also help determine whether individuals with the trait are heterozygous (two different alleles) or homozygous (two of the same allele). Some points to keep in mind when analyzing a pedigree are:
With autosomal recessive inheritance, all affected individuals will be homozygous recessive.
With dominant inheritance, all affected individuals will have at least one dominant allele. They will be either homozygous dominant or heterozygous.
With sex-linked inheritance, more males (XY) than females (XX) usually have the trait. Sex-linked inheritance is usually recessive.
In a pedigree, squares symbolize males, and circles represent females. A horizontal line joining a male and female indicates that the couple had offspring. Vertical lines indicate offspring which are listed left to right, in order of birth. Shading of the circle or square indicates an individual who has the trait being traced. In this pedigree, the inheritance of the recessive trait is being traced. A is the dominant allele, and a is the recessive allele.