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Heredity Notes

Heredity Notes. Chapter 5. 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.

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Heredity Notes

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  1. Heredity Notes Chapter 5

  2. 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.

  3. 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. • Genes work together to make the physical appearance. The physical appearance that results is called the phenotype. • See the traits studied by Mendel on p. 128

  4. Alleles • Different forms a gene can have for a trait. • For example, the trait plant height has two alleles: tall and short. What do you think the alleles are for the trait seed shape? • Letters are used to represent the alleles • Purebred: same alleles for a trait • Hybrid: different alleles from each parent (hybrid= mix or combination) • round and wrinkled

  5. 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)

  6. Now you try… round R r wrinkled green G g yellow leaf junctions L tips of branches l  All traits have complete dominance.  Use Table 1 on page 128 to help you.

  7. Now try this… Flower colors: purple, white (Purple has complete dominance over white.) • Identify the trait. • Identify the alleles. • How is each allele represented? • Flower color • Purple and white • Purple=P and white=p

  8. 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

  9. Let’s try this… Seed colors: yellow, green (Yellow has complete dominance over green.) • Identify the trait. • Identify the alleles. • How is each represented? • What are the possible ways alleles can combine? • Seed color • Yellow and green • Yellow=Y, green=y • YY, yy, Yy

  10. 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.

  11. Now you try… R W B W  All traits have incomplete dominance.

  12. Now you try… Coat colors: black, white (Black and white share incomplete dominance.) • Identify the trait. • Identify the alleles. • How is each represented? • What are the possible ways alleles can combine? • Coat color • Black and white • Black=B and white=W • BB, WW, BW

  13. 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.)

  14. Traits with Complete Dominance tall tall short PP purple pp wrinkled RR round Rr

  15. Traits with Incomplete Dominance red pink WW BB BW white

  16. Think about it… How can two organisms with different genotypes have the same phenotype?

  17. 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

  18. Now you try… How is each genotype represented? TT Tt tt PP Rr rr • Homozygous tall • Heterozygous tall • Short • Homozygous purple • Heterozygous round • Wrinkled

  19. Punnett Squares Used to show all possible combinations of alleles and predict probability of possible outcomes of crossing two genotypes. • Perform the cross. • Analyze results. • How many are tall? • Short? • Homozygous? • Heterozygous? • This represents Mendel’s first experiment. • Look at the parent allele above and left of each blank in the square. • Write both alleles, putting the capital letters first. • You can also bring each letter down from the top and over from the left. 4 out of 4 T T t t T T T T T T 0 out of 4 • The alleles from one parent are written on the top of the square. • Alleles for the other parent are written on the side of the square. 0 out of 4 4 out of 4 t t t t t t

  20. Punnett Squares • Let’s try another one. This time let’s show Mendel’s second experiment. • Perform the cross. (Capital letters should be written before lower case.) • Analyze results. • How many are tall? • Short? • Homozygous? • Heterozygous? • This led to many more experiments by Mendel. T t T t T T T t t t • The alleles from one parent are written on the top of the square. • Alleles for the other parent are written on the side of the square. 3 out of 4 T T T 1 out of 4 2 out of 4 2 out of 4 t t t

  21. Punnett Squares • Remember, a Punnett square shows probability. • Results can be expressed as ratios, fractions, or percents. (We will use fractions & percents.) 1:3 ¼ purple 25% purple 2:2 ½ purple 50% purple 3:1 ¾ purple 75% purple

  22. Punnett Squares (Alleles: tall, short) • Try crossing a heterozygous tall plant with a short plant. • Identify the genotypes for each parent. Tt tt • ½ or 50% • ½ or 50% • ½ or 50% • ½ or 50% T t t t Tt tt • Set up and perform the cross. • Analyze the results: • What are the chances of tall? • Short? • Homozygous? • Heterozygous?

  23. Punnett Squares • Now cross homozygous round with heterozygous round. • Identify the genotypes for each parent. (Alleles: round, wrinkled) RR RR • 100% • 0% • ½ or 50% • ½ or 50% R R R r Rr Rr • Set up and perform the cross. • Analyze the results: • What are the chances of round? • Wrinkled? • Homozygous? • Heterozygous?

  24. Punnett Squares • Cross green seeds with heterozygous yellow. • Identify the genotypes for each parent. (Alleles: yellow, green) Yy Yy • ½ or 50% • ½ or 50% • ½ or 50% • ½ or 50% y y Y y yy yy • Set up and perform the cross. • Analyze the results: • What are the chances of Yellow? • Green? • Homozygous? • Heterozygous?

  25. 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.

  26. Incomplete Dominance • Let’s cross a red snapdragon with a white snapdragon. • Identify the genotypes for each parent. (Alleles: red, white) RW RW • 0% • 0% • 100% R R W W RW RW • Set up and perform the cross. • Analyze the results: • What are the chances of red? • White? • Pink?

  27. Incomplete Dominance • Now let’s cross a pink snapdragon with another pink. • Identify the genotypes for each parent. (Alleles: red, white) RR RW • ¼ or 25% • ¼ or 25% • ½ or 50% R W R W RW WW • Set up and perform the cross. • Analyze the results: • What are the chances of red? • White? • Pink?

  28. Incomplete Dominance • Finally, we’ll cross a black mouse with grey mouse. • Identify the genotypes for each parent. (Alleles: black, white) BB BB • ½ or 50% • 0% • ½ or 50% B B B W BW BW • Set up and perform the cross. • Analyze the results: • What are the chances of black? • White? • Grey?

  29. 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

  30. AA , AO Type B Type AB OO

  31. Predicting Blood Type • Try crossing a type AB with type O. • Identify the genotypes for each parent. (Alleles: A, B, O) AO BO • ½ or 50% • ½ or 50% • 0% • 0% A B O O AO BO • Set up and perform the cross. • Analyze the results: • What are the chances of type A? • Type B? • Type AB? • Type O?

  32. Predicting Blood Type • Now cross genotype AO with genotype BO. • Identify the PHENOTYPES for each parent. AB BO • ¼ or 25% • ¼ or 25% • ¼ or 25% • ¼ or 25% A O B O AO OO • Set up and perform the cross. • Analyze the results: • What are the chances of type A? • Type B? • Type AB? • Type O?

  33. 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. O A B O ? • In the square, you will need the genotype for type O blood. • This means that offspring would have to get one O allele from each parent. • Now think of the possible alleles to complete the second parent’s genotype. A O B ? OO O

  34. 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

  35. 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.

  36. 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 Y Y X X X X X X

  37. Sex-Linked Disorders • Caused by alleles inherited on sex chromosomes • Color-blindness: a recessive allele on the X chromosome • 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 XC

  38. XX , XY Normal Vision Carrier Colorblind

  39. Predicting Colorblindness • Predict the result of crossing a normal female with a colorblind male. • Identify the genotypes. XXC XXC X X XC Y • Set up and perform the cross. • Analyze the results: • What are the chances of a child who is colorblind? • What will be special about daughters these parents might have? XY XY 0% They will be carriers.

  40. Predicting Colorblindness • Now try crossing a carrier female with a male who has normal vision. • Identify the genotypes. XX XXC X XC X Y • Set up and perform the cross. • Analyze the results: • What are the chances of a child who is colorblind? • What are the chances of a daughter who is colorblind? • What are the chances of a child who does not have the gene at all? XY XCY 25% 0% 50%

  41. 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.

  42. Pedigrees • A pedigree is visual tool for following a trait through generations of a family; it is similar to a family tree.

  43. Common Pedigree Symbols

  44.  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?

  45. 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.)

  46. Selective Breeding • Breeders of animals and plants are looking to produce organisms that will possess desirable characteristics. - high crop yields - resistance to disease - high growth rate - 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.

  47. How does selective breeding work?

  48. 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.

  49. 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.)

  50. 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.

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