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Presentation on theme: "FUNDAMENTALS OF GENETICS"— Presentation transcript:


2 GENETICS Genetics is a field of Biology that is devoted to understanding HOW characteristics are passed on from parents to offspring. Gregor Johann Mendel – the father of genetics

3 GREGOR MENDAL 1843 – Gregor Mendal joined the monastery at the age of 21. His task was to tend to the garden, which allowed him to observe and think about the growth of many generations of plants. 1851 – He entered the U. of Vienna to study math and science, where he studied statistics Statistics helped him in the field of Heredity – the transmission (passing on) of characteristics from parents to offspring.

4 GARDEN PEAS Mendel is remembered most for his work with Pisum sativum, Garden Peas. Seven Characteristics of garden peas were observed. For each characteristic, two contrasting traits were observed A trait is a genetically determined variant of a characteristic.

5 GARDEN PEAS The Seven Characteristics:
Plant Height (traits: Long & Short) Flower Position along stem (traits: axial & terminal) Pod Color (traits: green & yellow) Pod appearance (traits: inflated and constricted) Seed Texture (traits: round and wrinkled) Seed Color (traits: yellow & green) Flower Color (traits: purple & white)

6 GARDEN PEAS Mendel collected the seeds of his pea plants and recorded each plant’s traits and seeds He planted the seeds FLOWER COLOR: He observed that purple flowering plants came from most of the seeds that he had collected from purple plants But there where some white flowering plants that came from purple plant seeds.

Mendel observed that while tall plants grew from most of the seeds that were obtained from tall plants But Mendel also observed some short plants grow from seeds that were also obtained from tall plants. Mendel wanted to find an explanation for such variation.

8 MENDEL’S METHODS Mendel observed how traits were passed from one generation to the next by controlling how the plants were pollenated. Pollination – when the pollen grains from the male reproductive part of the flower (anthers) is transferred to the female reproduction part of the flower (stigma). Self pollination occurs when pollen is transferred from anthers to stigma of the same plant Cross-pollination occurs between two different plants

9 MENDEL’S METHODS To Control his Results:
Mendel removed the anthers of a plant and cross pollinated the plants by manually transferring pollen from the flower of a second plant to the stigma of the antherless plant. This way he prevented self pollination and controlled the specific traits of the parents.

10 MENDEL’S EXPERIMENTS Mendel’s Experiments: he first studied each characteristic and it’s contrasting traits individually. He began growing plants that were true-breeding, or pure for each trait. True-breeding plants always produce offspring with that trait when they self pollinate. Mendel produced true-breeding plants, by self pollination, for each of the characteristics and for each trait. So in the end he had 14 true breeding plants.

11 MENDEL’S EXPERIMENTS He began to cross-pollinate pairs of plants w/ opposite traits of a characteristic. He called the first true-breeding parents the P generation. He cross-pollinated a plant true-breeding for yellow pods with another plant true-breeding for green pods

12 MENDEL’S EXPERIMENTS Mendel recorded the number of each type of offspring that resulted from the cross-pollination of the P generation. He called the offspring of the P generation the F1 generation – F for Filial which has its roots in latin for son or daughter. He then allowed the flowers of the F1 generation to self- pollinate and then collected the seeds. He called the offspring of the F1 generation the F2 generation. Mendel performed hundreds of crossings and documented the results of each generation.

13 Mendel’s Results & Conclusions
The resulting F1 generation from the cross of the green pod plant and the yellow pod plant resulted in only green pod plants Mendel saw that no yellow pod plants developed even though one parent had been true-breeding for yellow pods. Mendel then allowed the F1 generation to self pollinate: Three – Fourths (75%) of the F2 generation produced green pods and One-Fourth (25%) produced yellow pods He concluded and hypothesized that something inside the pea plants controlled the characteristics and traits that were observed. He reasoned that a pair of control factors must control each trait.

14 Mendel’s Results & Conclusions
Recessive and Dominant Traits : Whenever Mendel crossed traits – one of the P traits failed to appear in the F1 plants. An every case, the trait reappeared in the F2 generation at a ratio of 3:1. This pattern lead Mendel to hypothesize that one factor in the pair may prevent the other from having an effect. Dominant Trait masks or dominates the appearing characteristic Recessive Traits is only expressed when paired with another plant or animal displaying that recessive trait

15 Results and Conclusions
The Law of Segregation: Mendel concluded that each reproductive cell or gamete, receives one factor from each parent. When the gametes combine during fertilization, the offspring have two factors for each characteristic. The Law of Segregation states that a pair of factors is segregated, or separated, during the formation of gametes.

16 Results and Conclusions
The Law of Independent Assortment: Mendel also crossed plants that differed in two characteristics, such as flower color and seed color. The results from these crosses showed that the traits produced by dominate factors do not necessarily appear together. A white flowering plant can produce green pods. Mendel hypothesized that the factors for individual characteristics are not connected.

17 RESULTS & CONCLUSIONS REMEMBER that in meiosis, the random separation of homologous chromosomes is called independent assortment. The Law of Independent Assortment states that characteristics separate independently of one another during the formation of gametes.

Support for Mendel’s Conclusions: Most of what Mendel found is supported by what biologists now know about molecular genetics. Molecular genetics is the study of the structure and function of chromosomes and genes. A gene is the segment of DNA on a chromosome that controls a certain hereditary trait. Remember that chromosomes occur in pairs so genes also occur in pairs. An allele is the name given to one of the alternate forms of a gene that governs a characteristic.

19 Section 2: Genetic Crosses
Differentiate between the genotype and phenotype of an organism Explain how probability is used to predict the results of genetic crosses Use a Punnett square to predict results of monohybrid and dihybrid genetic crosses Explain how a testcross is used to show the genotype of an individual whose phenotype expresses the dominant trait Differentiate a monohybrid cross from a dihybrid cross

20 Genotype and Phenotype
Genotype is the organism’s genetic make up. It is the alleles that the organism inherits from the parents. When you think of Genotype – Think GENES Example: The genotype for a white-flowering plant would be pp. The genotype of a purple flowering plant would be Pp or PP.

21 Genotype & Phenotype Phenotype is the organism’s appearance. When you hear phenotype – think PHYSICAL. Example: The phenotype of PP or Pp would be purple-flowers. The phenotype of pp would be white flowers.

22 GENOTYPE & PHENOTYPE Genotype doesn’t necessarily mean that phenotype and visa versa. A plant with the genotype to be tall can be short because of environmental factors. When both alleles of a pair are alike, the organism is said to be homozygous. This means that they can be homozygous dominant (PP) or homozygous recessive (pp). When the alleles are different, they are said to be heterozygous. An example of heterozygous is Pp.

23 Probability Probability is the likelihood that a specific event will occur. (the chances that “something” will happen). Probability may be expressed as a decimal number, a percentage, or a fraction. Probability is determined by the following equation: Probability = the # of times an event is expected to happen The # of times an event could happen

24 Predicting Results.. A monohybrid cross is a cross in which only one characteristic is tracked. Monohybrids are the offspring of the monohybrid cross. A Punnett square is an aid in the prediction of the probable distribution of inherited traits in the offspring. Homozygous x Homozygous We are going to cross a Pea Plant that is homozygous for Purple flowers (the alleles are PP) and a pea plant that is homozygous for White flowers (the alleles are pp). The alleles that are carried by each parent’s gamete are represented by the letters on the outside of the boxes:

25 Predicting Results The combinations within the four boxes represent the possible genotypes that can result from the cross of the homozygous pea plants. The outcome was Pp in every case, so the probability of the offspring having Pp is 100%. The probability of the flower being purple is 100% as well

26 PREDICTING RESULTS.. We are going to cross a Vegasaurous Rex that is homozygous dominant (allele is CC) for crazy curly hair and another Vegasaurous Rex (allele is Cc) that is heterozygous for crazy curly hair.

27 Predicting Results… The two possible genotypes from this cross are CC and Cc. The probability of an offspring having the genotype CC is 2/4 or 50%. You could expect about 2/4 or 50% of the offspring to have Cc. The probable phenotype of this cross is crazy curly hair. Thus, there is a 100% probability that the offspring will have crazy curly hair.

28 Predicting Results… If it were the other way around: cc x Cc , then the probability of an offspring having the genotype cc is 2/4 or 50%. You can expect the other half, 50% or 2/4 to have Cc. So the phenotype results would change – the genotype Cc would exhibit crazy curly hair and the cc would not.

29 Predicting results… Heterozygous x Heterozygous
In rabbits the allele for brown coat color (B) is dominant over white coat color (b). Now, we are going to cross to rabbits that are heterozygous Bb. They are both brown rabbits.

30 Predicting results… As you can see, ¼ or 25% of the offspring predicted will have the genotype BB Another 25% or ¼ of the offspring predicted will have the genotype bb and the rest, 50% or ½ will have the genotype Bb. The phenotype results in 75% or ¾ being brown rabbits while the rest, 25% or ¼ will have white fur.

31 Predicting Ratios… Ratio:
Genotype Ratio: the ratio of the genotype that appear in the offspring Example: 1 BB: 2Bb: 1bb Phenotype Ratio: the ratio of the phenotype that appear in the offspring Example: 3 brown: 1 white

32 TESTCROSS Is performed when the genotype of an individual is unknown.
In a test cross the individual with the unknown genotype is crossed with a homozygous recessive individual. Example: Brown Bunnies. Is it BB or Bb? Cross it with Homozygous Recessive Bunny If all the offspring appear Brown then the bunny is Homozygous Dominant (BB) If half the bunnies are white, then the bunny is Heterozygous Dominant (Bb)

33 INCOMPLETE DOMINANCE When the dominant allele completely masks recessive allele is called complete dominance. But there are some traits, where the recessive allele comes through a little bit. (Incomplete dominance) For Example: When crossing a certain type of Flower: Red Flowers (RR) and White Flowers (rr) The F1 generation are all (Rr) but the phenotype is Pink. In humans, skin color, eye color and hair shades are an example of incomplete dominance

34 Codominance Occurs when BOTH alleles for a gene are expressed in a heterozygous offspring In codominance – NIETHER allele is dominant or recessive, and they don’t blend as in incomplete dominance. Example: is in Blood Types

35 Predicting Results of Dihybrid Crosses
A dihybrid cross is a cross in which two characteristics are tracked. Dihybrids are the offspring of these crosses Naturally, with the addition of a trait, more combinations are possible.

36 Homozygous x Homozygous
By using a Punnett square, we are going to predict the possible offspring of a cross between two Pea plants with are Homozygous for Round, Yellow Seeds (RRYY) & Wrinkled, Green Seeds (rryy)

37 Homozygous x Homozygous
Predicting Results… Homozygous x Homozygous Assort the alleles RY, RY, RY, RY ry, ry, ry, ry Each box gets filled by the letters above it and to the left In this case each box has RrYy, so ach plant would have round yellow seeds.

38 Heterozygous x Heterozygous
Predicting Results Heterozygous x Heterozygous So, now lets take two pea plants from the F1 generation that is heterozygous yellow round (RrYy) This cross would result in NINE different genotypes This cross would result in FOUR different phenotypes

39 Heterozygous x Heterozygous
9/16 would have the phenotype of round, yellow seeds (genotypes: RRYY, RRYy, RrYY, RrYy) 3/16 would have the phenotype of round, green seeds (genotypes: RRyy & Rryy) 3/16 would have the phenotype of wrinkled, yellow seeds (genotypes: rrYY, rrYy) 1/16 would have the phenotype of wrinkled, green seeds (genotype: rryy)


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