Genetics and Inheritance
Download as PPTX, PDF28 likes25,986 views
The document provides a comprehensive overview of genetics and inheritance, focusing on key concepts such as genes, alleles, dominant and recessive traits, and the principles of segregation and independent assortment. It details Gregor Mendel's experiments with pea plants, explaining how he formulated the foundation of heredity through concepts like purebreds, hybrids, complete dominance, incomplete dominance, and co-dominance, as well as dihybrid crosses. Additionally, the document discusses multi-allele systems, sex linkage, and polygenic inheritance, providing definitions and explanations for various genetic terms and concepts.
Genetics and Inheritance
- 1. M R S J A N D Y Genetics and Inheritance
- 2. A Quick Review A gene is a section of DNA that is transcribed and translated into a single protein Each chromosome has up to 25,000 genes Humans have 46 chromosomes. (23 homologous from mom and 23 homologous from dad)
- 3. Gregor Mendel A monk who began studying pea plants in 1843 Discovered and described the basic principles of heredity (how genes are passed from parents to offspring) Mmm… peas….
- 4. Mendel’s Experiments Mendel started fertilizing pea plants by hand He realized that if he bred a “tall” plant with another “tall” plant that the offspring would all be tall. Parents called P1 generation Offspring called F1 (fillial) He called the parents “purebreds” as they produced offspring that looked exactly like them Studied seed shape, plant height, pod color, flower color… X P1 F1
- 5. Mendel’s Experiments He then bred pure plants with different characteristics (green pod plant with yellow pod plant) and the offspring (F1) all turned out green! Where did the yellow pods go? He called these offpsring hybrids (offspring produced by breeding two different pure lines) He then bred these F1 plants to produce an F2 generation ¼ of the F2 generation plants had yellow pods and ¾ had green pods. What gives? Clearly he thought something strange was going on… F1 F2 X
- 6. Mendel’s Hypothesis Mendel then hypothesized that there are two possibilities for each trait (green or yellow pods) He called the green pods a dominant trait because it was a more powerful trait that showed up more often He called the yellow pods a recessive trait because it sometimes disappeared and showed up less often He then realized that if an offspring had one dominant (green) and one recessive trait (yellow), the dominant trait would show up (green pods) If the offspring had two recessive traits, the recessive trait would show up (yellow pods)
- 7. Mendel’s Hypothesis He did the same experiment except with plants that had purple and white flowers and saw the same pattern! Purple = dominant White = recessive Because the dominant (purple) trait always covered up the recessive (white) trait he called this complete dominance
- 8. Stop, Pause and Think! Think about the following… 1)What are sections of DNA that contain heredity information ? 2) How does a purebred differ from a hybrid? 3) In a cross that displays completed dominance, if an offspring carries 1 dominant factor and 1 recessive factor, which trait will the offspring have?
- 9. Stop, Pause and Think! 1) What are sections of DNA that contain heredity information ? genes 2) How does a purebred differ from a hybrid? A purebred the offspring always has the same traits as the parent A hybrid is the result of breeding two different purebreds. 3) In a cross that displays completed dominance, if an offspring carries 1 dominant factor and 1 recessive factor, which trait will the offspring have? The dominant trait
- 10. Stop, Pause and Think! You should be able to define the following (Write down in notebook) 1) Gene 2) Purebred 3) Hybrid 4) Complete dominance 5) Dominant trait 6) Recessive trait
- 11. Principal of Segregation Each chromosome has 2 copies of a gene or trait (one on each chromatid) These two chromatids separate, or segregate during meiosis when gametes are formed Each parent contributes one of its copies of a trait to its offspring The chances of contributing either factor are equal (50/50)
- 12. Alleles We now know that the units of heredity are genes, and the different forms of the genes are called alleles If the offspring has two dominant alleles, the offspring will appear to show the dominant trait If the offspring has one dominant allele and one recessive allele, the offspring will show the dominant trait If the offspring has two recessive alleles, the offspring will show the negative trait Green + Green = Green + Yellow = Yellow + Yellow =
- 13. Alleles Each individual carries one copy of a gene (allele) from their mother and one copy of a gene (allele) from their father Chromosome #3 from mother Chromosome #3 from father
- 14. Representing Genes and Alleles Scientists use abbreviations to show dominant and recessive alleles They use the same letter for the dominant and recessive allele for each trait Dominant allele is capitalized: Green pods (G) Recessive alleles are lower case: Yellow pods (g)
- 15. Genotype vs Phenotype Genotype: which copies of the gene the organism has What the genes code for Phenotype: which trait does the organism show What you see
- 16. Determining Genotype If you know the phenotype can you determine the genotype? If the pea plant has purple flowers: could have one dominant and one recessive (Pp), or two dominant (PP) alleles If the pea plant has white flowers: must have two recessive (pp) alleles If the organism has two of the same allele, the organism is called homozygous PP = homozygous dominant pp = homozygous recessive If the organism as one of each allele, the organism is called heterozygous (Pp)
- 17. Punnett Squares A way to visualize test crosses (breeding two organisms) Can be used to determine the probability of genotypes and phenotypes of offspring
- 18. Punnett Squares Now you try! ? ? ? ?
- 19. Stop, Pause and Think! Think about the following… 1) What does the principle of segregation state? 2) What is an allele? 3) What is the difference between a genotype and a phenotype? 4) Define homozygous and heterozygous.
- 20. Stop, Pause and Think! 1) What does the principle of segregation state? Members of each pair of genes separate when gametes are formed 2) What is an allele? Different representations of a gene 3) What is the difference between a genotype and a phenotype? Genotype is the representation of the alleles; ex: BB or bb Phenotype is the physical representation of the trait; ex: Black or white 4) Define homozygous and heterozygous. Homozygous is when both alleles are the same: ex: BB or bb Heterozygous is when you have one dominant and one recessive allele; ex: Bb
- 21. Stop, Pause and Think On your vocabulary sheet define the following 1) Allele 2) Genotype 3) Phenotype 4) Homozygous 5) Heterozygous • In your notes write down what the Principle of Segregation States
- 22. Incomplete Dominance In the pea plants Mendel studied, 1 allele was clearly dominant over the other However, this is not always the case! Some alleles show incomplete dominance (blend of traits instead of one or the other) Ex: red flowers and white flowers make pink flowers In incomplete dominance the phenotype of a homozygous dominant individual will be different than the phenotype of the heterozygous individual CRCR = CRCW= CWCW =
- 24. Co-dominance Co-dominance occurs when both alleles are visible in the phenotype (but not mixed like incomplete dominance!) Ex: This Camellia flower is not pink, instead its petals have red and white parts
- 25. Stop, Pause and Think! How could you tell the difference between an organism and its offspring that show complete dominance and an incomplete dominance? What is the difference between incomplete dominance and co-dominance?
- 26. Stop, Pause and Think! How could you tell the difference between an organism and its offspring that show complete dominance and an incomplete dominance? Organisms with complete dominance will only show two variations of a trait. Organisms with incomplete dominance will show three variations of a trait (one mixed) What is the difference between incomplete dominance and co-dominance? Incomplete dominance is when two traits are mixed (red + white = pink) Co-Dominance is when two traits are both expressed (red + white = part red and part white)
- 27. Stop, Pause and Think! On your vocabulary sheet define the following 1) Incomplete Dominance 2) Co-Dominance
- 28. Multi-allele Systems Some traits are the result of more than 2 alleles at a locus (location of an allele on a chromosome) Ex: ABO blood system IA = produces A antigen on blood cell produces B anti-body in blood serum IB = produces B antigen on blood cell produces A anti-body in blood serum i = produces no antigen on blood cell produces both A and B anti-bodies in blood serum IA and IB are co-dominant i is recessive
- 29. The ABO Blood System Clots when exposed to B-antigen Clots when exposed to A-antigen Does not clot when exposed to antigens Clots when exposed to A or B- antigens
- 30. Law of Independent Assortment Mendel showed that dominant traits do not always show up together (don’t always see green pods and purple flowers in the same plant) Law of Independent Assortment: 2 or more pairs of alleles separate independently during the formation of gametes Ex: equal chances of inheriting blonde hair allele/brown eyes allele or blonde hair allele/blue eyes allele Traits are inherited separately from each other
- 31. Sex Linkage Autosomal trait o A gene carried on one of the 22 pairs of non-sex chromosomes Sex-linked trait A gene carried on one of the pairs of sex- chromosomes If female XX if male XY If X chromosome codes for the allele, then females will have 2 copies of the allele and males will only have one copy Y-linked trait Only males (XY) will have a copy of the allele Very rare in humans
- 32. Showing Sex-Linkage Symbols are written as superscript of the sex chromosome: Xa - X chromosome carrying the recessive allele XA - X chromosome carrying the dominant allele X - No superscript is used for the normal (wild type) allele If you see a different ratio of the trait in males and females its probably a sex-linked trait!
- 33. Pedigrees Pedigrees are used to determine mode of inheritance when few individuals, but several generations are involved Assume genetic trait discussed is rare, so individuals marrying into the family are not assumed to carry the trait Symbols: female not affected male not affected female affected male affected female carrier male carrier
- 35. Stop, Pause and Think! On your vocabulary sheet define the following 1) Multi-allele system
- 36. Polygenic Inheritance Most traits are not limited to two possibilities (green or yellow) Most traits are a continuum (many act together to determine phenotype) Ex: height, skin color Polygenetic Inheritance: two or more genes act additively on a trait
- 37. Stop, Pause and Think! Think about the following… 1) What is an example of a trait that is the result of multi-allele systems? 2) Which blood type is homozygous recessive? 3) How would you be able to tell if a disease was a sex linked trait by looking at a pedigree?
- 38. Stop, Pause and Think! 1) What is an example of a trait that is the result of multi-allele systems? Blood type 2) Which blood type is homozygous recessive? O blood 3) How would you be able to tell if a disease was a sex linked trait by looking at a pedigree? If the trait is present in a higher ratio in one sex when compared to the other
- 39. Stop, Pause and Think On your vocabulary sheet define the following 1) Multi-allele system 2) Law of Independent Assortment 3) Autosomal trait 4) Sex-linked trait 5) Pedigree 6) Polygenic Inheritance
- 40. Dihybrid Crosses Dihybrid cross involves the cross of two organisms while looking at two different genes Can demonstrate independent assortment Cross organism with two homozygous dominant and two homozygous recessive genotypes YYRR x yyrr can produce: YYRR and YyRr = yellow round YYrr = yellow wrinkled yyRR and yyRr = green round yyrr = green wrinkled Produces these genotypes in a 9:3:3:1 ratio
- 41. Diagramming a Dihybrid Cross
- 42. How to do a Dihybrid Cross Lets cross two plants one with (R = red flowers) (T = tall) RrTt and RrTt Draw out a 4 x 4 Punnentt square
- 43. How to do a Dihybrid Cross Crossing RrTt and RrTt Start at the top and fill out the first allele of the top line (R and r) RRrr
- 44. How to do a Dihybrid Cross Crossing RrTt and RrTt Now do the same thing with the second alleles but alternating (T and t) R R r r T T t t
- 45. How to do a Dihybrid Cross Crossing RrTt and RrTt Now do the same thing with the second set of alleles RT Rt rT rt RT Rt rT rt
- 46. How to do a Dihybrid Cross Crossing RrTt and RrTt Starting at the top fill out the first alleles on the whole Punnett square R R r r R R r r R R r r R R r r RT Rt rT rt RT Rt rT rt
- 47. How to do a Dihybrid Cross Crossing RrTt and RrTt Do the same thing with the first allele on the vertical axis RR RR rR rR RR RR rR rR Rr Rr rr rr Rr Rr rr rr RT Rt rT rt RT Rt rT rt
- 48. How to do a Dihybrid Cross Crossing RrTt and RrTt Starting at the top fill out the second allele on the whole punnet square RRT RRt rRT rRt RRT RRt rRT rRt RrT Rrt rrT rrt RrT Rrt rrT rrt RT Rt rT rt RT Rt rT rt
- 49. How to do a Dihybrid Cross Crossing RrTt and RrTt Now do the same thing with the second allele on the vertical axis RRTT RRtT rRTT rRtT RRTt RRtt rRTt rRtt RrTT RrtT rrTT rrtT RrTt Rrtt rrTt rrtt RT Rt rT rt RT Rt rT rt
- 50. How to do a Dihybrid Cross Analyze the data! Make a tally of all possible phenotypes RRTT RRtT rRTT rRtT RRTt RRtt rRTt rRtt RrTT RrtT rrTT rrtT RrTt Rrtt rrTt rrtt RT Rt rT rt RT Rt rT rt Red/Tall – IIII IIII = 9 Red/Short- III = 3 White/Tall- III = 3 White/Short- I = 1 9:3:3:1 Ratio