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Biology

Biology. Chapter 11: Introduction to Genetics. Genetics – the branch of biology that studies heredity Heredity – the passing on of traits from parent to offspring Chromosomes – the genetic material that is passed from generation to generation Genes – segment of DNA that codes for a trait

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Biology

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  1. Biology Chapter 11: Introduction to Genetics

  2. Genetics – the branch of biology that studies heredity • Heredity – the passing on of traits from parent to offspring • Chromosomes – the genetic material that is passed from generation to generation • Genes – segment of DNA that codes for a trait • Alleles – different forms of genes • 2 Types of Alleles • Dominant • observed trait, or the trait that is expressed • Capital Letter (T) • Recessive • trait that is masked or hidden • Lower Case Letter (t)

  3. You inherit 2 alleles for each gene. One from mom and one from dad. • Homozygous – two same alleles for a trait • Homozygous Dominant – 2 dominant alleles (TT) • Homozygous Recessive – 2 recessive alleles (tt) • Heterozygous – 2 different alleles for a trait (Tt)

  4. Two organisms can look alike but have different gene combinations • Phenotype – the trait you see • Genotype – the actual gene combination • You can’t always know an organisms genotype simply by looking at its phenotype • Genes are lined up on chromosomes • A chromosome can contain a thousand or more genes

  5. 11-1 The Work of Gregor Mendel • Gregor Mendel • Carried out the first important studies of heredity • Father of genetics • He was a monk in an Austria Monastery that was a great scientific research center • First person to predict how traits are transferred from generation to generation

  6. He studied pea plant traits • Pea plants reproduce sexually and contain both male and female gametes • Self Pollination – male and female gametes from the same plant unite • Pure-Bred – the offspring of 1 parent and has identical genes (clone) • Cross Pollination – male and female gametes from different plants unite • Hybrid – the offspring of 2 parents that have different forms of a trait • Mendel carefully controlled his experiments and only studied one pea trait at a time • One trait he studied was height of a pea plant

  7. Monohybrid Cross – a genetic cross involving one trait Tall Pea Plant Short Pea Plant Self Pollinate Tall Pea Plant Short Pea Plant P generation Cross Pollinate 4 Tall Pea Plants F1 generation Self Pollinate F2 generation 3 Tall and 1 Short

  8. From this monohybrid cross Mendel concluded… • Pea plant had two alleles of the gene that determined its height • The allele for Tall is Dominant - T • The allele for short is recessive - t • If the plant was tall its genotype was TT or Tt • If the plant was short its genotype was tt

  9. The Law of Segregation • To explain why in the F1 generation the trait for shortness disappeared and then reappeared in the F2 generation Mendel formulated his Law of Segregation • He concluded that the F1 plants had a allele for tallness and an allele for shortness • Because they have two different alleles they can produce 2 different types of gametes (tall and short) • During fertilization these gametes randomly pair to produce 4 possible combinations of alleles • Law of Segregation – the members of each pair of alleles separate when gametes are formed • Punnett Square – used to predict and compare the genetic variations that will result from a cross

  10. Tt • Alleles separate during gamete formation. t T

  11. Mendel's Cross Mom’s genotype: Tt Dad’s genotype: Tt Genotypic Ratio: TT – 1, Tt – 2, tt – 1 Ratio: 1:2:1 Phenotypic Ratio: Tall - 3, Short - 1 Ratio: 3:1

  12. Mendel’s Dihybrid Crosses • Mendel did another cross which he used pea plants that differed from each other in two traits rather than one • Dihybrid Cross – a cross involves two traits • Will the two traits stay together in the next generation or will they be inherited independently of each other?

  13. Round Yellow Pea Seed Wrinkled Green Pea Seed Self Pollinate P generation: Round Yellow Pea Seed Wrinkled Green Pea Seed Cross Pollinate 16 Round Yellow Pea Seeds F1 generation: Self Pollinate 9 Round Yellow 3 Round Green 3 Wrinkled Yellow 1 Wrinkled Green F2 generation:

  14. The Law of Independent Assortment • Law of Independent Assortment – genes for different traits can segregate independently during the formation of gametes • The gametes, R (round), r (wrinkled), Y (yellow), and y (green), will separate from each other and recombine in 4 different ways RY, Ry, rY, and ry • If the alleles were inherited together, only two kinds of pea seeds would have been produced RY (round yellow) and ry (wrinkled green) • This Law does not hold true all of the time. If the genes are on different chromosomes it is true, but if they are on the same chromosome it is not true.

  15. Beyond Dominant and Recessive Alleles • Not all genes show simple patterns of dominant and recessive alleles • In most organisms genetics is more complicated, because the majority of genes have more than two alleles. • Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes

  16. Incomplete Dominance • Incomplete Dominance – the phenotype of the heterozygote is intermediate between the dominant and recessive • Ex. Red Snapdragon (RR) x White Snapdragon (WW) Pink Snapdragon (RW) • Neither allele is completely dominant over the other

  17. Codominance • Codominant Alleles – the phenotypes of both homozygotes are produced • Both allele are expressed equally • Example • White Horse WW X Tan Horse TT • Roan Horse WT

  18. Multiple Alleles • It is common for more than two alleles to control a trait in a population • Multiple Alleles – traits controlled by more than two alleles Example 1- A rabbit's coat color is determined by a single gene that has at least four different alleles.

  19. KEY C = full color; dominant to all other alleles cch= chinchilla; partial defect in pigmentation; dominant to ch and c alleles ch = Himalayan; color in certain parts of the body; dominant to c allele c = albino; no color; recessive to all other alleles Different combinations of alleles result in the colors shown here.

  20. Full color: CC, Ccch, Cch, or Cc Chinchilla: cchch, cchcch, or cchc Himalayan: chc, or chch AIbino: cc

  21. Example 2 - ABO Blood Groups • Human blood types are determined by the presence or absence of certain molecules on the surface of red blood cells • The best known are ABO groups and Rh groups

  22. ABO Groups • 3 Alleles for blood types IA (A), IB (B), i (O) • IA (A), IB (B) are dominant • i (O) is the recessive • 4 Blood Types

  23. Rh Groups • Stands for rhesus monkey • + allele is dominant • - allele is recessive

  24. Universal Donor – O blood and is most common • Universal Recipient – AB blood and is rare • If blood types are not matched during a blood transfusion, the red blood cells will clump together causing death • Blood typing is also useful in determining parentage

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