AP Biology Unit 5: Heredity

Meiosis

Meiosis

The process of cell division that is used in gamete formation. It forms haploid (n) gametes from diploid (2n) parent cells, which helps maintain the proper number of chromosomes in the offspring. When a haploid egg is fertilized by a haploid sperm, the resulting zygote has the correct diploid number of chromosomes. It has two rounds of cell division that results in the creation of four genetically different haploid gametes.

Haploids

Contains only one set of genetic info in single copies of each chromosome. The joining of two of these gametes in fertilization produces a diploid zygote.

Diploids

Contains two sets of genetic info in homologous chromosome pairs.

Gametes

Reproductive cells like sperm and eggs.

Meiosis I

The first round of cell division in meiosis, it is referred to as a reduction phase and has four stages: prophase I, metaphase I, anaphase I, and telophase I.

Prophase I

The nuclear membrane breaks down and the chromosomes condense and become visible. Then homologous chromosomes pair up in a process called synapsis to form tetrads. Once the tetrads are formed, homologous chromosomes may exchange genetic info through the process of genetic recombination (crossing-over) can occur.

Tertrads

The foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.

Genetic Recombination

Cell division that produces gametes.

Metaphase I

Chromosomes line up in homologous pairs along the center of the cell.

Anaphase I

Pairs of homologous chromosomes separate. There are the same number of chromosomes at the end of the end of this stage as there is at the beginning.

Telophase I

Two new nuclei are formed. When followed by cytokinesis, this results in two haploid cells.

Meiosis II

The second round of cell division in meiosis, it has four stages: Prophase II, Metaphase II, Anaphase II, and Telophase II. At the end of it, there are four haploid gametic cells.

Prophase II

Chromosomes condense and become visible.

Metaphase II

Chromosomes line up in a single line along the center of each cell.

Anaphase II

Sister chromatids separate and move to opposite ends of the cell. Each sister chromatid will have its own centromere once this separation has occurred. At the end of this stage, these separated sister chromatids are now considered separate chromosomes.

Telophase II

Splits each of the two cells in half, resulting in four cells.

Genetic Diversity

The frequency of genetic recombination events can be used to estimate the distance between genes that are on the same chromosome. Genes that are further apart have a higher recombination frequency while genes that are closer have a lower recombination frequency. The frequency of genetic recombination between genes on the same chromosome can be used to generate genetic maps that show the relative positions of genes on chromosomes. It can also be generated during metaphase I. Each pair of chromosomes lines up and assorts independently, with different pairs having the paternal chromosome on one side and the maternal chromosome on the other side.

Genes

Info is transferred from generation to generation through these units of chemical info.

Mendel’s Law of Segregration

States that an organism carries two variations of every trait (called alleles), one from each parent, and these alleles separate independently into gametes. This separation occurs during anaphase of meiosis. When homologous chromosomes separate during anaphase I, the alleles on these chromosomes will segregate into separate gametes. One variation (or allele) for a trait ends up in each gamete. When two gametes join during fertilization, the resulting offspring then again has two alleles for the trait.

Mendel’s Law of Independent Assortment

States that genes for different traits segregate independently of one another. The independent assortment occurs during metaphase I of meiosis.

Pedigree

A chart that illustrates the inheritance of a trait through several generations. Horizontal lines between two individuals in this show that they have had offspring together. Offspring of these individuals are indicated by vertical lines. Circles represent females, and squares represent males. Shaded circles or squares represent individuals who possess the trait shown in this. Examining the pattern of inheritance of a trait shown in this can give clues to the traits’ possible mode for inheritance.

Dominant Traits

Tend to be expressed in at least one parent and their offspring because only one allele for the trait is required for it to be expressed.

Recessive Traits

Often will be expressed in offspring, but not in either parent, because the parents are heterozygous carriers for the trait.

Sex-Linked Recessive Traits

Usually appear more often in males than in females.

Genotype

The genetic makeup or alleles for the trait in an organism (AA, Aa, or aa).

Phenotype

The physical expression of the genotype in an organism (purple flowers or white flowers).

Homozygous

Having two copies of the same allele for a trait (AA or aa).

Heterozygous

Having two different alleles for a trait (Aa); hybrid.

Dominant

Requires only one copy of the allele for the trait to be expressed in the phenotype; they are expressed in organisms that are heterozygous.

Recessive

Requires two copies of the allele for the trait to be expressed in the phenotype; they are not expressed in organisms that are heterozygous.

Linked Genes

Genes that are close together on the same chromosome and tend to be inherited together more often than unlinked genes. During prophase I of meiosis, genetic recombination may occur between these. The farther apart two of these are on a chromosome, the more likely a genetic recombination event will occur because there is more space in which recombination may happen.

Map Units

Recombination frequencies can be used to create genetic maps that show the distance between genes on a chromosome. The distance between genes is measured with these. A recombination frequency of 10% between two genes would place those two genes 10 of these apart.

Autosomes

Most genes are located on these, chromosomes that are not directly involved in sex determination.

Sex Chromosomes

Are nonhomologous in humans; females have two X chromosomes and males have one X and one Y chromosome. When looking at a pedigree that shows more males with the trait than females, it is likely the trait is coded for by a gene that is located on one of these.

Sex-Linked Recessive Alleles

Due to males having one X and one Y chromosome, traits that are coded for by these are more likely to be expressed in males since they only have one X chromosome. Females could also express these, but since they have two X chromosomes, they would need to inherit the allele for the trait from both parents in order to express it. Examples are hemophilia and color blindness.

Sex-Linked Dominant Traits

These are very rare. If a male has one, all of his daughters will inherit the trait because all females inherit an X chromosome from their father. If a female has one, her sons and daughters will have a 50% chance of inheriting the trait.

Multiple Gene Inheritance

Some traits are produced by multiple genes acting together to produce the phenotype. Because more than one gene is involved in producing the trait, these traits would not follow the ratios predicted by Mendelian laws. Examples of traits produced by multiple genes are height and eye color. For example, say height in a plant is determined by three genes (A, B, and C), and each dominant allele present in the three genes has an additive effect on the plant’s height. The more dominant alleles a plant inherits, the taller the plant will be. A plant with the genotype AABBCC would be very tall, a plant with the genotype aabbcc would be very short, and a plant with the genotype AaBbCc, AAbbCc, or other genotypes would result in a height in the middle of the range.

Nonnuclear Inheritance

Genes on mitochondrial or chloroplast DNA do not follow the inheritance patterns seen in genes located on nuclear DNA. During gamete formation, the eggs produced in animals and the ovules produced in plants are much larger than the sperm (in animals) or pollen (in plants) that are produced. Since the eggs and ovules are larger, they contribute far more mitochondria and mitochondrial DNA (mtDNA) than the sperm or pollen do. For this reason, traits on nonnuclear DNA in the mitochondria or chloroplast demonstrate maternal inheritance.

Phenotypic Plasticity

The environment can affect gene expression and the resulting phenotype for an organism. For example, in the flowers of the hydrangea plant, a basic soil pH results in flowers with a pink color, while an acidic pH results in blue flowers. In humans, exposure to UV light can stimulate the expression of genes involved in the production of melanin. The ability of the same genotypes to produce different phenotypes in response to different environmental factors is called this.