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Introduction to Genetics

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Presentation on theme: "Introduction to Genetics"— Presentation transcript:

1 Introduction to Genetics

2 Heredity The passing on of characteristics from parents to offspring

3 Traits Characteristics that are inherited

4 Genetics The study of heredity
Genes: chemical factors that determine traits Can occur in 2 different forms called alleles

5 Gregor Mendel Father of genetics
1st to succeed in predicting how traits would be transferred from one generation to the next Used pea plants Pea plants reproduce sexually Produce male and female sex cells (gametes) Fertilization: male and female gametes unite, resulting in a zygote (fertilized egg)

6 Controlled plant crosses by removing male parts
Mendel studied only 1 trait at a time (7 total traits) True-breeding pea plants: Produced identical offspring Ex: tall or short, green or yellow

7 Organisms can look alike but have different combinations of alleles
Crossed a tall plant with a short plant Parent generation (P1) Offspring called hybrids (F1) Have different forms of a trait Showed characteristics of only 1 parent Observed traits are dominant Traits that seem to disappear are recessive Allele for tall plants is dominant to the allele for short plants Inherit one gene from mom and one from dad Organisms can look alike but have different combinations of alleles Different forms of a gene for each variation of an inherited trait

8 Phenotype: way an organism looks and behaves
Physical characteristics Ex: height (tall, short), skin color (red, white, blue..etc), head size, eye color…etc Genotype: Allele combination an organism contains Genetic make-up Assigned a letter to represent the trait Ex: TT, Tt, tt

9 Homozygous: 2 alleles for the trait are the same
Called true-breeding Ex: TT, CC, HH, tt, cc, hh Heterozygous: 2 alleles for the trait differ from each other Called hybrids Ex: Tt, Cc, Hh, Ss

10 Punnett Squares Way to find expected proportions or probability of possible genotypes in the offspring of a cross Probability Likelihood an event will occur Used to predict outcomes of a genetic cross Way alleles separate is random Cannot give actual outcome

11 Monohybrid cross (mono-: means one)
Focus on 1 trait Ex: height T= tall, t= short texture S= smooth vs. s= rough, Crossed a tall plant with a short plant

12 Mendel’s Crosses 1st generation plants (F1) were all tall
Self-pollinated to make 2nd generation plants (F2) 2nd generation plants: ¾ were all tall plants ¼ were short plants

13 ex: First generation plants (F1): all tall

14 ex: Second generation plants (F2)
Punnett squares are good for showing all the possible combinations of alleles and the probability of inheriting a trait

15 Bell Ringer: Which 2 combinations of alleles could produce a trait controlled by a dominant allele? What combination of alleles could produce a trait controlled by a recessive allele? If a heterozygous plant for seed color (Rr) is crossed with a homozygous recessive plant (rr), what is the probability of each seed color being produced? Draw a punnett square to show work. Organisms that have identical alleles for a particular trait are: a. heterozygous b. polygenic c. diploid d. homozygous

16 Dihybrid Crosses (di-: means 2)
Involve two traits Mendel: used seed color and seed texture Crossed round yellow seeds (RR and YY) with wrinkled green seeds (rr and yy) Round is dominant to wrinkled Yellow is dominant to green 1st generation (F1) plants: round and yellow (RrYy) 2nd generation (F2) plants: round yellow (9) round green (3) wrinkled yellow (3) wrinkled green (1)

17 Rule of Dominance Some alleles are dominant and others are recessive
Pea plants with 2 alleles for tallness were tall Pea plants with 2 alleles for shortness were short Dominant allele: observed trait Upper-case letters = Dominant allele (always put capital letter 1st for the genotype) Ex: allele for tallness (T) Tall Plants: TT or Tt Recessive allele Lower-case letters = Recessive allele Always listed after the dominant allele Will only appear if no dominant allele is present Ex: allele for shortness (t) tt  short plant Disappears

18 Law of Segregation Every individual has 2 alleles of each gene
When gametes are produced, each gamete receives 1 of these alleles Randomly pair to produce 4 combinations of alleles during fertilization Tall plant: Short plant: TT x tt = Tt Yellow plant x Green plant: YY x yy = Yy

19 F1 generation Tt Tt Tt X gametes T t T t TT Tt Tt tt F2 generation

20 Law of Independent Assortment
Ex: Does a gene that determines whether a seed is round or wrinkled have anything to do with seed color? Genes do not have to be carried together Travel independently Genes for different traits can segregate independently during formation of gametes Accounts for the wide variety of genetic combinations in offspring Genes for different traits are inherited independently of each other

21 Incomplete Dominance “blend” or mixing of traits
neither allele is completely dominant phenotype of heterozygous individuals is intermediate (in the middle) mix of purebred phenotypes ex: Red flowers (RR) x White flowers (R´R´) = Pink flowers (RR´)

22 Codominance phenotypes of both homozygotes are produced
both are dominant both alleles contribute to the phenotype but do not blend together both alleles are expressed equally ex: chickens Black chickens (BB) x White chickens (WW) = checkered or speckled chickens (BW) ex: red cow (RR) x white cow (WW) = roan cow (pinkish brown  RW) ex: Sickle-Cell anemia

23 Multiple Alleles Genes with more than 2 alleles
Ex: rabbit coat color, blood type, eye color

24 Polygenic Inheritance
inheritance pattern of a trait that is controlled by 2 or more genes may be on the same or different chromosome each gene can have 2 or more alleles ex: skin color, fruit fly eye color

25 Bell Ringer: a. segregation b. true-breeding
1. According to the principle known as _______, genes that segregate independently do not influence each other’s inheritance. 2. The separation of alleles during gamete formation is called ____. a. segregation b. true-breeding c. meiosis d. crossing-over 3. Which of the following structures assort independently? a. genes b. crossovers c. chromosomes d. genotypes 4. Genes on the same chromosome a. never separate b. sometimes separate c. always separate d. don’t show linkage

26 Applying Mendel’s Principles
Thomas Hunt Morgan (1900’s) Studied fruit flies Observed fruit flies produce a lot of offspring quickly Mendel’s principles apply to all living organisms

27 Linkage and Gene Maps Each chromosome is a group of linked genes
Chromosomes assort independently (not individual genes) Genes can separate by crossing-over Gives genetic diversity More likely to be separated the further apart they are on the chromosome

28 Gene map Shows locations of known genes on a chromosome

29 Simple Dominant Heredity
Only need to inherit 1 dominant allele to display the trait Tongue rolling Free earlobes Hitchhiker’s thumb Dimples Cleft chin Freckles

30 Bell Ringer: a. polygenic trait b. punnett square
Which of the following shows the relative locations of each known gene in an organism? a. polygenic trait b. punnett square c. gamete d. gene map 2. Match the type of inheritance with its correct description: incomplete dominance codominance multiple alleles polygenic traits _______ both alleles contribute to the phenotype of the organism _______ more than two possible alleles for a trait exist in a population _______ traits controlled by two or more genes _______ one allele is not completely dominant over the other allele

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