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CHAPTER 10: Introduction to Genetics This is the study of how traits or characteristics are passed from parent to offspring 1 2 Mendel: •Was interested in how organisms passed traits (characteristics) from parent to offspring •Decided to work with garden peas because of several characteristics •Pollination (fertilization, sexual reproduction) could easily be controlled •Peas produce lots of offspring (seeds) •A generation only takes 1 year to produce •Peas have many traits with only 2 phenotypes (appearance) and no intermediate phenotypes 3 He chose 7 traits to study 4 His first step was to develop true breeding lines for each of the traits he studied: •There were 14 true breeding lines in all •This took several years •True breeding means that if 2 true breeding individuals are crossed (mated) that all the offspring will have the parents trait…in other words – if 2 individuals with yellow seeds are crossed, all the offspring will have yellow seeds 5 His next step was to artificially cross individuals from 2 true breeding lines with opposite traits (ex: yellow seeds x green seeds), producing a hybrid line: •Grow the true breeding plants •Cross individuals with opposite traits by hand •Harvest seeds •Plant seeds the following spring •Grow hybrid plants (F1 hybrids) •Observe the traits of the offspring FOR ALL 7 traits the F1 individuals all showed only one of the parental traits ( ex: for a yellow seed x green seed cross, all the F1 seeds were yellow) 6 The chart below show Mendel’s results for all 7 traits he studied. 7 Mendel called the trait that appeared in the F1 generation dominant, and the trait that was not visible recessive. His next question: had the recessive trait disappeared, or was it masked by the dominant trait? 8 PART 1 QUESTIONS: 1. Why did Mendel choose to work with peas? 2. What is a true breeding line of a species? 3. What is a hybrid line? 4. What did Mendel call the trait that appeared in the F1 generation? 5. What did Mendel call the trait that was not visible in the F1 generation? 9 To find the answer, he allowed the plants of the F1 generation to self pollinate: •Saved the seeds of the F1 generation •Planted the following spring •Observed the characteristics of the F2 plants •Found the about ¼ showed the recessive trait 10 PART 2 QUESTIONS: 1.What did Mendel observe about the recessive trait in the F2 generation? 11 MENDEL CONCLUDED THAT: •The pea plants contained 2 copies of a ‘factor’ that controlled each trait •Factors occurred in different varieties (ex: tall and short) •A parent gave one copy of its 2 factors to its offspring •If plants contained 1 or 2 copies of the dominant factor, the plant had the dominant trait •2 copies of the recessive factor were needed to produce the recessive trait •The probability that a parent will provide each factor is 50% 12 How do Mendel’s results from the 19th century (1800’s) fit with what was discovered about DNA in the 20th century (1900’s)? •Mendel’s traits are determined by genes which are located on chromosomes •Most genes have 2 or more alleles, variations of genes that produce variations of a trait (FOR EXAMPLE: an allele for black fur and an allele for brown fur in mice – both affect fur color, but produce different colors) •Most organisms have 2 copies of each chromosome (one from the mother and the other from the father). Organisms with 2 copies of each chromosome are called diploid. Egg and sperm cells have 1 copy of each chromosome and are called haploid •These chromosomes have the same genes, in the same order, but may have different alleles for a gene. They are called homologous. 13 The human genome has 46 chromosomes: •22 pairs of autosomes, each member of a pair contains the same genes •1 pair of sex chromosomes (XX = female, XY = male 14 15 These images for all the human chromosomes can be found at: https://public.ornl.gov/site/gallery/gallery.cfm?topic=47&restsection=HGPArchive 16 PART 3 QUESTIONS: 1. What is the modern term used for what Mendel called factors? 2. What is the relationship between the terms gene and allele? 3. How many copies of each chromosome do most sexually reproducing organisms have? 4. What vocabulary term describes the above condition? 5. Describe the arrangement of genes on homologous chromosomes. 17 The cellular basis of reproduction: •A new individual is formed when an egg and sperm fuse •Each egg or sperm contains 1 copy of each chromosome and are described as haploid •Eggs and sperm are produced by a special type of cell division called meiosis, which cuts the number of chromosomes in half (Haploid) •Special sequence of steps •Assures each egg or sperm has one copy of each chromosome, although which copy ends up in which gamete is random (independent assortment) •After egg and sperm fuse, the new organism has one copy of each chromosome from each parent, for a total of 2 copies of each chromosome 18 MEIOSIS: •There are 2 cell divisions •The first provides each new cell with 1 duplicated chromosome (2 sister chromatids attached by a centromere) •Duringthe2nd division, the centromere disappears and each cell ends up with one copy of each chromosome •After meiosis has ended, there are 4 cells, each with one copy of each chromosome (these cells are called haploid) 19 There are differences in meiosis between males and females •Male meiosis produces 4 equal sperm cells •Female meiosis produces 1 egg cell and 3 polar bodies •Most of the contents of the cell (cytoplasm and organelles) end up in the egg •The polar bodies remove the extra DNA 20 During meiosis, homologous chromosome exchange pieces of DNA. This process is called crossing over The farther apart 2 genes are on a chromosome, the more likely they will be separated during meiosis by crossing over Genes close together usually don’t separate and are called linked genes 21 PART 4 QUESTIONS: 1. How many copies of a homologous chromosome does an egg or sperm contain? 2. What is the type of cell division that produces eggs and sperm called? 3. How does the process named above affect the number of chromosomes in a cell? 4. What is crossing over and when does it occur during meiosis? 5. Describe how meiosis is different for males and females. 22 Biologists often want to determine the probable outcome of a cross between 2 individuals This is done by modeling the cross using a Punnett square First – some vocabulary: •Individuals with 2 identical alleles for a trait are called homozygous •Individuals with 2 different alleles for a trait are called heterozygous •Dominant alleles are denoted with capital letters •Recessive alleles are denoted with lowercase letters FOR EXAMPLE: a pea plant heterozygous for height would be identified as height Tt, this is defined as the plant’s genotype for 23 MOST GENES SEPARATE INDEPENDENTLY DURING MEIOSIS: • the probability of a particular homologous chromosome ending up in a specific gamete is 0.5 •Therefore the probability of an allele ending up in a specific gamete is 0.5 •The probability of 2 different homologous chromosomes ending up in the same gamete is equal to the product of the probability of the 2 events occurring independently •The number of genetically different gametes an organism can produce can be determined by 2n, where n = the number of homologous chromosomes. 24 PART 5 QUESTIONS: 1. Use the vocabulary word allele to define homozygous. 2. Use the vocabulary word allele to define heterozygous. 3. What do you know about an individual cow with the genotype Rr, when ‘R’ denotes an allele for a red coat and ‘r’ denotes an allele for a white coat. 25 The genetic contribution of each parent is determined by meiosis The allele that ends up in an egg or a sperm is random Heterozygotes will produce 50% of the eggs or sperm with each allele FOR EXAMPLE: A pea plant heterozygous for height (Tt) will produce 50% T alleles and 50% t alleles Cross 2 pea plants heterozygous for height: Tt x Tt Genetic contribution of 1 parent (gametes) Genetic contribution of 1 parent (gametes) T t T TT Tt t Tt tt The offspring from this cross will be: •75% tall (25% homozygous dominant (TT) and 50% heterozygous (Tt) •25% short (homozygous recessive (tt) 26 STEPS TO WORKING GENETICS PROBLEMS: 1. Determine the genotypes of the parents 2. Determine the alleles in the gametes the parents will produce 3. Write the gametes at the top and down the side of the Punnett square 4. Fill in the boxes for the progeny 5. Determine the genotypic ratio of the progeny 6. Determine the phenotypic ratio of the progeny This information can be used to answer questions about a genetic cross 27 GENETICS PROBLEM EXAMPLE: In mice black fur (B) is dominant over brown fur (b). A female brown mouse is mated with a heterozygous black male mouse. Show the results of the cross. Genotype Phenotype Alleles Mother and Father and 1. How many genotypes will be present in the progeny? 2. How many phenotypes will be present in the progeny? 3. What fraction of the offspring will have each genotype and phenotype? 28 Incomplete Dominance •Heterozygotes look different from a homozygotes •Blending of phenotypes RR x RIRI RRI http://chsweb.lr.k12.nj.us/psidelsky/Chapter10(Gen).htm Codominance: Both alleles of a gene pair are expressed. Example: Human Blood Types • Human blood type is determined by codominant alleles. There are three different alleles: IA, IB, and i. The IA and IB alleles are codominant, and the i allele is recessive. • The possible human phenotypes for blood group are type A, type B, type AB, and type O. • Type A and B individuals can be either homozygous (IAIA or IBIB, respectively), or heterozygous (IAi or IBi, respectively). Type O is IOIO. ;) S-B-8-1_Non-Mendelian Heredity PowerPoint Codominance: More Examples Rhododendron coloration http://upload.wikimedia.org/wikipedia/commons/f/f3/Codominance_Rhododendron.jpg Roan coloration in cows (red coat with white blotches) http://en.wikipedia.org/wiki/File:Light_Roan_Shorthorn_Heifer_DSCN1872b.j pg S-B-8-1_Non-Mendelian Heredity PowerPoint 1. Distinguish between incomplete dominance and codominance. 2. How many phenotypes are seen when codominance is possible? 3. Red color (R) in snapdragons shows incomplete dominance to white snapdragons (RI). The heterozygotes are pink. If pink snap dragons are crossed with a white snapdragon, what are the genotypic/ phenotypic ratios? Multiple Alleles • When 4 or more possible phenotypes exist within a population – Blood type – Eye & hair color – Fur in mammals http://www.learner.org/interactives/dna/genetics8.html Polygenic Inheritance http://www.nature.com/nrg/journal/v10/n12/box/nrg2670_BX1.html Poly-many genic-gene • More than one gene on more than one loci or on more than one chromosome. • Give the appearance of gradation – Hair & skin color – Height – Body size http://scienceblogs.com/geneticfuture/a_ruler_beats_genetics_in_pred/tall_short1.jpg EPISTASIS When a gene alters the expression of a different gene. EXAMPLE: albanism (albino) •Albino individuals have a normal homozygous or heterozygous genotype for pigmentation. •A recessive allele for a different gene prevents the formation of the pigment melanin Sex-Linked Traits • Genes located on a sex (X or Y) chromosome ♀XX & ♂XY – Some forms of hemophilia – Red-green colorblindness http://www.ccs.k12.in.us/chsteachers/Amayhew/Biology%20Notes/sexlinked%20notes.htm INDEPENDENT ASSORTMENT: genes on different chromosomes and genes far away on the same chromosome are inherited independently. This happens during meiosis. 37 38 Independent Assortment produces a wide variety of gametes. There is more than one way to determine the gametes produced by an individual during meiosis. Consider an individual with the genotype AaBb Technique 1: AaBb GAMETES: AB aB Ab ab 39