Genetics and Evolution SBI3U

GENETICS 4.1 The Nature of Heredity Heredity itself is the process of traits being passed from parents to offspring. There are several important terms to understand before we speak of asexual and sexual reproduction. Genetics is the study of heredity. Genetic material is a term that describes all the material in an organism that contains the genetic information. In a chromosome that DNA. material is referred to as DNA. A gene is the segment of a DNA molecule that contains codes for a specific trait, which is a found at a precise location in the chromosome. That location is called the locus. Asexual Reproduction: Reproduction : the form of reproduction in which a new individual is produced from a single parent by cell division. This means that the cell does not require a partner to reproduce. The daughter cell is an exact genetic replica of the parent cell. The advantages to asexual reproduction include: the parent cell does not have to seek a mate, it does not have to form specialized mating techniques or have special reproductive organs. Sexual Reproduction: Reproduction : The production of offspring from the fusion of two cells. The sex cells usually come from two different organisms. The offspring produced from two parents is not genetically identical to either. They contain equal amounts of genetic information from either parent. Advantages to sexual reproduction: ensures diversity which can then make the animal adapt to a changing environment, or immune to diseases. Disadvantages: producing mating calls or having brightly coloured feathers can attract a mate but also can attract predators. Sex is biologically costly and potentially risky. The Cell Cycle At all times, cells are doing one of two things: 1. Growing 2. Dividing The cell cycle is this pattern of growth and division.

Interphase: Growth consists of the G1, S and G 2 phase. The G1 is the preparation for copying DNA (rapid growth), S is the synthesis, or actual copying, of DNA. The DNA at

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this point is in the form of chromatin and the G2 phase is the preparation for cell division. Mitotic phase is the division of the cell (either asexually [mitosis] or sexually [meiosis]). The genetic material divides. Cyokinesis: Division of the cytoplasm and organelles into two separate cells (daughter cells) Cell Division: Terms (Refer to pg 143) Chromatin : uncondensed form of DNA, long strands (this stage of DNA exists in Interphase) Chromosomes: the condensed form of DNA ‐ forms during Mitosis/Meiosis Sister Chromatids: Chromatids: two identical sister chromatids (duplicated during Interphase) are attached by the centromere to form a chromosome during Prophase Centromere: the attaching device Centrosome: The structure that will form spindle fibres Diploid: 23 Chromosomes doubled = 46. (2n) Haploid : 23 Chromosomes single = 23 (n) Somatic: all cells that grow (excluding gametes) Gametes: cells used specifically for fusion with another gamete (sperm + ovum = zygote) Humans have 23 Chromosomes (doubled) = 46. They have a set of 23 from the mother, and a set of 23 from the father. 4.2 Cell Division in Asexual Cells Mitosis and Cytokinesis form the process of asexual cell division. The genetic material in the nucleus of a eukaryotic cell is divided equally into two new nuclei. The rest of the cell divides during Cytokinesis. The purpose of Mitosis is to duplicate the 46 single stranded chromosomes (into 92) and then divide so each daughter cells is still 2 n, or 46 chromosomes (diploid). All somatic cells undergo mitosis.

Mitosis = PMAT (Prohpase, Metaphase, Anaphase, Telophase) 1. Prophase: the first phase of mitosis. The chromosomes really start to form, the nuclear membrane disappears and spindle fibres are formed by the centrioles. 2. Metaphase: The spindle fibres align the chromosomes in the middle. 3. Anaphase: the centromeres divide and the sister chromatids (referred to as







Cytokinesis: The cytoplasm of the eukaryotic cell divides to form two new cells. Cell then reenters Interphase, and prepares to go through this all again!

4.3 ‐ Cell Division in Sexual Cells The process of sexual cell division is meiosis + Cytokinesis. Purpose of sexual reproduction: The resulting daughter cells have half the number of chromosomes as the parent cells. Parent cells are diploid (2 n) and daughter cells are haploid (n). These haploid cells will undergo a similar process to mitosis to produce identical daughter cells. These are called “gametes” – or sex cells. A female gamete (ovum) will fuse with a male gamete (sperm) to form a zygote that has (2 n) Terms to know: Homologous chromosomes: matching pairs of chromosomes, similar in size and carrying information for the same genes. However, one chromosome might have a different version of the gene (eg blue eyes) whereas its homologous pair might have another version (eg green eyes). These are called alleles.







Meiosis is made up of two phases, each with 8 stages, separated by Cytokinesis. Meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase I Meiosis II: Prophase II, Metaphase II, Anaphase II, Telophase II Meiosis I Prophase I: Dna condense into chromosomes from chromatin. Homologous chromosomes (corresponding chromosomes from mom and dad) synapse and cross over, exchange DNA for diversity. The lined up chromosomes form a tetrad. The nuclear membrane breaks down, meiotic spindle forms and centrosomes appear at the poles of the sell. The chromosomes attach to the spindle fibres at the centromeres. Metaphase I: Homologous chromosomes (eg. Ch 1 from mom and Ch 1 from dad) line up beside each other along the metaphase plate (centre of the cell)

Anaphase I: Homologous chromosomes move to opposite side of the cell.

Telophase I: Cell begins to pinch off (cleavage) and meiotic spindle breaks down. The chromosomes do not extend (different than mitosis), and the nuclear membrane does not reform. Cytokinesis: Organelles and cytoplasm are divided among daughter cells, and the cell membrane pinches off. These two new daughter cells are haploid (n), because they only have one set of chromosomes. Meiosis II Meiosis II is very similar to Mitosis in that it takes the daughter cells from Meiosis 1, which are haploid, but duplicates of 23 chromosomes, and divide them asexually so that each new cell has 23 single chromosomes, and is haploid. Prophase II: Meiotic spindle reforms and chromosomes return to spindle Metaphase II: Chromosomes line up beneath one another along the metaphasic plate.







Cytokinesis: Organelles and cytoplasm are divided among the daughter cells. The cell membrane pinches off. Back to Interphase: the cells restart their normal activities. The Difference Between Mitosis & Meiosis

Section 4.4 – Abnormal Meiosis and/or Fertilization Sometimes errors can occur during the division process. One of these errors is called non‐disjunction. Non‐disjunction can occur in Anaphase I, or Anaphase II. Non‐Disjunction occurs when homologous chromosomes fail to move to opposite poles of the cell during meiosis. This results in an abnormal number of chromosomes in a daughter cell. Look at figure 1 on page 161.







Section 5.1 – Mendelian Inheritance Terms to know: Review chromosome, locus, and allele Gene: Gene: A segment of DNA that provides the instructions for the building and regulation of a functional molecule in the cell. Homozygous/Heterozygous: Recall that we spoke of homologous chromosomes (1M – 1D). Homologous chromosomes have the same characteristics and shape. They have a copy of every gene in that chromosome. In one chromosome the gene may be the same or different to the same gene in another chromosome. The version of the genes is called the allele. If the alleles are the same the individual is said to be homozygous , if they are different they are said to be heterozygous . Genotype & Phenotype: Genotype: The combination of alleles a person has at a locus. For example: Bb (B = brown eyes, b = blue eyes) Phenotype: the physical characteristics produced by the genotype (eg Brown eyes) Mendel’s Crosses Mendel: a monk who crossed pea plants because they have many traits with two specific forms. Mendel could see how they appeared or disappeared in progeny. First, Mendel had to produced pea plants purebred for specific traits for consistency. Mendel crossed a purebred with another purebred that had a different characteristic. These two plants were called his parental generation, or P generation. The hybrid offspring of the P‐generation cross were the filial generation, or F 1 generation. The offspring only differed from each other through one characteristic, making them monohybrid. The results of Mendel’s first cross revealed that one of the two traits completely disappeared. When Mendel allowed the F1 generation of plants to cross, and produce F2, he noticed that the “disappearing” trait reappeared. He measured the ratio of the F 2 cross to be 3:1 (3 being the trait that appeared in F 1, 1 being the “disappearing” trait). Mendel’s conclusion led to the first law of Mendelian Inheritance









The question is, in heterozygous individuals (recall above – a pair of homologous chromosomes with different alleles) which allele is expressed? The answer is based on dominance & rececessiveness of alleles: Dominant Allele: The allele that, if present, is always expressed. Recessive Allele: The allele that is expressed only if it is not in the presence of the dominant allele. The individual must be homozygous for the recessive allele. In the genotype, the dominant allele is represented by an upper case letter (eg brown eyes = B) and the recessive allele is represented by a lower case letter (eg blue eyes = b) E.g. an individual could be: 1. Heterozygous (Bb) 2. Homozygous for Dominant Allele (BB) 3. Homozygous for Recessive allele (bb) In options 1 & 2, the individual will have brown eyes, because the Dominant Allele is present. In option 3 the individual will have blue eyes, because the recessive allele is not masked by any dominant one. The Punnett Square The Punnett Square is a diagram that summarizes every possible combination of each allele from each parent. It is the tool to determine the probability of a single offspring having a particular genotype. For example, in the diagram to the left, the ratio would be: 1GG:2Gg:1gg Here is an example Punnett Square problem: (Pg. 189) Two heterozygous yellow seed plants (Yy) are crossed. Determine the genotype and phenotype ratios of the F2 generation. 1. Define your alleles. Y = yellow seed, y = green seed. 2. Determine the genotypes of the parents (Yy x Yy) 3. Determine the potential gametes each plant could produce: Plant 1: Y, y Plant 2:







5.2: Variations in Heredity Incomplete Dominance and Codominance Mendel’s experiments had proved that the blending theory (not req. for exam) was wrong. He had discovered complete dominance: one of the alleles is expressed, despite the presence of the other allele. Incomplete Dominance: Neither allele dominates the other. Both have an influence on the phenotype of the individual. (Eg. Purebred white flowers mixed with purebred red flowers product 1 red, 2 “pink” and 1 white) Codominance: Both alleles are expressed fully to produce offspring with a third phenotype (Red bull crossed with a white cow produces 1 red calf, 2 roan calves and 1 white calf) 5.3 Sex Linkage – Following the X and Y Chromosomes (Read this Chapter) Some traits are only found on the sex chromosomes. Some disorders have recessive alleles that are found on the X chromosome. Men have an XY chromosome but the Y cannot mask the X chromosome, therefore he will have the disorder. 5.7 Multi‐Trait Inheritance Dihybrid Crosses Mendel decided to do test crosses on plants with two traits. For example: Mendel crosses a homozygous round, yellow plant with a homozygous wrinkled, green plant to produce: RY

RY







Linkage & Recombination Linkage: when two genes are fond on the same chromosome Recombination: When chromosomes cross over during Prophase 1, the genes are switched around and there is a reassortment of the genes on each chromosome. The percent recomb. Is the percent that the two genes cover switching (Practice Problems on this) EVOLUTION Section 7.1 – Biological Change Over Time Mutations The original sources of genetic variation are mutations, which are changes in genetic information. Mutations create new genes that provide a continual supply of new genetic information. We must also understand the term fitness. “Fitness” is the animal’s ability to reproduce. Mutations can happen for two reasons: 1. The exposure to external stimuli 2. Error during the copying of DNA There are three types of mutations: 1. Neutral mutations: mutations that do not really effect the organism 2. Harmful mutations: mutations that decrease the individual’s fitness 3. Beneficial mutations: mutations that increase the individual’s fitness. Beneficial mutations are favoured by natural selection and accumulate over time. Mutations are the source of new alleles in a population. The frequency of alleles will change due to selection of alleles based on fitness.







Hypothesis: Similarity between species is due to the fact that all of these species are derived from a small number of ancestral species. Observation 2: Homologous vs. Analogous Features Observation: Certain organisms had homologous features (similar structure, different function), whereas certain organisms had analogous features (different structure, similar function)  Example of homologous feature

An example of analogous features is dove vs. butterfly wings Hypothesis: Organisms with homologous features likely share a more recent common ancestor than organisms with analogous features. Observation 3: Vestigial Features Observation: Certain organisms had features that had no useful purpose but were homologous to fully functioning structures in related species (e.g. hipbone in whales) Hypothesis: Species come from ancestors that once used these features. Observation 4: Artificial Selection & Evolution Darwin found that all of the species had inherited variations that can be selected for to change the species in desirable ways.







Inferences from his Theory: Individuals within a population compete for resources, and some individuals will inherit characteristics that give them a better chance of surviving. The overall theory is that over time the population changes, as advantageous heritable characteristics become more common generation after generation. 8.1 Natural Selection Patterns of Selection Stabilizing Selection: This is the selection for the most common phenotypes present in a population, because the species is already well adapted to its environment. Selection pressure acts to maintain the evolved state. Directional Selection: Selection for individuals with less common variations of a trait. The species moves to a new environment. Less common variants are selected for an in effort to adapt to the new environment. Disruptive Selection: This is selection for individuals with variations at either extreme of a trait. Variants at both ends of the spectrum are selected for in an effort to adapt to the new environment. Sexual Selection: This is selection for the traits that influence the mating success of an individual. Sexual selection leads to sexual dimorphism, or differences in physical appearance between male and female. Evolution is often a compromise among selective pressures. For example, colourful









Genetic Drift: Changes to allele frequency as a result of chance. These changes are much more pronounced in small populations 1. Bottleneck Effect: events (catastrophes) result in a random, drastic reduction in a population size, leading to significant change, in relative allele frequency within the population. 2. The Founder Effect: a few individuals from a larger population leave to establish a new population, leading to significant changes in relative allele frequency. Gene Flow: People from an existing population leave and immigrate into another population. This changes the gene frequencies. 8.2 Speciation The question is: how does a new species get here in the first place? Microevolution: Microevolution: is the gradual change in allele frequencies and phenotypic traits in populations and species. Continued microevolution over time can eventually lead to the formation of a new species. Speciation: is the formation of a completely new species through progressive microevolution What is a species? A species is defined as any group of organisms that is able to interbreed under normal







Speciation  any event that causes the reproductive isolation of two populations 1. Allopatric Speciation: Populations become geographically separated from one another 2. Sympatric Speciation: Populations become genetically isolated from one another through mutations but remain in the same location 8.5 Macroevolution Macroevolution: large scale evolutionary changes leading to the production of new genera or even taxa (species) Gradualism: A theory that attributes large evolutionary changes in species to the accumulation of many small and ongoing changes and processes Punctuated Equilibrium: The theory attributes most evolutionary changes due to rapid spurts of change followed by long periods of little, or no change

Genetics and Evolution SBI3U

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