Mission 1: Meiosis. Mission objectives are listed below.
You are already familiar with meiosis. Chapter 10 takes you deeper into the topic. Instead of examining single genes with two alleles, we will look at multiple genes that control a single trait. The guiding question is “Could there be a range of traits from one extreme to the other?”
You are already familiar with meiosis. Chapter 10 takes you deeper into the topic. Instead of examining single genes with two alleles, we will look at multiple genes that control a single trait. The guiding question is “Could there be a range of traits from one extreme to the other?”
During prophase I, the process of synapsis brings two homologous chromosomes together in a pair called a bivalent. Recall that homologous chromosomes are similar in length, centromere placement, and the same genes at the same loci. They are not identical. One chromosome in the bivalent comes from the mother and the other chromosome comes from the father.
Chromatids are one strand of a chromosome. The chromosome pair is made up of two sister chromatids. Mixing genetic material between non-sister chromatids (mother and father) occur when chromatids intertwine and break. In order for crossing over to take place, identical breaks must occur at exactly the same position in adjacent non-sister chromatids.
The place where non-sister chromatids cross over is called a chiasma (plural: chiasmata). Many chiasmata can form along all four chromatids. During any single crossing over event, hundreds of thousands of genes can be traded this way. A single bivalent can have several chiasmata producing crossing over in more than one chromatid. This yields extensive genetic variation in the formation of gametes. This is why siblings (unless identical twins) never get the same combination of their parents’ alleles.
The bivalent is pulled apart in anaphase I, which means one half of the homologous pair is pulled to one end of the cell. In anaphase II, the sister chromatids of each chromosome are separated and pulled to ends of the cell. During telophase II, a new nuclear membrane forms around the genetic material.
Chromatids are one strand of a chromosome. The chromosome pair is made up of two sister chromatids. Mixing genetic material between non-sister chromatids (mother and father) occur when chromatids intertwine and break. In order for crossing over to take place, identical breaks must occur at exactly the same position in adjacent non-sister chromatids.
The place where non-sister chromatids cross over is called a chiasma (plural: chiasmata). Many chiasmata can form along all four chromatids. During any single crossing over event, hundreds of thousands of genes can be traded this way. A single bivalent can have several chiasmata producing crossing over in more than one chromatid. This yields extensive genetic variation in the formation of gametes. This is why siblings (unless identical twins) never get the same combination of their parents’ alleles.
The bivalent is pulled apart in anaphase I, which means one half of the homologous pair is pulled to one end of the cell. In anaphase II, the sister chromatids of each chromosome are separated and pulled to ends of the cell. During telophase II, a new nuclear membrane forms around the genetic material.
Independent Assortment. Recall that Mendel’s law of Independent Assortment states that when gametes are formed, the separation of a pair of alleles between daughter cells is independent of the separation of another pair of alleles. One allele does not follow another when it is passed on to a gamete. Each allele in a pair can mix with either allele in another pair.
Why do traits get passed on independently from each other? The orientation of bivalents during metaphase I is a random process. For humans, the total (theoretical) number of possible combinations during random orientation is 2^23 because there are 23 chromosomes in each gamete. The theoretical probability of a woman producing the exact same egg twice is 1 in 8,388,608. It is likely the same or similar probability of a male producing the exact same sperm. The possible combinations are infinite.
Why do traits get passed on independently from each other? The orientation of bivalents during metaphase I is a random process. For humans, the total (theoretical) number of possible combinations during random orientation is 2^23 because there are 23 chromosomes in each gamete. The theoretical probability of a woman producing the exact same egg twice is 1 in 8,388,608. It is likely the same or similar probability of a male producing the exact same sperm. The possible combinations are infinite.
Mission 2: Inheritance. Mission objectives are below.
Remember when we completed monohybrid and dihybrid crosses? Monohybrid crosses examined one trait whereas dihybrid crosses examined two traits. In this section, we will look at multiple traits.
Remember when we completed monohybrid and dihybrid crosses? Monohybrid crosses examined one trait whereas dihybrid crosses examined two traits. In this section, we will look at multiple traits.
Genes that are not linked should segregate independently, which means they should be able to pass to the next generation with or without the other. They show no dependence on each other and no preference either way. During the shuffling of alleles in meiosis, they are equally distributed between gametes. As a result, there should be predictable ratios.
There are autosomal genes, which means they’re located on any of the 22 autosomes, not the sex chromosomes. A trait that is sex-linked is located on the 23 pair (the sex chromosomes). Where a gene is located determines whether the trait it controls is more common in males than females. When a trait is more common in one sex than the other, the chances are that the trait is sex-linked and the locus is either on the X or the Y, or both. If there is no pattern, then the trait is likely autosomal.
Any two genes that are found on the same chromosome are said to be linked. Linked genes are passed on to the next generation together. See the Drosophila data. Linked genes have a special notation used in test crossing. To determine whether a fly’s phenotype is the result of a homozygous or heterozygous genotype, a test cross is completed using a known homozygous recessive.
Recombinant is used to describe the new chromosome and the resulting organism. Recombinants have allele combinations that do not match either parent’s genotype. The way recombinants form is through crossing over. Without this process, certain alleles would always be inherited with certain other alleles for the reason that they are linked.
There are autosomal genes, which means they’re located on any of the 22 autosomes, not the sex chromosomes. A trait that is sex-linked is located on the 23 pair (the sex chromosomes). Where a gene is located determines whether the trait it controls is more common in males than females. When a trait is more common in one sex than the other, the chances are that the trait is sex-linked and the locus is either on the X or the Y, or both. If there is no pattern, then the trait is likely autosomal.
Any two genes that are found on the same chromosome are said to be linked. Linked genes are passed on to the next generation together. See the Drosophila data. Linked genes have a special notation used in test crossing. To determine whether a fly’s phenotype is the result of a homozygous or heterozygous genotype, a test cross is completed using a known homozygous recessive.
Recombinant is used to describe the new chromosome and the resulting organism. Recombinants have allele combinations that do not match either parent’s genotype. The way recombinants form is through crossing over. Without this process, certain alleles would always be inherited with certain other alleles for the reason that they are linked.
Below is a gene linkage practice problem. We will work on it in class. You need to be able to form and identify recombinants like this.
Polygenic Inheritance. This involves two or more genes influencing the expression of one trait. With two or more allele pairs are found at different loci, the number of possible genotypes is greatly increased. Most human traits are too complex and show too many combinations to be determined by one gene.
With two alleles (dominant and recessive) of a single gene, possible phenotypes are limited. When there are multiple alleles for a single trait, the phenotype possibilies increases. Ex: ABO blood type has 3 alleles and four possible phenotypes. When an array of possible phenotypes can be produced, it is called continuous variation. Skin color is such a trait. The intensity of pigment in skin is the result of the interaction of multiple genes. When variation is not continuous, it is referred to as discontinuous variation, or discrete variation.
When there are many intermediate phenotype possibilities, the trait shows continuous variation, which becomes a bell curve when results are plotted on a graph.
With two alleles (dominant and recessive) of a single gene, possible phenotypes are limited. When there are multiple alleles for a single trait, the phenotype possibilies increases. Ex: ABO blood type has 3 alleles and four possible phenotypes. When an array of possible phenotypes can be produced, it is called continuous variation. Skin color is such a trait. The intensity of pigment in skin is the result of the interaction of multiple genes. When variation is not continuous, it is referred to as discontinuous variation, or discrete variation.
When there are many intermediate phenotype possibilities, the trait shows continuous variation, which becomes a bell curve when results are plotted on a graph.
We will look at chi squared tests at the end of the chapter, but it would help you to prepare beforehand by watching this video.
Mission 3: Gene Pools & Speciation. Mission objectives are below.
Gene pools are relatively stable over time, but not always. New alleles can be introduced as a result of mutation and old alleles can disappear when the last organism carrying that allele dies. Some alleles prove to be advantageous and are more frequent as a result. Immigrations (coming in) and emigrations (leaving) usually result in a change in allele frequencies. When there is a change in allele frequency, it can be determined that some degree of evolution has taken place.
Reproductive Isolation of Populations. Members of the same species (same gene pool) can be stopped from reproducing because there is a barrier between them. Barriers can be geographical, temporal, behavioral or related to hybridization infertility. Geographical isolation happens when land/water formations prevent males and females from finding each other, which makes interbreeding impossible. Temporal isolation refers to incompatible time frames that prevent populations or their gametes from encountering one another. Behavioral isolation can occur when one population’s lifestyle and habits are incompatible with those of another population.
Find two examples of each kind of reproductive isolation.
Speciation. Gradualism is one of two theories about the pace of evolutionary change. It was the prevailing the idea that species slowly change through a series of intermediate forms. However, there were gaps in the fossil record that did not support the theory. Punctuated equilibrium holds that long periods of relative stability in a species are punctuated by periods of rapid evolution. According to this theory, gaps in the fossil record are not gaps at all, as there was no long sequence of intermediate forms. Events such as geographic isolation and the opening of new niches within a shared geographic range can lead to rapid speciation.
Read up on directional, stabilizing, and disruptive selection.
Reproductive Isolation of Populations. Members of the same species (same gene pool) can be stopped from reproducing because there is a barrier between them. Barriers can be geographical, temporal, behavioral or related to hybridization infertility. Geographical isolation happens when land/water formations prevent males and females from finding each other, which makes interbreeding impossible. Temporal isolation refers to incompatible time frames that prevent populations or their gametes from encountering one another. Behavioral isolation can occur when one population’s lifestyle and habits are incompatible with those of another population.
Find two examples of each kind of reproductive isolation.
Speciation. Gradualism is one of two theories about the pace of evolutionary change. It was the prevailing the idea that species slowly change through a series of intermediate forms. However, there were gaps in the fossil record that did not support the theory. Punctuated equilibrium holds that long periods of relative stability in a species are punctuated by periods of rapid evolution. According to this theory, gaps in the fossil record are not gaps at all, as there was no long sequence of intermediate forms. Events such as geographic isolation and the opening of new niches within a shared geographic range can lead to rapid speciation.
Read up on directional, stabilizing, and disruptive selection.