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Exploring Mendelian Genetics: Traits, Inheritance, and Patterns

This resource delves into the principles of heredity as established by Gregor Mendel through his experiments with pea plants. Key concepts include the identification of traits determined by genes, the laws of segregation and independent assortment, and the definition of genotypes and phenotypes. It highlights how alleles dictate physical features and the significance of Punnett squares in predicting offspring traits. This foundational understanding of genetics is essential for grasping how traits are passed down through generations.

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Exploring Mendelian Genetics: Traits, Inheritance, and Patterns

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  1. Heredity – or What did Mendel do? Biology

  2. Mendel’s main points: • Physical traits are determined by “factors” (genes) passed down by both parents. • “Factors” are passed down in predictable patterns from one generation to the next.

  3. Other points • Mendel grew pea plants to determine what “traits” were passed on to offspring. (chose 7 traits to study) • He did not know about genes.

  4. Mendel’s peas – flower color only

  5. Mendel’s Generations • Parent Generation – the original breeding plants (parents) • F1 Generation – the first “offspring” that resulted. (children) • F2 Generation – the “offspring” that result from mating 2 of the F1 Generation plants (grandchildren) Go back to picture

  6. Vocabulary • Traits – features that make up who you are (determined by your genes) • Phenotype – physical makeup (example – brown hair, pointy ears) • Genotype – genetic makeup (example – genes on chromosomes)

  7. Alleles • An alternative form of a gene located at a specific position on a specific chromosome. (photo next slide) • Organisms have 2 alleles for each trait: HH, Hh, or hh (each letter is an allele) • Your chromosomes occur in pairs (called homologous pairs)

  8. Alleles for one trait – flower color One allele comes from father, other allele comes from mother

  9. Mendel’sLaw of Segregation • Alleles for a trait separate when gametes form during meiosis. • Resulting gametes have only one allele for each trait.

  10. What this means…… • 1. A gene exists in more than one form – gene for flower color might be either white or purple. • 2. You inherit randomly a “form” of the gene from each parent – these “forms” are called alleles.

  11. What this means…… • 3. When you form sex cells, alleles separate and resulting eggs or sperm contain only 1 allele (your body cells have both alleles) • 4. If Alleles are different (purple vs. white), one is dominant, the other is recessive.

  12. Genotypes • Dominant versus Recessive alleles • Possibilities = Homozygous dominant, Heterozygous, Homozygous recessive • What are these?? Hh, hh, HH • Dominant trait “hides” recessive

  13. Practice • T = tall t = short • TT = • Tt = • tt = • Which is homozygous dominant? • Which is heterozygous? • What’s the other one called?

  14. Mendel’s Law of Independent Assortment • Allele pairs separate independently of each other during gamete formation. • Example – the gene for hair color is passed on to offspring independently from eye color. • Also – which alleles you get from each parent is random

  15. How chromosomes line up before division is random Tetrads – group of 4 chromosomes

  16. Inheritance • You inherited genes from both your father and your mother. • Meiosis – because of “crossing over”, each time your parents had a child, different traits were passed on. • Crossing over – results in variation.

  17. Crossing over

  18. Review • When you cross parents (P generation) you get a ____ generation. • What you physically look like is your _____. • Write an example of heterozygous. • Write an example of homozygous recessive. • Alternate forms of the same gene are called _______.

  19. Review • Your chromosomes occur in homologous pairs. Where did you get these? • Which of Mendel’s laws says that different alleles separate independently of each other during meiosis? • What’s Mendel’s other law called? • What causes variation among you and your siblings? stop

  20. How to determine traits of offspring • Punnett squares are used to find the “possible” traits an offspring can have. • Cross a father with a mother • Each resulting “box” = probability of an offspring having that trait (genotype and phenotype)

  21. Monohybrid Cross – Parents

  22. Monohybrid Cross

  23. Mendel’s peas (flower color) (PP) (pp) (All Pp) Phenotype: 3 purple, 1 white Genotype: 1 PP, 2 Pp, 1 pp

  24. Monohybrid crosses – Punnett squares * * *(Possible alleles to pass down)

  25. Practice Cross a homozygous male for pointy ears with a heterozygous female for pointy ears. (use letter “e”) Pointy ears are dominant over non-pointy ears. Phenotype ratio: Genotype ratio:

  26. Answer E E Genotype ratio: 2:2 or 50% EE, 50% Ee (2 = EE, 2 = Ee) But…reduce ratio to 1:1 EE EE E e Ee Ee Phenotype ratio: 4:0 pointy ears or 100%

  27. Make the following crosses • 1. HH X hh • 2. Aa X AA • 3. Ff X ff • 4. BB X Bb • 5. dd X dd • 6. Ee X ee • 7. Long haired male with a short haired female. Long is dominant over short. Male is heterozygous. Use “h’s”. • 8. Pointy earred male with pointy earred female. Pointy is dominant. Male is homozygous, female is heterozygous. Use “e’s”. stop

  28. Special situation – Incomplete Dominance • Heterozygous offspring show a “blending” of the parents phenotypes. • Example: red flower (RR) X white flower (rr) = pink flower (Rr)

  29. Incomplete Dominance (rr) (RR) (Rr)

  30. Codominance • Both alleles of the parents are expressed in the heterozygous offspring. • Example: Red cow (RR) X White cow (rr) = Red and white spotted cow (Rr)

  31. Codominance

  32. Multiple alleles • Trait is determined by genes with 3 or more alleles • Example: blood types. Type O = ii; recessive. Type A = IAIA or IAi Type B = IBIB or IBi Type AB = IAIB. A and B are codominant

  33. Linked genes • Genes that tend to be inherited together, because they are located close to each other on the chromosome. • A result of crossing over.

  34. Polygenic Traits • Traits that are a results of more than one gene. • Example of polygenic traits: skin color or eye color

  35. Polygenic traits

  36. Sex-linked traits • Female chromosomes = XX • Male chromosomes = XY • “Y” chromosome is shorter than “X” so cannot carry many traits. “X” chromosomes can carry more traits – are larger so can carry more genes.

  37. Sex-linked traits • Traits that are carried on the X or Y chromosomes. X-linked more common since has more room for genes. • If trait is recessive, male more likely to show it. (XY – Y cannot hide trait) • Examples: Hemophilia, colorblindness • XHXh XHXH XhXh XHY XhY

  38. Practice – sex-linked trait Use the letter “h”. Mother is a carrier for hemophilia. Father does not have hemophilia. Hemophilia is a recessive trait.

  39. Answer How do you know which is male vs. female? Name genotype and phenotype of each offspring?

  40. Another – sex-linked trait Use the letter “t”. Mother lacks a tail. Father has a tail. Tails are a dominant trait. stop

  41. Dihybrid Crosses AaBb *Parents (both Heterozygous for both traits). Different colors mean offpsring have different phenotypes. AaBb Phenotypic ratio: 9:3:3:1 Know this ratio for heterozygous parents

  42. How to set it up and read it… (GGbb) (ggBb)

  43. Answer - volunteer Phenotypic ratio:

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