He was a Chezch priest and math teacher.

Transcription

1 Genetics

2 The Study of Heredity This field of Genetics began with the work of Mendel in the early 19 th century. He was a Chezch priest and math teacher. He observed that many plants were true breeding or self pollinating and produce new plants identical to themselves. es

3 Reproduction of Plants

4 In plants the flower is the reproductive organ Male portions Stamen These contain ti the pollen or male haploid cells, on the anther. Pollen is the equivalent to sperm Female portions Pistil/Carpel Comprised of the sticky stigma which catches the pollen, the style leading to the ovule which h contains female haploid cells that are fertilized forming seeds

5 Mendel s Peas The offspring of these true breeders were the result of all genetic information coming from the same parent plant and thus producing identical offspring He noticed among his peas 7 distinct traits which h were expressed by specific characteristics

6 1. seed texture: smooth / wrinkled 2. seed color: green / yellow 3. Pod appearance: constricted / inflated 4. pod color: green / yellow 5. flower color: purple / white 6. flower position: axial / terminal 7. stem height: tall / short

7 Mendel hypothesized that something in the pea plants controlled the traits he studied and called these controls, factors. Td Today we call these factors, genes. Because each trait ithdt had two distinct t expressions there must be a pair of factors controlling each trait. Today we call these variations of a gene an allele. These factors were passed on or inherited from one generation to another

8 Today s interpretation of Mendel s first principal i Heredity is carried by DNA which is contained Heredity is carried by DNA which is contained in chromosomes occurring in pairs. Genes are small segments of the DNA, which therefore occur in pairs or alleles The separate allele of each pair may represent a different expression of the same trait. 2 alleles working together allow expression of a genetic trait.

9 Mendel began his experiments by first developing true strains for each trait and characteristic. He called these pure strains his parental or P 1 generations. He cross pollinated this generation of plants carefully to study each trait. This next generation of plants was called the first filial F 1 generation. Offspring that are the result of such crossbreeding are called hybrids.

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11 These hybrids were then allowed to selfpollinate giving birth to the F 2 generation. All of his results were carefully recorded Mendel observed that thesecond variant of each trait reappeared and in particular percentages or ratios

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13 Based on the fact that his F 1 generation expressed predominantly a single characteristic of the traits The secondary characteristics reappeared in the F 2 generation Mendel also hypothesized that one variant or allele must be dominant while the other was recessive. A dominant allele masks the expression of characteristics of the other alleles. A recessive allele shows no observable effect on an organism when paired with a dominant allele.

14 Genetics shorthand Genetic traits are often referred to in terms of their dominant and recessive alleles for that trait. The trait for height for example. Tall or T is dominant and expressed in short hand with an uppercase letter. Short or t is recessive and represented by the corresponding lower case letter. Since alleles occur in pairs there are the following possibilities: TT Tt tt tall tall short

15 Law of Segregation A pairof factors is segregated or separated during the formation of gametes. This idea came when Mendel examined his F 2 generation and noticed that the recessive trait re expressed itself He concluded that the paired factors (alleles) must be received separately by the zygote one from each parent for each gamete.

16 Genotype vs. Phenotype The genetic makeup of an organism, or the alleles that it inherits fromitsparents parents, is referred to as its genotype. When both alleles l of a pair are alike lk it is said to be homozygous. (TT) = homozygous dominant (tt) = homozygous recessive When the two alleles in a pair are different for that characteristic the organism is heterozygous (Tt)

17 The physical appearance of the organism as a result of its genotype is referred to as its phenotype. Example: Mendel s pea plant that is tall may have the genotype TT or Tt. Its phenotype or physical characteristic is still tall. The phenotype of tt would be short.

18 The role of statistics Mendel noticed in the F 2 generation of plants that the recessive trait reappeared with predictable statistical consistency This predictability is now a useful tool of geneticists Probability is the likelihood that a specific event will occur.

19 probability bilit = # times an event is expected to occur total # of opportunities for the event to occur Can be expressed as a: fraction ratio percentage decimal

20 In the case of Mendel s F 2 generation he observed that the phenotype for purple flowers occurred 3 out of every 4 opportunities. ¾ of the time 75% probability In a ratio of 3:1 With a probability of 0.75

21 These probabilities biliti are used in genetics to describe outcomes to assess validity of experimental results. Scientists will always come closer to achieving expected probabilities with a greater number of trials. To predict probabilities biliti we use a tool or diagram called a Punnett square.

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23 Using the Punnett Square Draw 4 small squares in the shape of a window. Write the possible gene(s) of one parent across the top the gene(s) of the other parent along the side of the Punnett square Fill in each box of the Punnett square by transferring the letter above and in front of each box into each appropriate box. As a general rule, the capital letter goes first and a lowercase letter follows.

24 These crosses which only involve one pair of contrasting trait are called monohybrid hbidcrosses Monohybrid Homozygous x Homozygous cross

25 MonohybridHomozygousx Heterozygous cross

26 Monohybrid heterozygous x heterozygous cross.

27 More Mendel. Mendel then turned to crossing plants based on 2 traits rather than just one His results showed dominant traits did not necessarily appear together. In other words a green seed pod (dominant) could appear in a plant with a white flower (recessive). This lead to the Law of independent Assortment

28 Law of independent Assortment Factors for different characteristics distribute to gametes independently of one another. These experiments can be explained by the punnett squares for dihybrid id crosses. Dihybrid crosses are crosses between two individual organisms involving two separate traits.

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30 A pea plant is heterozygous for both seed shape and seed color. S is the allele for the dominant, spherical shape characteristic; s is the allele for the recessive, dented shape characteristic. Y is the allele for the dominant, yellow color characteristic; y is the allele for the recessive, green color characteristic. What will be the distribution of these two alleles l in this plant's gametes?

31 SY Sy sy sy SY SSYY SSYy SsYY SsYy Sy SSYy SSyy SsYy Ssyy sy SsYY SsYy ssyy ssyy sy SsYy Ssyy ssyy ssyy

32 Beyond simple dominant /recessive alleles: l Some alleles are neither dominant or recessive. incomplete dominance, is a blending of allele trait. individuals dua heterozygous for the trait twould exhibit an intermediate (third) phenotype. Thus, the genotypic ratio is the same as the phenotypic ratio in cases of incomplete dominance. For example, in snapdragons, red flowered plants (C R C R ) white flowered plants (C W C W ). pink flowers (C R C W ) genes that code for the production of red and white pigments are both partially expressed.

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34 In some cats, the gene for tail length shows incomplete dominance. Cats can have no tails (T N ), long tails (T L ), or short tails (T N T L ). Cross a short tail cat and a cat with no tail. Cross a long tail cat and a short tail cat. What proportion of the offspring will have short tails? Long tails?

35 Codominance both alleles are expressed equally and fully in the phenotype. Examples: Roan color in cattle: C R C R =red C W C W =white C R C W roan Roan is a splotchy red and white in cattle

36 Blood types in humans A B O

37 An anemic condition called thalassemia is controlledby codominantalleles. alleles. Individualsthat are homozygous for the normal allele (T N ) do not have the disease. Inheritance of two abnormal alleles (T A T A ) produces a severe anemia known as thalassemia major. Individuals that are heterozygous (T N T A ) for the condition have a mild anemia known as thalassemia hl minor. A young couple comes to you, a genetic counselor, for advice. The young man is normal and has no form of anemia. His lovely wife has just been diagnosed with thalassemia minor. What are the chances they could produce a child with anemia?

38 Multiple Alleles A population o might have aemore oethan two oaees alleles for a given gene. In labrador retriever, coat color is determined by one gene with four different alleles. Five different colors result from the combinations of these alleles. l Even if more than two alleles exist in a population, any given individual can have no more than two of them: one from the mother and one from the father.

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40 Polygenic Traits produced by the interactions of morethan one gene Human skin color is controlled by more than 4 genes and would explain the wide range of phenotypes.

41 Environmentally determined variation in traits individuals could be genetically thesame but differ in appearance because of the environment within which theylive and develop. Some examples of this include: human height is influenced by prenatal and childhood nutrition as well as genetics.