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Genetics Genetics is the scientific study of heredity.

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Presentation on theme: "Genetics Genetics is the scientific study of heredity."— Presentation transcript:

1 Genetics Genetics is the scientific study of heredity.
Gregor Mendel born in 1822 was an Austrian monk who first formed the basic laws of heredity during the mid-1800’s. He is credited with advancing the study of genetics through his good fortune, scientific method, careful record keeping and application of mathematics to biological studies. Mendel is known as the “ The Father of Genetics” because his work laid the foundation to the study of heredity.

2 The Good Fortune Mendel inherited a purebred stock of peas that produced identical offspring as a result of self pollination of pea plants. (male and female parts on the same plant) Mendel based his laws on his studies of the garden peas and was able to observe differences in multiple traits over many generations because pea plants reproduce rapidly He chose to study seven traits with just two contrasting characters (as opposed to intermediate forms)

3 Good Science (Tall plants used as an example)
Mendel noticed that some plants always produced offspring that had a form of a trait exactly like the parent plant. He called these plants “purebred” plants. Mendel cross pollinated purebred plants with opposite forms of a trait. He called these plants the parental generation; or P generation. He called the offspring the F1 generation; or first filial and he observed that they were all tall, as if short trait had disappeared.

4 Good Science (Tall plants used as an example)
Since the F1 offspring plants received one tall and one short gene from the parents plants then they are said to be “hybrid” for that trait. (But why were all the F1 plants tall if they had inherited one tall gene and one short gene?) Mendel then crossed two of the offspring tall plants produced in the F1 generation. He called this second generation of plants the second filial; or F2 generation. To his surprise he observed that this generation had a mix of tall and short plants even though none of the F1 parents were short.

5 Mendel’s Law of Segregation
Mendel’s first law, the Law of Segregation, has three parts. From his experiments he concluded: Plant traits are handed down through “hereditary factors” in the sperm and egg. Because offspring obtain hereditary factors from both parents, each plant must contain two factors for every trait. The factors in a pair segregate (separate) during the formation of sex cells (meiosis), and each sperm or egg receives only one member of the pair. We now call the “hereditary factors” genes.

6 Dominant and Recessive Genes
In the first experiment when Mendel crossed a purebred tall with a purebred short, all the offspring were tall. Mendel reasoned that one factor (gene) in a pair may mask, or hide, the other factor. Although the F1 offspring all had both tall and short factors, they only displayed the tall factor He concluded that the tallness factor masked the shortness factor.

7 Dominant and Recessive Genes, cont.
Today scientists refer to factors that control traits as genes. The different forms of a gene are called alleles. Dominant alleles are the alleles that mask or hide other alleles, such as the tall allele. Recessive alleles, such as the short allele, that are masked, or covered up (hidden), whenever the dominant allele is present. Mendel’s Principle of dominance states that one factor in a pair may prevent the other from being expressed. The dominant gene masks the expression of the recessive gene.


9 P generation plants in this experiment were both purebreds with the same type of gene
TT (tall ) and tt (short) Capital letter of the dominant trait is used for the dominant factor and the lower case letter is used for the recessive trait The purebred dominant is called homozygous dominant-TT The pure bred recessive is called homozygous recessive- tt The F1 plants in this study had one factor from each purebred parent and are called heterozygous - Tt or hybrid

10 To explain the expression of traits in the F2 plants Mendel developed two more principals
The principle of segregation states that the members of each pair of genes separates ,or segregate, when gametes are formed The principle of independent assortment states that two or mores pairs of genes segregate independently of one another during the formation of gametes. The process of meiosis is the mechanism for these principles

11 More Terms Allele is the term used to describe either member of a pair of genes that determines a single trait. T is the dominant allele t is the recessive allele Genotype is the genetic makeup of the organism. It’s gene-type (genes it received) Phenotype is the trait that’s actually expressed in an organism’s appearance. (physical appearance)

12 I t is important to note that Mendel made all his observations, developed his experimental methods and mathematical conclusions before detailed knowledge of cell structure and division had been discovered .

13 Genetics and Probability
Probability – is the likelihood that a particular event will occur. Example: If you flip a coin it may land heads up or tails up. The chance, or probability, of either outcome are equal. Therefore, the probability that a single coin flip will come up heads is 1 chance in 2 , that is ½ , or 50%. If you flip the coin 3 times in a row the probability each time is still ½ So ½ X ½ X ½ = 1/8 The principles of probability can be used to predict the outcomes of genetic crosses.

14 Punnett Square Developed in the early 1900’s by Reginald Punnett, the Punnett square is a useful tool in visualizing the outcome of various types of gene combinations. Monohybrid Cross

15 A monohybrid cross is one involving just one trait
A dihybrid cross involves two traits

16 In a test cross the unknown genotype of a dominant phenotype can be determined by crossing it with a homozygous recessive (recessive phenotype)

17 Genetics and Probability
The genotypic ratio for the F2 offspring is just like our coins 1:2:1 The phenotypic ratio is 3:1

18 Genetics and Probability
The phenotypic ratio for the F2 offspring in a dihybrid cross is 9:3:3:1

19 Steps To Working Genetic Crosses
1. Set up A Legend 2. Set-up Parental Genotype. 3. Construct a Punnett Square 4. Align Parentals Genotype 5. Fill in Punnett Square. 6. Record Results.

20 1.) Setting Up A Legend A legend is an explanatory list of the symbols on a map or chart

21 Setting Up A Legend Your legend for genetic crosses will show both the phenotype and genotype. The Phenotype (Physical appearance) will be on the left. The Genotype (genetic make-up) will be on the right. Tall = TT Tt Short = tt (phenotype) (genotype) or Red = RR Rr White = rr

22 2.) Set-up parental genotypes
Write down your "cross"(mating).  Write the genotypes of the parents in the form of letters Tt x tt

23 3.) Setting Up A PUNNETT SQUARE
When given enough info about two parent organisms, we can use this window pane to predict the genotypes & phenotypes of their offspring.

24 4.) Align Parentals Genotype 1st Parent
Take the genotype letters of one parent, split them and put them on the left, outside the rows of the punnett square

25 Align Parental Genotype 2nd Parent
Now take the two letters of the second parent's genotype, split 'em up, and place them above each of the two columns of the punnett square.

26 Fill in Punnett Square Filling in the top-left box:
One from the left, one from the top.

27 Filling in the bottom-left box:
One from the left, one from the top.

28 Filling in the top-right box:
One from the left, one from the top.

29 Filling in the bottom-right box:
One from the left, one from the top.

30 5.) Record Results Show Phenotype and Genotype in your results
Phenotype: Show the results as Percent (%) and ratio. Genotype: Show the results for each different genotype. Phenotype: 50% Tall, 50% Short (2:2) Genotype: 2Tt, 2tt

31 It was Walter Sutton in 1903 who made the link between Mendel's principles and his own observation of meiosis in grasshoppers. The chromosome theory he proposed states that hereditary factors or genes, are carried on chromosomes. The pairing of homologous chromosomes followed by their independent segregation during meiosis explained all of Mendel's observations.

32 Dihybrid Crosses: Setting Up Punnett Square
Go to Dihybrid pwpt

33 Incomplete Dominant Crosses
Not all Phenotypes are the result of dominant or recessive genes . Incomplete Dominance is the incomplete expression of the dominant allele in a heterozygote, where both alleles are expressed in the phenotype. That is, there is a blending of traits where the alleles act together to yield an intermediate phenotype

34 Flower Color in Snapdragons: Incomplete Dominance
Red flowers - two alleles allow them to make a red pigment White flowers - two mutant alleles; can’t make red pigment Pink flowers - have one normal and one mutant allele; make a smaller amount of red pigment

35 Flower Color in Snapdragons: Incomplete Dominance
Red-flowered plant X White-flowered plant (homozygote) (homozygote) Pink-flowered F1 plants (heterozygote)

36 Flower Color in Snapdragons: Incomplete Dominance
Pink-flowered plant X Pink-flowered plant (heterozygote) (heterozygote) White-, pink-, and red-flowered plants in a 1:2:1 ratio

37 Phenotype: 0% Red, 100% Pink, 0% White (0:4:0) (R:P:W)
R = allele for red flowers W = allele for white flowers red x white ---> pink Legend: Red = RR White= WW Pink= RW RR x WW Phenotype: 0% Red, 100% Pink, 0% White (0:4:0) (R:P:W) Genotype: 4RW

38 Non Mendalian Genetics
Codominance is a condition when both alleles are expressed neither one is dominant BBxWW = BW

39 Example of Co-Dominance
Black\white spotted crossed with

40 Multiple Allele Traits
Many traits have more then two alleles in the population One important multiple allele trait is blood type Type A is Codominant with type B Type O is recessive to both A and B Resulting in: Type A = AA or AO Type B = BB or BO Type AB = AB Type O = OO

41 Human ABO Blood Groups AA AO BB BO OO AB or IAIA IAi IBIB IBi ii IAIB
Gene “I” specifies which sugar is found on the outside of red blood cells 3 common alleles are present in the human population: IA = N-acetyl-galactosamine IB = galactose i (also referred to as o) = no sugar present 6 possible genotypes AA AO BB BO OO AB or IAIA IAi IBIB IBi ii IAIB

42 Immunology 101 Sugar on the blood cell is an antigen* (A, B, A and B, or none) Your immune system thinks your own antigens are fine Your immune system makes antibodies against non-self antigens Antibodies recognize and target cells with antigens for destruction *something that elicits an immune response



45 Legend: Type A = IAIA or IAi. Type B = IBIB or IBi. Type AB = IAIB
Legend: Type A = IAIA or IAi Type B = IBIB or IBi Type AB = IAIB Type 0 = ii Example: IAi x IAIB IA i IAIA IA IAi Phenotype = 50% Type A; 25% Type AB; 25% Type B 0% Type O (2:1:1:0) (A:B:AB:O) IAIB IBi IB Genotype = 1IAIA; 1IAi; 1IAIB; 1IBi

46 Polygenic Traits Polygenic traits are those traits that are determined by several genes. Examples of these are skin color, eye color and height

47 Drosophila melanogaster
Thomas Hunt Morgan working with genetics most famous animal, the fruit fly ,was the first to demonstrate that a specific gene was located on a specific chromosome.

48 The Sex Chromosomes Nettie Stevens was the first to propose
that the odd smaller Y chromosome that paired with the X determined an organism's sex. To be male you must have an X and a Y to be female you need two X’s. 50% chance of Male, 50% chance of Female

49 The Sex Chromosomes The mismatched chromosomes are called the sex chromosomes. (note the smaller size of the Y) All the other (usually homologous) chromosomes are called autosomes.

50 Sex linked Traits A sex linked trait is one determined by alleles carried only on the X chromosome A sex linked trait has no corresponding allele on the Y chromosome.

51 Sex-Linked Traits As with all organisms humans have traits that are only carried on the X chromosome. Examples of these are colorblindness and hemophilia, a disorder that prevents normal blood clotting

52 Gene linkage is a situation in which two or more genes occur on the same Chromosome and are thus inherited together. Crossing over is the exchange of alleles between two homologous chromosomes ( it is a useful tool in gene mapping)

53 Sex-linked Traits Traits (genes) located on the sex chromosomes
Example: fruit flies (red-eyed male) X (white-eyed female)

54 Sex-linked Traits Sex Chromosomes Notice there is no gene on the
Y chromosome Sex Chromosomes XX chromosome - female XY chromosome - male fruit fly eye color

55 Example:. fruit flies. (red-eyed male) X (white-eyed female) XRY
Example: fruit flies (red-eyed male) X (white-eyed female) XRY * XrXr Remember: the Y chromosome in males does not carry traits. XR Y Legend: red eyed= RR Rr white eyed= rr Male= XY Female= XX XRXr XrY Xr XRXr XrY Xr Phenotype 50% red eyed and female 50% white eyed and male Genotype 2 XRXr, 2 XrY

56 PEDIGREE CHARTS Pedigree charts show a record of the family of an individual. It can be used to study the transmission of a hereditary condition. It is particularly useful when there are large families and a good family record over several generations. = males and O = females

57 Autosomal Pedigrees An autosomal pedigree is probably one of the most common pedigrees that you will find.  If a (recessive) trait is inherited on any chromosome except for the sex chromosomes (X or Y), then you are studying an autosomal pedigree.

58 Symbols used in pedigree charts
In a marriage with five children, two daughters and three sons. The second son is affected by the condition.

59 Above is a pedigree chart of a family showing four generations.
A total of 20 individuals.

60 Sex Linked Pedigrees A sex linked pedigree
deals with a trait that is found on the X Chromosome. Since males are XY, they will only carry one allele; therefore the allele that they possess will directly be reflected in the genotype. Females, on the bother hand, are XX, so they carry two alleles per trait.  A female who is heterozygous for a sex-linked trait is said to be a carrier. Symbol Meaning         Dominant Male Dominant Female Recessive Male          Recessive Female Carrier Female (for a sex-linked trait)


62 Human Genetics A pedigree is a graphical representation of a genetic inheritance Key

63 Key

64 Tay Sachs Disease Key

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