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Chapter 11 Notes – Introduction to Genetics

I. The Work of Gregor Mendel A. Gregor Mendel – father of genetics(Scientific study of Heredity); studied plants

B. He experimented with garden pea plants. 1. observed Traits (a trait is a specific characteristic, such as seed color or plant height, that varies from one individual to another.) 2. studied self-pollinating plants (sperm cells in pollen fertilize the egg cells in the same flower and the seeds that are produced inherit their characteristics from the single parent plant) 3. studied cross pollinating plants (sperm cells in pollen from the flower on one plant fertilize the egg cells of the flower on another plant and the seeds that are produced inherit their characteristics from two parent plants)

C. He found that self-pollinating plants would produce offspring identical to themselves. This is known as true breeding.

II. Genes and Dominance A. Mendel studied 7 different pea plant traits (e.g. seed color, plant height etc). Each trait had 2 contrasting characters B. He crossed true breeding plants with contrasting characters for the same trait and studied their offspring. C. The original pair of plants is the P (parental) generation. The offspring of the P generation is the F1 generation (“f” means filial, Filius is Latin for “son”). D. The offspring from crosses between parents with different traits are hybrids. He ALWAYS found that all of the offspring had the character of only one of the parents and character of the other parent seemed to have disappeared. One parental (P) trait disappeared in the Filial (F1) generation. E. Mendel concluded: 1) The biological inheritance is determined by factors that are passed from one generation to the next. We now call these chemical factors “genes.” Alleles – different forms/versions of a gene that controlled each trait. Each trait is controlled by 1 gene ------> 2 contrasting forms 2) His second conclusion was called the principle of dominance. It states that some alleles are dominant and some are recessive. Dominant allele for a particular form of a trait will always exhibit that form of the trait. Recessive allele for a particular form of a trait will exhibit that form only when the dominant allele for the trait is not present.

III. Segregation A. He continued experiments with self-pollination. Mendel self-pollinated the F1 hybrid plants and in F2 generation found some peas had traits of generation P (traits not found in their own parents).

B. The traits controlled by the recessive alleles had reappeared in F2 generation. Roughly 1/4 of the F2 plants showed the trait controlled by the recessive allele. Remaining ¾ F2 plants showed the trait controlled by dominant allele. Ratio of dominant to recessive is 3:1

C. During gamete (sex cells) formation, alleles segregate (separate) from each other so that each gamete carries only a single copy of each gene.

D. Each F1 plant produces two types of gametes—those with the allele for tallness and those with the allele for shortness. A capital letter T represents a dominant allele. A lowercase letter t represents a recessive allele.

E. The alleles are paired up again when gametes fuse during fertilization. The TT and Tt allele combinations produce tall pea plants; tt is the only allele combination that produces a short pea plant.

IV. Punnett Squares

A. A diagram used to determine the gene combinations that might result from a genetic cross

B. homozygous – having 2 identical alleles for a trait (BB or bb) – True breeding

C. heterozygous – having 2 different alleles for a trait (Bb) – hybrid

D. phenotype – physical characteristics of an organism (brown hair)

E. genotype – genetic makeup of an organism (BB or Bb)

F. Examples of crossing: Example #1 Trait – seed pod color G = green seed pod (dominant allele) g = yellow seed pod (recessive allele)

**Remember…there are two genes for each trait in an individual.** Cross a true breeding green seed pod with a true breeding yellow seed pod (cross- pollination) P1 GG x gg = 100% Gg (F1) (parents) (offspring) G. Example #2 Cross F1 generation (self-pollinate)

Gg x Gg = F2 GG Gg Gg gg

V. Genetics and Probability A. probability – the likelihood that a certain event will occur B. If you toss a coin, what is the probability of getting heads? 1 chance in 2 = ½ = 50% tails? 1 chance in 2 = 50% C. If you toss a coin three times in a row, what is the probability that it will land heads up all three times? ½ * ½ * ½ = 1/8 D. The principles of probability can be used to predict the outcome of genetic crosses. The probability predicts the average outcome of the event, and cannot predict the precise outcome of an individual event.

VI. Exploring Mendelian Genetics A. Independent Assortment- does the segregation of one pair of alleles affect the segregation of another pair of alleles? He followed two different genes as they passed from one generation to the next. 1. The Two-Factor Cross: F1 – crossing only homozygous dominant with homozygous recessive. 2. The Two-Factor Cross: F2 – crossing the F1 generation that are both heterozygous

B. alleles for seed shape segregated independently of those for seed color and do not influence each other's inheritance. The results were very close to the 9 : 3 : 3 : 1 ratio that the Punnett square shown above predicts. C. The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms.

D. A Summary of Mendel’s Principles 1. The inheritance of biological characteristics is determined by genes. In sexually reproducing individuals, these genes are passed from parents to offspring. 2. Principle of Dominance – some alleles are dominant and some are recessive 3. Principle of Segregation - in most sexually reproducing organisms, each adult has two copies of each gene. These genes are segregated from each other when gametes are formed. 4. Principle of Independent Assortment – the alleles for different genes usually segregated independently of one another (i.e. hair color and eye color are not linked together)

E. Beyond Dominant and Recessive Alleles The genetics is more complicated as not all genes show simple patterns of dominant and recessive alleles. The majority of genes have more than two alleles. Many important traits are controlled by more than one gene.

1. Incomplete Dominance – where one allele is not completely dominant over another; the heterozygous phenotype is in between the two homozygous (parents) phenotypes. Ex: a cross between red-flowered (RR) and white-flowered (WW) plants consists of pink- colored flowers (RW).

2. Codominance – both alleles are expressed; i.e. in certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens have a color described as “erminette,” speckled with black and white feathers.

3. Multiple Alleles – when genes have more than two alleles. This does not mean that an individual can have more than two alleles. It only means that more than two possible alleles exist in a population. e.g. is human genes for blood type (A, B, AB, O)

Genotype IAIA or IAi IBIB or IBi IAIB ii Phenotype A B AB O

4. Polygenic Traits – traits controlled by interaction of two or more genes (often show a wide range of phenotypes) i.e. at least three genes are involved in making the reddish-brown pigment in the eyes of fruit flies. Different combinations of alleles for these genes produce very different eye colors. Wide range of skin color in humans result from more than four different genes that control this trait. VII. Meiosis A. Mendel’s Principle require 2 things : Each organism must inherit a single copy of every gene from both it’s parents. When an organism produces gametes, those 2 sets of genes must be separated from each other so that each gamete contains just one set of genes.

B. Meiosis is a cellular reproduction in which number of chromosomes per cell is cut in half due to separation of homologous chromosomes of the diploid parent cell. The result is 4 haploid cells that are genetically different from one another and from the original cell.

C. gametes – sex cells; combine during sexual reproduction; examples are egg, sperm D. diploid – cell with 2 complete sets of chromosomes (2N); matching pairs are called homologous chromosomes; all body cells are diploid; result from mitosis E. haploid – cell with one complete set of chromosomes (N or 1N); examples are gametes; result from meiosis

F. Phases of Meiosis I 1. Meiosis I – Prior to meiosis each chromosome is replicated and homologous pairs of chromosomes come together. 2. Prophase I a. chromosomes become thick and visible b. homologous pairs (4 chromatids) form a structure called “tetrad” c. may exchange portion of chromatids (alleles) called as “crossing over” d. nucleus disappears; spindle fibers form 3. Metaphase I a. homologous pairs line up at the middle of the cell b. spidle fibers attach to the chromosomes 4. Anaphase I a. The spidle fibers pull homologous chromosomes toward opposite end of the cell. 5. Telophase I a. cytokinesis takes place b. each new cell is haploid (neither daughter cell has 2 complete sets of chromosomes) c. nuclear membrane may reform

G. Phases of Meiosis II – each of the daughter cells produced during Meiosis I will divide again during Meiosis II. Neither cell goes through chromosome replication 1.Prophase II a. nuclear membrane disappears b. chromatids attached at centromere c. centrioles and spindle fibers visible 2.Metaphase II a. chromosomes lined up at center of cell b. chromosomes attached to spindle fibers at centromere 3.Anaphase II a. spindle fibers contract b. sister chromatids separate c. chromatids pulled to opposite poles of cell 4.Telophase II a. chromatids are at separate poles of cell b. nuclear membrane reforms c. cytokinesis begins 5.four new haploid cells are produced; each contains one strand of each of the original pairs of homologous chromosomes 6.Meiosis allows for mixing of genes and variation among species.

VIII. Linkage and Gene Maps A. Mendel’s law of independent assortment - each pair of alleles segregates independently of other pair of alleles during gamete formation The two genes (color and shape) assort independently of each other and do not influence each other's inheritance What about the genes located on the same chromosome? Wouldn’t they be inherited together?

B. Thomas Morgan Researched on Fruit flies and explained “principle of Gene Linkage” Some genes are linked together and are inherited together. Linkage groups assort independently. He found Drosophila has 4 linkage groups. Coincidently, Drosophila also has 4 pairs of chromosomes. Conclusion – each chromosome is a group of linked genes. Chromosomes assort independently, Not individual genes

C. How did Mendel missed gene linkage? The 6 out of 7 genes Mendel studied, are located on different chromosomes, which segregate independently during gamete formation. A modern restatement of Mendel’s Second Law - genes located on different chromosomes assort independently during meiosis

D. Crossing-over during meiosis sometimes separates genes that had been on the same chromosome onto homologous chromosomes. Crossover events occasionally separate and exchange linked genes and produce new combinations of alleles. This is important because it helps to generate genetic diversity

E. Gene Maps – shows the locations of genes on a chromosome; the farther apart two genes are, the more likely they are to be separated by a crossover in meiosis.

Q. Would you expect more cross over events to occur between star eye and speck wing or between star eye and black body?