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Genetics Basics

Genetics Basics. A gene can be described in two ways, by:. Genotype: g enetic makeup; use letters to represent gene ( allele – different form of a gene; single letter) Ex: BB, Bb, bb Phenotype : p hysical trait; what you see - Ex Brown hair, blue eyes.

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Genetics Basics

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  1. Genetics Basics

  2. A gene can be described in two ways, by: • Genotype: genetic makeup; use letters to represent gene • (allele – different form of a gene; single letter) • Ex: BB, Bb, bb • Phenotype: physical trait; what you see - Ex Brown hair, blue eyes

  3. Alleles can sometimes be described as Dominant or Recessive: • Dominant: this allele dominates the other • use capital letters • Ex: Brown Hair (B) dominates blonde hair (b) • Recessive: this gene will only be expressed if 2 recessive alleles are present • use lower case letters - Ex: blonde hair is recessive (bb)

  4. A combinations of alleles can be Homozygous or Heterozygous: • Homozygous: same size letters (alleles) • BB, bb • Heterozygous: different size letters (alleles) • Bb • Also called hybrid

  5. Self-Quiz A different form of a gene • What is an allele? • What is the difference between genotype and phenotype? • When will a recessive gene be expressed? • Indicate whether the following are heterozygous or homozygous; Bb, BB, bb • Why are there always at least 2 alleles for a gene? Genotype represents genes, phenotype represents physical traits When both alleles are recessive (lower case) Bb-hetero, BB-homo dominant, bb homo recessive You receive one allele for a gene from each of your parents

  6. Punnett Square Basics

  7. Dominate vs. Recessive Punnett Squares • Remember: • BB = homozygous dominant • Bb = heterozygous • bb = homozygous recessive • Each parent donates 1 allele to their offspring • How can we predict what alleles offspring will inherit?

  8. Punnett Squares! Dad Mom Bb bb Offspring Bb bb bb Bb

  9. Ex 1: What is the predicted offspring of combining a parent that is Homozygousrecessive with one that is Heterozygous? bb x Bb b b B b Bb Bb 2/4 = 50% Bb 2/4 = 50% bb bb bb

  10. Ex 2: A heterozygous brown bird is crossed with a black bird. Brown is dominate to black. What is the predicted offspring of the baby birds (genotypes and phenotypes)? Bb x bb B b b b Bb bb 2/4 = 50% Bb brown 2/4 = 50% bb black Bb bb

  11. Self-Quiz S s • One cat carries heterozygous, long-haired traits (Ss), and its mate carries homozygous short-haired traits (ss). Use a Punnett square to determine the probability of one of their offspring having long hair. • In humans, brown eyes (B) are dominant over blue (b). A brown-eyed man marries a blue-eyed woman and they have three children, two of whom are brown-eyed and one of whom is blue-eyed. Draw the Punnett square that illustrates this marriage. What is the man’s genotype? What are the genotypes of the children? s s Ss ss Ss ss b b Man’s genotype- Bb Brown eyed children-Bb, Bb Blue eyed children-bb Bb B Bb b ?b ?b

  12. Intermediate Traits Punnett Squares

  13. Intermediate Traits • Sometimes, one trait does not dominate another. Sometimes, both traits will be expressed. There are two types of intermediate traits • Incomplete Dominance-a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. INBETWEEN trait  • Codominance-cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.  COMBINATION trait

  14. To complete a punnett square for an intermediate trait, use a combination of two different letters instead of upper vs. lower case (because there is no a need to show dominance)

  15. Ex: a red flower and a white flower combine to make pink flowered offspring • R – red flower = RR (or CR CR) • W – white flower = WW (or CwCw) • RW – pink flower = (or CRCw)

  16. A pink flower is crossed with a white flower. What are the genotypes and phenotypes of the predicted offspring? RW x WW R W W W 2/4 RW= pink = 50% 2/4 WW= white = 50% RW WW RW WW

  17. Roan Cow • An example of codominance • Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together).  A good example of codominance. • (the EOC loves to use this example, so learn it!)

  18. Multiple Alleles

  19. REVIEW • BASIC DOMINANCE-When the gene for one trait exists as only two alleles & the alleles play according to Mendel's Law of Dominance, there are 3 possible genotypes (combination of alleles) & 2 possible phenotypes (the dominant one or the recessive one). • INTERMEDIATE DOMINANCE-If there are only two alleles involved in determining the phenotype of a certain trait, but there are 3 possible phenotypes, then the inheritance of the trait illustrates either incomplete dominance or codominance. • MULTIPLE ALLELES-If there are 4 or more possible phenotypes for a particular trait, then more than 2 alleles for that trait must exist in the population.

  20. Examples • Basic-Hitchhiker’s thumb is a dominant trait. The possible genotypes are HH, Hh, hh. The possible phenotypes are Hitchhiker’s or no hitchhiker’s thumb. • Intermediate- • Codominance- The possible genotypes in cattle are RR, RW, WW. The possible pheotypes are red, white or roan. • Incomplete-The possible genotypes in flowers are BB, BR, RR. The possible phenotypes are blue, blue & red or red.

  21. Multiple Alleles • There are multiple alleles in a population causing 4 or more phenotypes of a trait to exist • KEEP IN MIND • There may be multiple alleles within the population, but individuals have only two of those alleles • Why? …because individuals have only two biological parents.  We inherit half of our genes (alleles) from mommy, & the other half from daddy, so we end up with two alleles for every trait in our phenotype.

  22. Example • Human Blood Type: • Blood type exists as four possible phenotypes: A, B, AB, & O. • There are 3 alleles for the gene that determines blood type: A, B, O • Alleles A & B are CODOMINANT • What does that mean? • Allele O is recessive • What is the genotype for O blood? Combination. You “see” both traits. i.e. AB blood. OO

  23. Blood Typing Basics • Blood types are also represented as: IA, IB, io or i- (this is how you will most likely see it expressed on your test. ) PhenotypeGenotype Type O OO (io) Type A AA, AO (IA IA, IB io) Type B BB, BO (IB IB, IB io) Type AB AB (IA IB)

  24. Example • A husband is homozygous for blood group B. He marries a woman with type O blood. What are the blood types of their predicted offspring? B B BO x OO O O BO BO 100% BO 100% type B BO BO

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