bio topic 10
10.2.4: Define Linkage group:
group of genes whose loci are on the same chromosome and therefore do not follow Mendel's law of independent assortment
10.2.3: Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.
Crossing over occurs during prophase I of meiosis. It follows the process of synapsis, during which homologous chromosomes pair up to form a bivalent. Non-sister chromatids break at the same loci on each homologue. They then exchange alleles to form an X-shaped structure. A chiasma is formed at the position where crossing over occurred and is what holds homologous chromosomes together until anaphase I. As these chromatids break at the same point, any gene loci below the point of the break will be exchanged as a result of recombination. This means that material between maternal and paternal chromosomes may be exchanged between them, resulting in new combinations of genes (called recombinants). The further apart two gene loci are on a chromosome; the more likely they are to be exchanged.
10.1.2: Outline the formation of chiasmata in the process of crossing over.
Crossing over occurs during prophase I of meiosis. It follows the process of synapsis, during which homologous chromosomes pair up to form a bivalent. Non-sister chromatids break at the same point on each homologue. They then exchange alleles to form an X-shaped structure. A chiasma is formed at the position when crossing over occurred and is what holds homologous chromosomes together until anaphase I.
10.1.1: Describe the behavior of chromosomes in the phases of meiosis.
During the S phase of interphase, the DNA is replicated to create two identical copies of the cell's genetic information. In Prophase I, the DNA supercoils to form chromosomes, each made up of two genetically identical sister chromatids joined at the centromere. Homologous pairs of chromosomes come together in a process called synapsis to form tetrads (bivalents). Crossing over then occurs at chiasmata, involving the exchange of genetic material between homologous chromosomes. During Metaphase I, the tetrads line up along the equator of the cell, and spindle fibers attach at the centromere. In Anaphase I, the spindle fibers contract and split the bivalent, separating the homologous chromosomes to opposite poles of the cell. During Telophase I, the chromosomes change back into chromatin and the cell divides via cytokinesis into two haploid daughter cells. Interkinesis is the optional rest period between meiosis I and meiosis II during which no DNA replication occurs. During Prophase II the chromosomes supercoil, each one still made up of two genetically identical sister chromatids. During Metaphase II, the chromosomes line up along the equator of cell in a single file. The sister chromatids are pulled apart at the centromere by spindle fibers during Anaphase II, and moved to opposite poles of the cell. In Telophase II, the chromosomes change back into chromatin and each cell undergoes cytokinesis, leaving a total of four haploid daughter cells.
10.2.5: Explain an example of a cross between two linked genes.
In fruit flies the genes for body color and wing length are linked. The two alleles for body color are gray (G), which is dominant, and black (g), which is recessive. The two alleles for wing length are long (L), which is dominant, and short (g), which is recessive. Because these genes are linked, they do not conform to the phenotypic ratios determined by dihybrid crosses. Instead, the phenotypic ratio will follow that of a monohybrid cross as the two genes are inherited together. Recombinant offspring will be present if crossing over occurs and exchanges the alleles between the homologous pairs. However, they will only exist in small numbers.
10.1.5: Explain the relationship between Mendel's law of independent assortment and meiosis.
Mendel's law of independent assortment states that the separation of one pair of alleles is independent of the separation of another pair of alleles during gamete formation. This is only relevant to unlinked genes (i.e. genes that are not located on the same pair of homologous chromosomes). During prophase I, homologous pairs of chromosomes pair up in a process called synapsis to form tetrads. These tetrads line up independently of each other (random assortment) in the equator of cell during metaphase I. During anaphase I, homologous chromosomes are separated and moved to opposite poles. Each pair of homologous chromosomes separates independently from the other pairs. This means that the alleles located on the chromosomes will also separate independently from other alleles. This does not occur, however, in gene linkage (i.e. when genes are located on the same chromosomes) as genes which are part of the same linkage group will separate together.
10.3.2: Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin color.
Polygenic inheritance is when a single characteristic is controlled by two or more genes. Examples of polygenic inheritance include human skin color and wheat grain color. Both of these traits are not exhibited in two distinct variations, but rather show continuous variation between extremes. Human skin color can vary from pale to very dark, depending on the amount of the pigment melanin. The amount of melanin is controlled by the alleles from at least three different genes. These alleles exhibit incomplete dominance, allowing for different combinations of alleles to produce different results. Hence, human skin color is determined by a combination of genes. Environmental factors may also contribute to skin color. The colors of grains of wheat also show continuous variation. Grains can range from white to dark red, depending on the amount of pigment they contain. Three genes control color, and each gene has two alleles (one coding for red pigment, the other coding for no pigment). Different combinations of these six alleles give rise to continuous variation between the extremes.
10.1.3: Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.
Random orientation of chromosomes in metaphase I allows for genetic variety in gametes. Random orientation means that homologous pairs orient themselves randomly, so that either the maternal or paternal homologue can orient towards either pole. The number of possible orientations is 2n, where n is the haploid number of a species. In humans, the haploid number is 23. This means that random orientation in metaphase I can produce over 8 million different gametes. This effect is compounded by crossing over during prophase I. This is when non-sister chromatids in a homologous pair exchange genetic information. This allows for the formation of recombinants. The formation of chiasmata is random, which means that two recombinants are not necessarily the same. Hence, random orientation of homologous pairs of chromosomes during metaphase I of meiosis and crossing over between non-sister chromatids during prophase I of meiosis can result in an effectively number of different gametes.
10.2.2: Distinguish between autosomes and sex chromosomes.
Sex chromosomes are pairs of chromosomes that are involved in sex determination (X and Y chromosomes in humans). Autosomes are chromosomes that are not involved in sex determination (22 pairs in humans).
10.1.4: State Mendel's law of independent assortment.
The separation of one pair of alleles is independent of the separation of another pair of alleles during gamete formation (i.e. meiosis).
10.3.1: Define Polygenic inheritance
a single characteristic that is controlled by two or more genes
