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Genetics Unit 7 General Biology

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Chromosome Number The Chromosomal Theory of Inheritance genes are located in specific positions on chromosomes. Homologous Chromosomes chromosomes come in pairs, one from the male parent and one from the female parent.

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Gene Map

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Chromosome Number Diploid a cell that contains both sets of homologous chromosomes. (2N) Diploid cells contain two complete sets of chromosomes and two complete sets of genes (one set from each parent). Haploid a cell only containing one set of chromosomes. (N)

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Meiosis Meiosis a process of reduction division in which the number of chromosomes is cut in half through separation of homologous chromosomes in a diploid cell. Meiosis takes place in two distinct divisions: Meiosis I and Meiosis II.

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Meiosis Interphase cells undergo DNA replication, forming duplicate chromosomes during the S phase. Meiosis I Prophase I each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Crossing over occurs in prophase I. Metaphase I chromosomes line up in the middle of the cell and attach to spindle fibers. Anaphase I spindle fibers pull chromosomes toward opposite ends of the cell. Telophase I and Cytokinesis nuclear membrane reforms and the cell divides into two cells.

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Meiosis I Crossing Over in prophase I, homologous chromosomes exchange portions of their chromatids. This produces new combination of alleles and allows for more genetic variation.

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Meiosis I

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Meiosis Meiosis II Prophase II meiosis I resulted in two haploid daughter cells with half the number of chromosomes as the original cell. Metaphase II the chromosomes line up in the middle of the cell. Anaphase II sister chromatids are separated and move toward opposite ends of the cell. Telophase II and Cytokinesis nuclear membranes form and meiosis II results in four haploid daughter cells.

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Meiosis II

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Gamete Formation In male animals, meiosis results in four equal-sized gametes called sperm.

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Gamete Formation In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction.

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Comparing Mitosis and Meiosis Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells. The physical processes that occur during meiosis II is identical to the physical processes that occur during mitosis.

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Gregor Mendel Gregor Mendel is known as the father of genetics. In 1866, he published his findings on the method and mathematics of inheritance in garden pea plants. Pea plants reproduce by self-fertilization. Self- fertilization occurs when a male gamete within a flower combines with a female gamete in the same flower. Mendel discovered that pea plants could be easily cross- pollinated. As Mendel bred his pea plants, he analyzed his results using mathematics to form hypotheses concerning how traits were inherited.

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Flower Anatomy

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Genetics The passing of traits from one generation to the next is called inheritance, or heredity. Genetics is the study of heredity.

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The Inheritance of Traits Mendel noticed that certain varieties of garden pea plants produced specific forms of a trait, generation after generation (like yellow and green seeds). To begin to understand how the traits were inherited, he used cross-pollination: Transferring male gametes from a true-breeding green- seed pea plant to the female organ of a flower from a true-breeding yellow-seed pea plant. He called this parent generation, the P generation.

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The Inheritance of Traits The offspring of the P cross, called the F 1 generation, all had yellow seeds Why didnt any of have green seeds if one of the parents had green seeds??? Mendel allowed the F 1 generation to self-fertilize and the offspring of the F 2 generation had mostly yellow seeds, but some green seeds too How did the green seeds reappear in the F 2 generation???

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Conclusions From The Experiment There must be two forms of the seed-color trait in the pea plants (yellow and green) These are called alleles, or difference forms of a single gene/trait The gene/trait: seed color Alleles: green or yellow Based on his observations, he decided that some alleles must be dominant over others. We called the allele that gets masked recessive.

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Representing Alleles Alleles that are dominant are represented with capital letters. Alleles that are recessive are represented by the same letter as the dominant allele for the trait, just lower- case. For example: If yellow seeds are dominant over green seeds. The dominant allele, yellow seeds: Y The recessive allele, green seeds: y

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You try In pea plants, if round seeds are dominant over wrinkled seeds The dominant allele, round seeds: The recessive allele, wrinkled seeds: In pea plants, if tall stems are dominant over short stems The dominant allele, tall stems: The recessive allele, short stems:

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Homozygous and Heterozygous Remember that each offspring has an allele for each trait from both parents. If both alleles are the same, we say that the offspring is homozygous for that trait. YY (homozygous dominant) yy (homozygous recessive) If the two alleles are different, we say that the offspring is heterozygous for that trait. Yy

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Genotype and Phenotype The organisms allele pairs (YY, Yy, or yy) is called its genotype. The observed characteristic or outward expression (yellow or green) of an allele pair is called the phenotype.

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The Laws During Mendels study of heredity in pea plants, he was able to develop two laws: 1.Law of Segregation 2.Law of Independent Assortment

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Law of Segregation The law of segregation states that two alleles for each trait separate during the formation of gametes (meiosis). During fertilization, two alleles for that trait unite.

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Monohybrid Cross A cross that involves hybrids for a single trait is called a monohybrid cross. This occurred during the self-fertilization of Mendels F 1 generation. Yy x Yy

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Dihybrid Cross The simultaneous inheritance of two or more traits in the same plant is a dihybrid cross. Dihybrids are heterozygous for both traits YyRr x YyRr

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Law of Independent Assortment The law of independent assortment states that a random distribution of alleles occurs during metaphase I of meiosis as chromosomes align down the center of the cell. Therefore, the genes of one trait do not influence the genes of another trait.

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Fig. 13-11-1 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I

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Fig. 13-11-2 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II

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Fig. 13-11-3 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1Combination 2Combination 3Combination 4

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Dr. Reginald Punnett In the early 1900s, he developed what is known as the Punnett square to predict the possible offspring of a cross between two known genotypes. Punnett squares can be used to determine possible genotypes and phenotypes of the cross. These can be represented as ratios: Genotypic ratio Phenotypic ratio

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Using a Punnett square 1.Create a box with 4 squares. 2.Identify the alleles for the trait/gene (T and t). 3.Identify the genotypes of the individuals being crossed. 4.Place the alleles for the genotypes in the appropriate places around the box. 5.Fill in the box by carrying the letter across and down.

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Single Factor Cross

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Two Factor Cross

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Complex Inheritance Patterns Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or genes. These types of inheritance patterns are called complex inheritance patters

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Incomplete Dominance Incomplete Dominance one allele is not completely dominant over another. In incomplete dominance, the heterozygous phenotype is somewhere in-between the two homozygous phenotypes.

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Codominance Codominance both alleles contribute to the phenotype. Example: AB blood type

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Multiple Alleles Multiple Alleles genes having more than two alleles. This does not mean that an individual can have more than two alleles, but it means that more than two possible alleles exist in a population for a given trait. Example: human blood type (A, B, AB, O)

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Multiple Alleles Human Blood Groups The ABO blood group has three alleles I A, I B, and i. Alleles I A and I B are codominant. These alleles produce molecules known as antigens on the surface of red blood cells. The i allele is recessive to both I A and I B and produces no antigen.

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Multiple Alleles Human Blood Groups

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Polygenic Traits Polygenic Traits controlled by two or more genes. Example: skin color of humanscontrolled by more than 4 different genes

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Applying Mendels Principles Mendels principles dont apply only to plants, but other organisms and humans too. In the early 1900s, Thomas Hunt Morgan found a model organism to advance the study of genetics: the common fruit fly. Fruit flies were an ideal organism for several reasons: They reproduced quickly and had many offspring Morgan and other biologists learned that Mendels principles applied not to just pea plants, but all lifesince DNA is universal and contains genetic information.

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Genetics and the Environment The characteristics or phenotypes of any organisms are not determined solely by the genes it inherits, but by the interaction between genes and the environment. Example: Genes may affect a sunflowers height and the color of its flowers, but these same characteristics are also influenced by climate, soil conditions, and availability of water.

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Karyotype Studies The study of genetic material doesnt involve genes alone. Scientists also study whole chromosomes by using images of chromosomes taken during mitosis. A stain is used to identify or mark identical places on homologous chromosomes. The pairs of homologous chromosomes are arranged in decreasing size to produce a diagram called a karyotype.

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Karyotype (male or female?)

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A pedigree is a diagram that traces the inheritance of a particular trait through several generations. A pedigree uses symbols to illustrate the inheritance: Males are represented by squares Females are represented by circles One who expresses a trait is dark or filled One who doesnt express the trait in unfilled One who is a carrier is half shaded (heterozygous)only done in recessive disorders Pedigree Charts

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A horizontal line between two symbols shows that these individuals are the parents of the offspring listed below them. Offspring are listed below them, oldest on the left to youngest on the right. A numbering system is used in which Roman numerals represent generations and Arabic numbers are used to describe birth order. Pedigree Charts

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Pedigree

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A pedigree shows an individuals phenotype You can analyze a pedigree to infer genotypes and whether the trait that is being inherited is a recessive or dominant genetic disorder Pedigrees are useful if good records have been kept within families. It allows genetic disorders in future offspring to be predicted. Analyzing Pedigrees

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View the pedigree below. Is this disorder dominant or recessive? Dominant or Recessive Disorder?

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View the pedigree below. Is this disorder dominant or recessive? Dominant or Recessive Disorder?

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Genetic Disorders: Recessive Disorders Many disorders seen in humans are caused by genetics. A recessive disorder is expressed when the individual is homozygous recessive for the trait. An individual that is heterozygous for a recessive disorder, and therefore doesnt express it, is called a carrier.

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Recessive Disorders

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Genetic Disorders: Dominant Disorders Not all disorders are caused by recessive inheritance. Some are cause by dominant alleles. Dominant disorders are not present in individuals that are homozygous recessive for the trait.

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Understanding Genetic Disorders through Genetics Counseling

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Meiosis and Nondisjuction Recall, that meiosis is the process used to form gametes (Diploid cell haploid cells) During meiosis I, homologous chromosomes are separated. During meiosis II, sister chromatids are separated. If homologous chromosomes or sister chromatids dont separate properly during meiosis, this is known as nondisjunction.

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Nondisjunction

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Downs Syndrome Trisomy 21 Characteristics of the disorder: Distinctive facial features Short stature Heart defects Mental disability

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Sex Determination Your gender is inherited based on your 23 rd pair of chromosomes, called sex chromosomes. 2 types: X and Y XX = female XY = male The other 22 pairs of chromosomes are called autosomes.

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Sex-Linked Inheritance Sex-Linked Genes genes located on the sex chromosomes are said to be sex-linked. Males have just one X chromosome, thus all X-linked alleles are expressed in males, even if they are recessive.

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Sex-Linked Inheritance