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Chapter 5

Chapter 5. Heredity. Gregor Mendel. The father of genetics and heredity Famous for his pea experiments Was a monk in a monastery when he did his ground breaking work. Died in 1884.

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Chapter 5

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  1. Chapter 5 Heredity

  2. Gregor Mendel • The father of genetics and heredity • Famous for his pea experiments • Was a monk in a monastery when he did his ground breaking work. • Died in 1884.

  3. The genetic experiments Mendel did with pea plants took him eight years (1856-1863) and he published his results in 1865. During this time, Mendel grew over 10,000 pea plants, keeping track of progeny number and type. Mendel's work and his Laws of Inheritance were not appreciated in his time. It wasn't until 1900, after the rediscovery of his Laws, that his experimental results were understood. • Why did it take so long to recognize his work?

  4. Are You Unique? • In the past ten thousand years, there have billions of people who have lived. • Have they all been different? Yes – But Why?

  5. In December 1999, the first human chromosome was completely sequenced. Chromosome #22 is one of the smallest human chromosomes and has 33.5 million base pairs of DNA.

  6. http://www.dnaftb.org/dnaftb/1/concept/ Concept Animation Audio video clip 2 Problem

  7. http://www.dnaftb.org/dnaftb/15/concept/ • animation

  8. http://www.dnai.org/a/index.html • Copying the code • Reading the code • Controlling the code

  9. Getting back to Mendel • Mendelian Genetics • Mendel was interested in the way traits were passed from parents to offspring. = HEREDITY • Mendel simplified his investigation by studying only one organism (the garden pea plant)

  10. Why Pea Plants? • They grow quickly • They are usually self pollinating • They come in many varieties • Self Pollinating – contains both male and female reproductive structures – thus pollen from one flower or plant can fertilize the eggs of the same flower or the eggs of another flower on the same plant.

  11. How Did He Do it? • Studied only one characteristic at a time (ie. Plant height, or pea color) • He chose plants with 2 forms of the characteristic to be studied. • Short vs. tall • Smooth peas vs. wrinkled peas • Purple flowers vs. white flowers

  12. He always chose true breeding plants – when self pollinatingthese always produce the offspring with the same characteristics. (SS or ss) or (TT or tt) as the parent – identical alleles for a trait • He started cross breeding 2 plants with different forms of the same trait via cross-pollination.

  13. Cross Pollination 1) the anthers on the stamen of one plant are removed (so the plant can not self-pollinate) 2)Pollen from another plant is used to fertilize the plant without anthers. * This allows Mendel to control which pollen fertilizes which plant

  14. Know This Nomenclature • True Breed Smooth (SS) x True Breed wrinkled (ss) = parents Cross pollination • The offspring of 2 true breeds = f1 generation (all Ss but all smooth) Self pollination • The offspring of an f1 generation due to self pollination = f2 generation (Ss) X (Ss)

  15. How did Mendel follow the scientific method? Ask a Question – How are traits inherited? Form a Hypothesis – If traits are inherited then their patterns can be predicted. Test the Hypothesis – Cross true-breeding plants and offspring. Analyze the Results – Identify patterns in inherited traits. Draw Conclusions – Traits are inherited in predictable patterns.

  16. How he deciphered heredity • Mendel chose to study only one characteristic, such as plant height, flower color, or seed shape. True breeding plants – a plant that only reproduces offspring with the same traits as the parent when it self-pollinates. (SS or ss) For example- a tall true breeding plant will always produce offspring that are tall.

  17. He then cross pollinated two true breeding plants with different forms of a single trait. For example- a true breeding tall plant with a true breeding short plant. • Cross Breeding – cut the anthers from the stamens of one plant (so that it can’t self-pollinate) then add pollen to it from another plant.

  18. Mendel’s first experiment • Crossed plants that had true bred smooth seeds (SS) with those that had true bred wrinkled seeds (ss). • The offspring from this first generation (f1) were all smooth. • This same thing occurred no matter what the trait tested – all the f1 generation possessed only one of the parent’s traits. • dominant trait – the one and only trait that shows itself in f1 generation • recessive trait – the trait that does not show itself in the f1 generation

  19. Mendel: Experiment 1 Remember – in sexual reproduction each parent donates 1 gene each. An SS can only donate “S” But a Ss can donate either an “S” or an “s”

  20. Mendel’s second experiment • Mendel allowed the first generation (f1) to self pollinate. • This time the f1 generation which was all smooth seeds (the dominant trait) produced a next generation (f2) which possessed some smooth seeds and some wrinkled seeds. • Somehow the recessive trait showed up again. • Every fourth seed was wrinkled.

  21. P parents F1 offspring F1 breeding F2 offspring

  22. Mendel’s Actual Results 1) P = smooth seeds crossed with wrinkled seeds F1 = all smooth seeds (so smooth is dominant and wrinkled is recessive) F2 = 5,474 smooth seeds and 1,850 wrinkled seeds is a ratio of 2.96 : 1 2) P = green seeds crossed with yellow seeds F1 = all yellow seeds (so yellow is dominant and green is recessive) F2 = 6,022 yellow seeds and 2,001 green seeds is a ratio of 3.01 : 1 3) P = purple flowers crossed with white flowers F1 = all purple flowers (so purple is dominant and white is recessive) F2 = 705 purple flowers and 224 white flowers is a ratio of 3.15 : 1 4) P = constricted pods crossed with inflated pods F1 = all inflated pods (so inflated is dominant and constricted is recessive) F2 = 882 inflated pods and 299 constricted pods is a ratio of 2.95 : 1 • 5) P = green pods crossed with yellow pods F1 = all green pods (so green is dominant and yellow is recessive) F2 = 428 green pods and 152 yellow pods is a ratio of 2.82 : 1 • 6) P = terminal flowers crossed with axial flowers F1 = all axial flowers (so axial is dominant and terminal is recessive) F2 = 651 axial flowers and 207 terminal flowers is a ratio of 3.14 : 1 • 7) P = dwarf stem crossed with tall stemF1 = all tall (so tall is dominant and dwarf is recessive) F2 = 787 tall stems and 277 dwarf stems is a ratio of 2.84 : 1

  23. Let’s Look at the Ratios Flower Color 3.15:1 (purple:white) Seed Color 3.00:1 (yellow:green) Seed Shape 2.96:1 (smooth:wrinkled) Pod Color 2.82:1 (green: yellow) Pod Shape 2.95:1 (inflated:constricted) Flower Position 3.14:1 (middle:end) Plant Height 2.84:1 (tall:short) 3:1

  24. Math Break (probability) If Mendel looked at 7 different traits each with only 2 forms, how many different plants are possible? Pea Shape Flower Color Pea Color yellow purple green Smooth yellow white green yellow purple green Wrinkled yellow white green

  25. The Answer Is…. 2 x 2 x 2 x 2 x 2 x 2 x 2 Or 2 = 7 128 different combinations What are the chances of getting a plant that is tall, with green wrinkled peas in a inflated yellow pod with purple flowers located at only the ends of branches (assume equal chance of each individual trait possibility)?

  26. Mendel’s Brilliant Conclusion The only way to explain the presence of only one trait in the f1 generation and the 3:1 ratios in the f2 generation was if each plant had 2 sets of instructions for each characteristic. Each parent donates one set of instructions known as genes Therefore, every fertilized egg (offspring) would have 2 forms of the same gene for every characteristic. The 2 forms are individually termed an allele. The combination of the 2 alleles in the offspring is termed the genotype.

  27. Since we know smooth seeds are dominant as they show up exclusively in the f1 generation we will label that gene with a capital S. The recessive gene we will label as a small s. In f1 if there were 4 offspring (or 10,000) their only possible allele combination is Ss. The allele combination is called the genotype. Thus 4 Ss which are all smooth seeds. Pure Breed Wrinkled Seeds Pure Breed Smooth Seeds Parents ss SS S gene (or allele) s gene (or allele) Offspring (f1) Ss genotype which makes all smooth seeds because smooth is dominant

  28. Second Generation – this generation is allowed to self fertilize. The possible male genotypes are Ss and the possible female genotypes are also Ss. Each can send an S allele or an s allele. Male Contribution Smooth Seeds Female contribution Smooth Seeds f1 Ss Ss or or S gene s gene S gene s gene Offspring (f2) SS Ss Ss ss

  29. F2 generation What is the Ratio? Smooth • SS • Ss • Ss • ss Smooth 3:1 smooth to wrinkled Smooth Wrinkled What are the percentages? 75% Smooth and 25% Wrinkled

  30. Terminology • Homozygous – possessing 2 of the same alleles for a particular trait (SS or ss or TT or tt) • Heterozygous – possessing different alleles for a particular trait (Ss or Tt) • Homozygous dominant = SS or TT • Homozygous recessive = ss or tt

  31. The Punnett Square • The Punnett square is a simple grid with the all the possible sperm/male alleles along one side, all the possible egg/female alleles along another side, and all the possible offspring genotypes filling the grid. • Fusion of those two gametes produces the genotype of the offspring (zygote) in the boxes. For a given trait, dominant traits are symbolized by CAPITAL letters and recessive traits by lower case letter.. Always use the same letter for dominant and recessive. DO NOT USE DIFFERENT LETTERS

  32. Therefore all genotypes from true breeding organisms are represented by 2 similar alleles –represented by similar letters (pp or PP, rr or RR, etc.) The cross between 2 true breeding plants one purple flowered and one white flowered would be (where purple is dominant and white is recessive): PP x pp

  33. The Punnett Square for PP x pp p p – termed allele Flower Color P f1 Pp Pp – termed genotype purple Purple – termed phenotype Pp P Pp purple purple Defn: phenotype – an organism’s inherited appearance

  34. The Punnett Square for Pp x Pp P p Flower Color P f2 PP Pp purple purple pp p Pp white purple

  35. Punnett Square Practice • What happens in f1 when you cross a Pure Breed Green Pod (GG) with a Pure Breed Yellow pod (gg)? • What happens in f2 when you allow a green pod genotype of Gg to self pollinate? • What happens when you cross a Gg green pod with a yellow pod gg? • The allele for a cleft chin, C, is dominant among humans. What would be the results from a cross between a heterozygous woman and a homozygous dominant man? • What about a Cc and a cc combination. • What is the ratio of offspring with a cleft chin to offspring without a cleft chin in #’s 4 and 5)? • If 2 adults both do not have cleft chins. What are the genotype and phenotype possibilities for their children? • If a child was born with a cleft chin. What possible genotype combinations could his parents have had?

  36. Mini-Lab Assume: Brown eyes are dominant over blue eyes. Take masking tape and label both sides of 2 coins. Label both coins with a B and a b, to represent a mixed genotype for brown eyes. In this example 2 people heterozygous for eye color produce offspring . • Flip each coin 50 times. Record your results for each allele from coin 1 and coin 2 and the subsequent genotype and phenotype of the offspring. • Flip each coin an additional 40 times and record your results as above.

  37. What are genes? • A gene can be defined as a region of DNA that controls a hereditary characteristic. It usually corresponds to a sequence used in the production of a specific protein or RNA. • In humans, Genes can be as short as 1000 base pairs or as long as several hundred thousand base pairs. It can even be carried by more than one chromosome. • Not as simple as Mendel’s pea plants

  38. The 46 human chromosomes = 23 pairs house almost 3 billion base pairs of DNA that contains about 30,000 - 40,000 protein-coding genes. The coding regions make up less than 5% of the genome (the function of the remaining DNA is not clear) and some chromosomes have a higher density of genes than others.

  39. Sex Chromosomes • Females are XX • Males are XY • What sex alleles can the female donate? • What sex alleles can the male donate? • Which parent is responsible for the determination of the gender of the child? • X only • X or Y Males Only

  40. seX- linked Traits

  41. Sex Linked GenesA particularly important category of genetic linkage has to do with the X and Y sex chromosomes.  These not only carry the genes that determine male and female traits but also those for some other characteristics as well.  Genes that are carried by either sex chromosome are said to be sex linked. • Men normally have an X and a Y combination of sex chromosomes (XY), while women have two X's (XX).  Since only men inherit Y chromosomes, they are the only ones to inherit Y-linked traits.  Men and women can get the X-linked ones since both inherit X chromosomes.

  42. Sex cell inheritance patterns for male and female children X-linked traits that are not related to feminine body characteristics are primarily expressed in the observable characteristics, or phenotype , of men.  This is due to the fact that men only have one X chromosome. Subsequently, genes on that chromosome that do not code for gender are usually expressed in the male phenotype even if they are recessive since there are no corresponding genes on the Y chromosome to dominate over them, in most cases.

  43. X-linkage in men In women, a recessive allele on one X chromosome is often masked in their phenotype by a dominant normal allele on the other.  This explains why women are frequently carriers of X-linked traits but more rarely have them expressed in their own phenotypes.

  44. X-linkage in women There are about 1,000 human X-linked genes.  Most of them code for something other than female anatomical traits.  Some of the non-sex determining X-linked genes are responsible for abnormal conditions such as: hemophilia red-green color blindness congenital night blindness high blood pressure duchene muscular dystrophy fragile-X syndrome.  Queen Victoria of England was a carrier of the genefor hemophilia.  She passed the harmful allele forthis X-linked trait on to one of her four sons and atleast two of her five daughters.  Her son Leopold hadthe disease and died at age 30, while her daughterswere only carriers.  As a result of marrying into otherEuropean royal families, the princesses Alice andBeatrice spread hemophilia to Russia, Germany, andSpain.  By the early 20th century, ten of Victoria'sdescendents had hemophilia.  All of them were men.  Queen Victoria(1819-1901)

  45. By comparison to the X chromosome, the much smaller Y chromosome has only about 26 genes and gene families.  Most of the Y chromosome genes are involved with essential cell house-keeping activities (16 genes) and sperm production (9 gene families).  Only one of the Y chromosome genes, the SRY gene, is responsible for male anatomical traits. 

  46. Because the Y chromosome only experiences recombination with the X chromosome at the ends (as a result of crossing-over), the Y chromosome essentially is reproduced via cloning from one generation to the next.  This prevents mutant Y chromosome genes from being eliminated from male genetic lines. Subsequently, most of the human Y chromosome now contains genetic junk rather than genes.

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