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Heredity Notes Chapter 10 Heredity & Genetics Heredity: passing of traits from parent to offspring Genetics: STUDY of heredity – Gregor Mendel: “Father of Genetics” Austrian monk who was first to trace a trait passing through generations. He was first to use probability in plant science. Mendel’s work was forgotten for many years, but when more scientists came across his work in their research and came to the same conclusions, he became known as the father of genetics. Traits Characteristics of an organism – Hair color, flower color, seed shape, etc. – Controlled by genes (sections of chromosomes) Each chromosome will have a gene for each trait. (A few exceptions.) Because chromosomes are in pairs, genes for traits are in pairs. The type of genes an organism has for a trait is called the genotype. To make the physical appearance, genes work together. The physical appearance that results is called the phenotype. See the traits studied by Mendel on p. 368-371 Alleles Different forms a gene can have for a trait. For example, the trait plant height has two alleles: tall and short. Look on pg. 371: What do you think the alleles are for the trait seed shape? round and wrinkled Letters are used to represent the alleles Purebred: same alleles for a trait Hybrid: different alleles from each parent (hybrid= mix or combination) Complete Dominance Complete dominance: one form of a gene can completely “cover up” the other form – Form that is seen = dominant – Form not seen = recessive – Example: In pea plants, purple flower color is completely dominant over white, so when both alleles are present, the flower color will be purple. Representing alleles in complete dominance: – ONE LETTER is used to represent both forms of a trait Dominant form determines letter Dominant form uses the capital letter Seed Shape, R=Round (round is dominant) Plant Height, T=Tall (tall is dominant) Recessive form gets the same letter, but lowercase Seed Shape, r=wrinkled (round is dominant) Plant Height, t=short (tall is dominant) Now you try… Trait Alleles Representation Shape of Seeds round R r wrinkled green Color of Pods yellow Position of Flowers Side of stem tips of stem G g S s All traits have complete dominance. Use Table 1 on page 371 to help you. Now try this… Flower colors: purple, white (Purple has complete dominance over white.) Flower Identify color the trait. Purple Identifyand the white alleles. Purple=P andallele white=p How is each represented? Combining Alleles Because chromosomes are in pairs, organisms will have pairs of alleles. When there are two different alleles, there are three ways those alleles can combine. – Two dominant alleles – Two recessive alleles – One dominant and one recessive For example, the two alleles for flower color are purple (P) and white (p). The possible combinations are: – PP, pp, and Pp (always write the capital letter first) What are the possible ways that the alleles for seed shape can combine? (R=round, r=wrinkled) – RR, rr, Rr Let’s try this… Seed colors: yellow, green (Yellow has complete dominance over green.) Identify the trait. Seed color Identifyand the green alleles. Yellow How is each represented? Yellow=Y, green=y What YY, yy,are Yythe possible ways alleles can combine? Incomplete Dominance One allele doesn’t completely cover another, so both forms of the gene show at the same time. – Example: In snapdragons, red and white flower color share incomplete dominance, so when both alleles are present, the flower color will be pink. Representing alleles in incomplete dominance – Each allele uses its own letter, and they are all capital Remember, when there are two different alleles, there are three ways those alleles can combine. Now you try… Trait Alleles Flower color red R white W brown B W Fur color white Representation All traits have incomplete dominance. Now you try… Coat colors: black, white (Black and white share incomplete dominance.) Identify the trait. Coat color Identify thewhite alleles. Black and How is each Black=B and represented? white=W What are BW the possible ways BB, WW, alleles can combine? Genotype & Phenotype Genotype: genetic make-up an organism has for a particular trait – THINK: type of gene=genotype – Represented with a pair of letters because genes for traits are in pairs. (TT, Tt, tt, etc.) Phenotype: physical appearance resulting from the forms of the genes an organism has – THINK: physical appearance=phenotype (tall, short, etc.) Traits with Complete Dominance Trait Plant Height Alleles: tall, short Flower Color Alleles: purple, white Seed Shape Alleles: round, wrinkled Genotype Phenotype TT Tt tt PP Pp pp tall tall short rr RR Rr purple purple white wrinkled round round Traits with Incomplete Dominance Trait Flower Color Alleles: red, white Fur Color Alleles: black, white Genotype Phenotype RR RW WW red pink BB BW WW white black gray white Think about it… How can two organisms with different genotypes have the same phenotype? Genotype Phenotype TT Tt tt tall tall short Homozygous & Heterozygous Genotypes are represented with letters. Those letters can be matched or unmatched. Homozygous: a genotype with alleles that are the same. –TT, tt, PP, pp, RR, rr Heterozygous: a genotype with alleles that are are different. –Tt, Pp, Rr, RW Now you try… How is each genotype represented? TT 1. Homozygous tall Tt 2. Heterozygous tall tt 3. Short PP 4. Homozygous purple Rr 5. Heterozygous round rr 6. Wrinkled Punnett Squares Used to show all possible combinations of Performand the cross. alleles predict probability of possible Analyze results. Look You can at the alsoparent bring allele each two genotypes. outcomes of crossing 4top out of 4 – Howdown many areof tall? letter above and left from the each T T tt – Short? 0 out of 4 left. blank and over in the from square. the The alleles from one – Homozygous? 0 out of 4 Write both alleles, putting parent are written onof 4 – Heterozygous? 4 out the first. t the capital top of letters the square. This represents Alleles the other Mendel’sfor first experiment. parent are written on the side of the square. t T T Punnett Squares Let’s try another one. This time let’s Perform the cross. show Mendel’s second (Capital letters should be experiment. written before lower case.) Tt Tt Analyze results. The alleles from one out of 4 – How many tall? 3 parent are are written on – Short? 1 out of 4 the top of the square. T – Homozygous? 2 out of 4 Alleles for the other – Heterozygous? 2 out of 4 parent are written on t This led to many more the side of the square. experiments by Mendel. T t Punnett Squares Remember, a Punnett square shows probability. Results can be expressed as ratios, fractions, or percents. (We will use fractions & percents.) RATIO FRACTION PERCENT (purple to white) 1:3 ¼ purple 25% purple 2:2 ½ purple 50% purple 3:1 ¾ purple 75% purple Punnett Squares Try crossing a heterozygous tall plant with a short plant. Identify the genotypes for each parent. Tt Tt tt Tt tt tt – ½up or 50% Set and perform the cross. – ½ or 50% Analyze the results: – – – – (Alleles: tall, short) ½ or 50% What are the chances of tall? ½ or 50% Short? – Homozygous? – Heterozygous? Punnett Squares Now cross homozygous round with heterozygous round. Identify the genotypes for each parent. RR (Alleles: round, wrinkled) RR RR Rr Rr Rr Set up and perform the cross. – 100% – 0% Analyze the results: –½ or 50% What are the chances of round? –½ or 50% Wrinkled? – Homozygous? – Heterozygous? Punnett Squares Cross green seeds with heterozygous yellow. Identify the genotypes for each parent. yy (Alleles: yellow, green) Yy Yy yy yy Yy Set – ½up or and 50% perform the cross. – ½ or 50% Analyze the results: –½ or 50% What are the chances of Yellow? –½ or 50% Green? – Homozygous? – Heterozygous? Incomplete Dominance No allele completely dominates over another, so both alleles represented with CAPITAL LETTERS. (Letters are usually written in alphabetical order.) Flower color: – 2 alleles: Red (R), White (W) – Since both forms can show simultaneously, the heterozygous genotype (RW) would have a pink phenotype. Incomplete Dominance Let’s cross a red snapdragon with a white snapdragon. Identify the genotypes for each parent. RR RW RW RW RW WW Set up and perform the cross. – 0% – 0% Analyze the results: – – – (Alleles: red, white) 100% What are the chances of red? White? Pink? Incomplete Dominance Now let’s cross a pink snapdragon with another pink. Identify the genotypes for each parent. RW RR RW RW WW RW Set and perform the cross. – ¼up or 25% – ¼ or 25% Analyze the results: – – – (Alleles: red, white) ½ or 50% What are the chances of red? White? Pink? Incomplete Dominance Finally, we’ll cross a black mouse with grey mouse. Identify the genotypes for each parent. BB BB BB BW BW BW Set and perform the cross. – ½up or 50% – 0% Analyze the results: – – – (Alleles: black, white) ½ or 50% What are the chances of black? White? Grey? Codominance When both alleles for a gene are expressed equally, codominance occurs. – Example: a white rooster and black hen cross to form offspring that have feathers that are black and white (look spotted) Codominance We represent codominance by using the capital letter F for the trait feathers and a superscript B or W to tell you the color. – FB – feather black – FW – feather white Codominance Now let’s cross a white rooster with a black hen. Identify the genotypes for each parent. W W F F B F (Alleles: feather black, feathers white) FBFW FBFW FB 0/4and or 0% Set– up perform the cross. – 0/4 orthe 0%results: Analyze – 4/4are or 100% – What the chances of black feathers? – White feathers? – Black & White feathers FBFW FBFW Codominance Now let’s cross a black and white rooster with a black hen. Identify the genotypes for each parent. FB FW FB FB 2/4and or 50% Set– up perform the cross. – 0/4 orthe 0%results: Analyze – 2/4are or 50% – What the chances of black feathers? – White feathers? – Black & White feathers (Alleles: feather black & white, feathers black) FBFB FBFW FBFB FBFW Codominance Now let’s cross a black and white rooster with a black and white hen. Identify the genotypes for each parent. FB FW FB FW 1/4and or 25% Set– up perform the cross. – 1/4 orthe 25% Analyze results: – 2/4are or 50% – What the chances of black feathers? – White feathers? – Black & White feathers (Alleles: feather black & white, feathers black) FBFB FBFW FBFW FWFW Multiple Alleles Traits can be controlled by more than two alleles. This results in more possible phenotypes. There are multiple alleles for human blood type. 3 alleles: A, B, O Complete the list of possible combinations. – AA, AB, AO, BB, BO, OO O is recessive to A and B A and B can show simultaneously (at same time) This results in 4 possible phenotypes: A, B, AB, and O blood types Genotype(s) Phenotype AA, AO Type A BB, BO Type B AB Type AB OO Type O Predicting Blood Type Try crossing a type AB with type O. Identify the genotypes for each parent. AB (Alleles: A, B, O) AO BO AO BO OO Set and perform the cross. – ½up or 50% – ½ or 50% Analyze the results: – 0% What are the chances of type A? – 0% Type B? – Type AB? – Type O? Predicting Blood Type Now cross genotype AO with genotype BO. Identify the PHENOTYPES for each parent. AO AB BO AO OO BO Set and perform the cross. – ¼up or 25% – ¼ or 25% Analyze the results: –¼ or 25% What are the chances of type A? –¼ or 25% Type B? – Type AB? – Type O? Working Backwards You can use a Punnett square to help answer questions by working backwards. Try this: If a parent has type A blood, could he have offspring with type O blood? Explain. In the square, you will need the genotype for type O blood. This means that offspring O A B ? would have to get one O allele from each parent. Now think of the possible O alleles to complete the second parent’s genotype. O A B ? O OO Polygenic Inheritance Traits can be produced by the combination of many genes—they act together to produce a trait. –Produces wide variety of phenotypes Human hair color, eye color, skin color, height Milk production in cows Wheat grain color Mutations & Genetic Disorders A mutation is any permanent change in the DNA of a cell’s gene or chromosome. This can result in a change in the way a trait is expressed. – Can be caused by outside factors like X-rays, sunlight, and some chemicals. – Can also result from an error in DNA replication (copying). Not all mutations are harmful; they can even be helpful. Mutations allow variety within species. Mutations can be passed to offspring only if mutation is copied to a sperm cell or egg cell. Just like any other trait, genetic disorders can be passed down. Some disorders, like cystic fibrosis, are caused by recessive genes. Sex Determination One pair of chromosomes determine sex (XX in females, XY in males) Females always contribute an X egg Males can contribute an X-containing sperm or a Y-containing sperm X X X Y Sex-Linked Disorders Caused by alleles inherited on sex chromosomes Color-blindness: a recessive allele on the X chromosome XC – Females that have the gene on one chromosome are not colorblind. The normal allele is dominant over the colorblindness allele. They are “carriers.” – Females have two X chromosomes, so they are colorblind only when trait is on both chromosomes. – Males have only one X, so they are colorblind when the trait is on that chromosome Genotype(s) Phenotype XX , XY Normal Vision XXC Carrier XCXC, XCY Colorblind Predicting Colorblindness Predict the result of crossing a normal female with a colorblind male. Identify the genotypes. XX XC Y Set up and perform the cross. 0% Analyze the results: XXC XXC XY – What are the chances of a child who They will be carriers. is colorblind? – What will be special about daughters these parents might have? XY Predicting Colorblindness Now try crossing a carrier female with a male who has normal vision. Identify the genotypes. X XC XX XXC XY XCY XY Set up and perform the cross. 25% Analyze the results: – What are the chances of a child who 0% is colorblind? – What are the chances of a daughter who is colorblind? 50% – What are the chances of a child who Genetics in Humans Some situations do not provide the opportunity to perform controlled crosses, such as when studying human genetics. In these situations, we have to analyze existing populations. Scientists have devised an approach called pedigree analysis to study the inheritance of genes in humans. Pedigree analysis is also useful when studying a population when data from several generations is limited or when studying species with a long generation time. Pedigrees A pedigree is visual tool for following a trait through generations of a family; it is similar to a family tree. Common Pedigree Symbols Use the pedigree to help you complete the following. Why are some shapes filled in and others not? Why are some of the females carriers while others are not? Why is a pedigree useful? Creating a Pedigree Using the symbols, create a pedigree that represents your family, including your parents and your siblings. (If you’re up for a challenge, try including your parents’ siblings and your grandparents.) Selective Breeding Breeders of animals and plants are looking to produce organisms that will possess desirable characteristics. - high crop yields - high growth rate - resistance to disease - many other characteristics To accomplish this, the organisms with desirable characteristics are chosen for breeding. Over time, the desirable characteristics become more common in the population. This intentional breeding for certain traits (or combinations of traits) over others is called selective breeding or artificial selection. How does selective breeding work? Examples of Selective Breeding Wheat has been selectively bred for higher yields, shorter stems to reduce wind damage and greater resistance to diseases. Turkeys with the desired characteristics (large breast muscles) are bred, passing along their genes to their offspring. Bananas have been selectively bred to be sweet and seedless. Examples of Selective Breeding Selecting for different traits over hundreds of years of breeding has caused different dog breeds to have distinctive characteristics although all the different breeds belong to the same species. Top row- Alaskan Malamute, Basset Hound, Llasa Apsa; Middle row- Beagle puppy, Shar Pei, Chow Bottom row- Pekinese, Tibetan Terrier, Pug.) Examples of Selective Breeding English shorthorn cattle, which provided for good beef, but lacked heat resistance, were crossed with Brahman cattle from India, which were highly resistant to heat and humidity. This produced the Santa Gertrudis breed of cattle, which has both of these characteristics. English Shorthorn: Good beef, no heat resistance Brahman: Poor beef, good heat resistance. Santa Gartrudis: Good beef, good heat resistance. Advances in Genetics Genetic Engineering: Biological or chemical methods can be used to change an organism’s genes. This only works because there is one language of life: DNA from one organism will work in others. – Recombinant DNA methods insert useful segments of DNA into the DNA of another organism. First used insert DNA into bacteria that caused them to make insulin. Genetically modified (GM) plants: Flavr Savr Tomato, antifreeze potatoes There is significant controversy surrounding the use of genetic modification. The possible benefits are limitless, but no one can predict possible consequences. Gene therapy can be used to treat diseases, including hereditary diseases. A normal allele is placed into a virus and the virus acts to replace defective hereditary material.