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Introduction to Genetics Chapter 11 (M) Genetics and Inheritance Genetics the study of heredity that deals with the transmission of traits or characteristics from one generation to another Inheritance The reception of traits by transmission from parent to offspring Prehistoric Times Little is known when humans first recognized the importance of genetics The breeding of horses, cattle, and various breeds of dogs began around 8000 B.C. and 1000 B.C. Plants such as corn, wheat, and rice was cultivated in Mexico around 5000 B.C. The History Of Genetics Pythagoras 500 B.C., a Greek philosopher, stated that human life began with male and female fluids The History Of Genetics Aristotle furthered this idea and suggested that these fluids, or “semens,” were actually purified blood—therefore, blood must be part of heredity. One Bizarre Theory •The theory of Homunculus -17th century •sex cells contained a complete miniature adult, perfect in form •This statement was popular way into the 18th century Small individual Blending Hypothesis 1800’s This stated that both the genetic material from the mother and father were blended in order to produce an offspring Parents Red Flower X Yellow Flower Offspring Orange (offspring all orange) Exceptions Red yellow ,etc Theory discarded, If blending occurred all extreme characteristics would disappear from the population Gregor Mendel-1800’s •Considered to be the “father of genetics •Austrian monk, teacher and mathematician •Expt. approach to genetics •Particulate Hypothesis Particulate Hypothesis Parents pass on to their offspring separate and distinct factors (genes) that are responsible for inherited traits Used pea plants to study this Why peas? Rapid reproduction rate Presence of distinctive traits Closed structure of flowers (each pea plant has male (stamens) and female (carpal) sexual allows selffertilization organs) Cross-fertilization One plant is fertilized by another Terminology P Generation parents F1 Generation first filial F2 Generation second filial PXP F1 F1 F1 F2 Allele Alternative forms of a gene which determines a trait. Alleles cont. Uppercase (Capital) letters for dominant traits Lowercase letters for recessive traits Ex: Tall = T short = t, expressed in pairs TT, Tt, tt Phenotype Physical appearance Genotype Genetic makeup Dominant trait that is easily observed Recessive trait that is often masked Homozygous2 alleles for a trait are identical TT or tt Heterozygous – 2 alleles for a trait are not identical Tt Inheritance Follows Rules of Chance Mendel began experiments to track the inheritance of characters in pea plants Results led him to formulate several hypotheses Seven Traits Studied by Mendel Terminology P Generation parents F1 Generation first filial F2 Generation second filial PXP F1 F1 F1 F2 Mendel’s Experiment Crossed pure purple and a pure white flower (P generation) =F1 generation All F1 plants (purple) are crossed by self pollination = F2 generation yields ¾ purple and ¼ yellow Mendel’s Hypotheses 1. 2. 3. 4. There are alternate forms of genes For each character an organism has two alleles Alleles are either dominant or recessive Alleles segregate during formation of gametes Law of Dominance When organisms pure for contrasting traits are crossed, all their offspring will show the dominant trait Probability Fractions or ratios that will predict that an event will occur Punnett Square Diagram which shows the possible outcome of a cross Monohybrid Cross Using a single trait – crossing a pure bred Tall (TT) with a pure bred short (tt) plant Mating 2 heterozygous black (Bb) rabbits Test Cross • Individual with dominant phenotype not possible to predict the genotype run a test cross with individual with recessive phenotype to determine the allele Dominant Phenotype purple flower (genotype PP or Pp) Recessive Phenotype white flower ( genotype pp) Test Cross Law of Independent Assortment Each pair of alleles segregates into gametes independently Independent Assortment Mendel followed the inheritance of two different characters ( dihybrid cross) The allele for yellow seeds (Y) is dominant to the allele for green seeds (y). The allele for round seeds (R) is dominant to the allele for wrinkled seeds (r). He crossed true-breeding plants that had yellow, round seeds (YYRR) with truebreeding plants that had green, wrinkled seeds (yyrr). Dihybrid Cross Crosses Involving Two Traits: Color: Yellow, Green Shape: Round, Wrinkled Yellow-Round Yellow-Wrinkled Green-Round Green-Wrinkled The Y and R alleles and y and r alleles stay together F1 offspring would produce yellow, round seeds. The F2 offspring would produce two phenotypes in a 3:1 ratio, just like a monohybrid cross. If the two pairs of alleles segregate independently of each other Four classes of gametes (YR, Yr, yR, and yr) would be produced in equal amounts. These combinations produce four distinct phenotypes in a 9:3:3:1 ratio Practice problems Cross a homozygous yellow, homozygous round plant with a green, wrinkled plant Cross a homozygous yellow, homozygous round plant with a heterozygous yellow, wrinkled plant Other Patterns of Inheritance 11.3 Variation in Inheritance Incomplete Dominance Codominance (multiple alleles) Polygenic inheritance Incomplete Dominance Both alleles contribute to a phenotype of a heterozygous individuals to produce a trait not like either parent. Phenotype intermediate between two pure traits Ex: Snapdragons, Andalusian chick. Snapdragon Flower Color Alleles often written as capital letters with superscripts ex. CRCR (red) x CWCW (white) Incomplete Dominance revealed in Heterozygous Individual A cross between a white-flowered plant and a red-flowered plant will produce all pink F1 offspring Self-pollination of the F1 offspring produces 25% white, 25% red, and 50% pink offspring. Codominance (multiple alleles) Two dominant alleles are expressed at the same time Ex: Roan coat color in horses C R C R , C WC W, C R C W R R C C W W C C R W C C (Red) (White) (Roan) If a roan cow (RW) is mated to a roan bull (RW), what are the phenotypes of the offspring? Multiple Alleles Most genes have more than two alleles in a population The ABO blood groups in humans are determined by three alleles, IA, IB, and i. Both the IA and IB alleles are dominant to the i allele The IA and IB alleles are codominant to each other Because each individual carries two alleles, there are six possible genotypes and four possible blood types. ABO Blood Group System Type A Type B Type AB Type O IA IA or IB IB or IA IB ii IA i IB i Problems A man homozygous for type A blood marries a woman who is heterozygous for type B blood. What are the possible genotypes and phenotypes of their children? A couple has a child with type O blood. If one parent is type O, what are the possible genotypes of the other parent? Polygenic inheritance When two or more genes effect a single characteristic For example, skin color in humans is controlled by at least three different genes. Imagine that each gene has two alleles, one light and one dark, that demonstrate incomplete dominance. An AABBCC individual is dark and aabbcc is light A cross between two AaBbCc individuals (intermediate skin shade) would produce offspring covering a wide range of shades The range of phenotypes forms a normal distribution. Importance of Environment In some cases environment plays an important part in the expression of genes Ex: Temperature (Siameses cat fur), Nutrition (height & growth of individual) However human blood type is not influenced by environment. Influence of Environment Meiosis 11.4 Meiosis A type of cell division that produces four cells Each cell hash half the number of chromosomes as the parent cell. In animals, meiosis occurs in the sex organs—the testes in males and the ovaries in females Meiosis Each species has its own number of chromosomes Humans 46 or 23 pairs Karyotype display of these 46 chromosomes Homologus Chromosomes chromosomes making up the pair that carry the genes for the same trait Homologus Chromosomes Two general types Autosomes 22 pairs found both in male & female Sex Chromosomes carry the gene that determines sex indicated by “X” or “Y” Sex Chromosomes Female 22 Autosomes + 1 Sex chromosome, XX 1 from father , 1 from mother Eggs 1X + 22 Autosomes Male XY X from mother, Y from Father Sperm half have 1X+ 22 autosomes, other half have 1Y+22 autosomes Diploid and Haploid Cells Diploid Cells Cells with 2 sets of chromosomes total number of chromosomes diploid # or 2n Humans 2n=46 in somatic cells Haploid Cells 1n =23 chromosomes in sex cells, sperm & egg (Gametes) Human Life cycle Meiosis Process by which haploid gametes are formed Alternation of meiosis and fertilization keeps the # of chromosomes same from generation to generation. Mitosis 2 offspring w/same # of chromosomes Meiosis Gametes are formed egg & sperm 4 offspring with ½ the # of chromosomes Exchange of genetic material crossing over Oogenesis production of egg 1 egg + 3 smaller polar bodies Spermatogenesis production of sperm 4 sperm cells Two Distinct parts of Meiosis Meiosis I Meiosis II Assortment of Chromosomes Crossing Over/ Genetic Recombination Comparison of Mitosis & Meiosis 1. By three events in Meiosis l Synapsis and crossing over 2. Separation of homologues 3. Homologous chromosomes physically connect and exchange genetic information At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell In anaphase II of meiosis, the sister chromatids separate Tetrads on the metaphase plate At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates Comparison of Mitosis & Meiosis