This document discusses Gregor Mendel's principles of inheritance and genetics. It covers Mendel's laws of segregation, dominance, and independent assortment. It also discusses exceptions to Mendelian genetics like incomplete dominance, codominance, multiple alleles, polygenic traits, epistasis, and gene linkage. Thomas Hunt Morgan's work with fruit flies provided evidence that genes are located on chromosomes, not assorting independently as Mendel believed, but rather assorting as linked genes on the same chromosome. Gene mapping using recombination rates helped establish the chromosomal theory of inheritance.
This Power Point Presentation is designed to explain Mendel's experiment on hybridization and dihybrid cross which considers inheritance of two traits at a time and to know whether they are inherited independently or are influenced by each other and also about Law of Independent assortment
This Power Point Presentation is designed to explain Mendel's experiment on hybridization and dihybrid cross which considers inheritance of two traits at a time and to know whether they are inherited independently or are influenced by each other and also about Law of Independent assortment
Dihybrid Crosses, Gene Linkage and RecombinationStephen Taylor
For the IB DP Biology course AHL: Genetics unit. To get the editable pptx file, please make a donation to one of my preferred charities. More information at http://sciencevideos.wordpress.com/about/biology4good/
Biology 103 Laboratory Exercise – Genetic Problems
Introduction
Although the science of genetics has become a highly sophisticated discipline dealing
with the interactions of hereditary factors at the molecular level, it has its roots in the
basic laws of heredity initially discovered and presented by Gregor Mendel more than
one hundred years ago. Mendel's success in discovering these laws was due largely to his
application of the simple rules of mathematical probability - the laws of chance - to his
observations concerning the inheritance of certain characteristics in the garden pea plant.
Reginald Punnett and the Punnett Square
The Punnett square is a diagram used by biologists to determine genotypic probability
within the offspring from a particular genetic cross. The Punnett square shows every
possible genotypic combination of maternal alleles with the paternal alleles for a genetic
cross. Punnett squares only give probabilities for genotypes, not phenotypes. The square
diagram was designed by the British geneticist, Reginald Punnett (1865-1967) and first
presented to the science community in 1905. Punnett’s Mendelism (1905) is considered
the first popular science book to introduce genetics to the public.
Solving Genetic Problems
R
R'
R
RR RR'
R'
RR' R'R'
Maternal alleles
A
A
a
Aa
Aa
Paternal
Alleles
a
Aa
Aa
The first step in solving a genetic problem is to establish the genetic symbols you will use
in your problem solution. Stay consistent by using these same symbols throughout the
problem solving process.
Represent dominant and recessive alleles (different forms of a gene) using traditional
genetic symbols. Dominant alleles should be represented with the capital version of an
alphabetic letter while using the lower case version to show recessiveness. For example:
B = black color, b = white color.
Each individual gene or trait is diploid (2n) in nature and therefore, must be represented
with two alleles. Continuing with the alleles mentioned previously, an individual may
have the genetic makeup BB, Bb, or bb when using those alleles.
Remember that gametes (sperm and egg) are haploid (n) and can only provide one allele
per trait. For example: B or b
An individual’s genotype contains the possible gametes that can be expected to be
produced by that individual. Much of genetics revolves around the probability of the
makeup of gametes. If the individual is homozygous, all of the gametes produced will
possess the same kind of allele. For example, an individual with the genotype BB would
be expected to produce only B gametes and individuals with genotype bb would produce
only b gametes.
If the individual is heterozygous, that is the individual’s genotype contains one dominant
allele and one recessive allele (Bb), the gametes produced will possess one or the other of
the two forms of the gene – B or b. ...
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Understanding the Mahadasha of Shukra (Venus): Effects and RemediesAstro Pathshala
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The Fascinating World of Bats: Unveiling the Secrets of the Night
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2. What we know… Inheritance of biological characteristics is determined by genes Principle of Dominance When there are two or more alleles for a gene, some are dominant while others are recessive Law of Segregation In sexually reproducing organisms, adult cells have two copies of each gene—one from each parent; these genes segregate when GAMETES are formed
3. Does the segregation of 1 pair of alleles affect the segregation of another pair of alleles?
4. Let’s look at Pea shape and pea color Round (R) vs. wrinkled (r) Yellow (Y) vs. green (y) Remember, there are 4 possible gametes If a parent is homozygous dominant (true-breeding dominant) for both shape and color, what will all possible gametes be? If a parent is homozygous recessive (true-breeding recessive) for both shape and color, what will all possible gametes be?
9. What is the phenotype of the F1 offspring? What is the genotype of the F1 offspring?
10. What this first cross told us… All F1 offspring were heterozygous for seed shape (round) and seed color (yellow) RrYy The F1 plant was made from fusing a gamete carrying RY and a gamete carrying ry Will the dominant alleles stay together or separate when making the F2 offspring?
11. Now cross these hybrid (RrYy) plants on a new 4X4 Punnett Square
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13. What does the F2 cross tell us? Are there combinations of alleles that we did not see in either of the parents? This means that the alleles for seed color separated independently than the alleles for seed shape Genes that segregate separately do NOT influence each other’s inheritance
14. Principle of Independent Assortment Genes for different traits can segregate independently during the formation of gametes. This principle helps account for many genetic variations in plants, animals and other organisms.
15. Mendel’s 4 Principle’s Inheritance of biological characteristics is determined by genes Principle of Dominance Law of Segregation Principle of Independent Assortment
16. Exceptions to Mendel Genetics more complicated Some alleles are neither dominant nor recessive Many traits are controlled by multiple alleles or multiple genes
17. Incomplete Dominance When red flowered (CRCR) plants were crossed with white flowered (CWCW) plants they made…pink flowers (CRCW) Which allele is dominant? neither Incomplete dominance: Case in which one allele is not dominant over another The heterozygous phenotype is somewhere between the two homozygous phenotypes
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19. Codominance Both alleles contribute to the phenotype Chickens Allele for black feathers is codominant with allele for white feathers Chicken looks speckled with black and white feathers Not like the blending of dominant phenotypes… BOTH dominant phenotypes show up In humans Gene for protein that controls cholesterol levels in the blood People with heterozygous form make both types of protien
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21. Multiple Alleles When a gene has more than two alleles NOT more than 2 alleles for a person but MORE than 2 alleles for the trait exist Coat color in rabbits A single gene for coat color At least 4 different alleles Simple dominance and make 4 possible coat colors Genes for human blood type 3 different alleles: IA, IB, I You can get different genotypes: IAIA IAi IAIB IBIB Ibi ii You can get different Phenotypes: Type A (dom) Type B (dom) Type AB (dom) Type O (recessive)
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26. Polygenic Traits “Poly” many “-genic” genes Traits controlled by two or more genes Several genes interact to produce a trait Wide range of phenotypes Skin color Four different genes Fruit Fly eye color Three genes make the reddish brown pigment
27. Epistasis When the expression of one gene effects the expression of another gene Ex. Fur color in mice…controlled by 2 separate genes Gene 1 Brown fur pigment (BB or Bb) is dominant over gray fur pigment (bb) Gene 2 Coat Pigment depositing gene Dominant gene (CC or Cc) means fur will get pigment and this is determined by gene one Recessive gene (cc) means that no pigment will be deposited on fur…whether the they have the gene for black or brown fur The gene for Pigment Deposition is the EPISTATIC gene because it alters the Phenotypic ration Sd
28. Homework Complete a Dihybrid cross for 2 mice that are heterozygous for both Brown fur (Bb) and Pigment deposition (Pp) Predict your phenotypic ratio… What are you final phenotypic ratios? Write a paragraph explaining your results.
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30. Genes and the Environment Genes provide the plan for development How the plan unfolds depends on the environment Example: Sunflower has genes for height and color of flowers But these traits are also influenced by climate, soil conditions and water availability
33. Applying Mendel’s Principles… Apply Mendel’s Principles to many organisms, including humans Thomas Hunt Morgan (1900’s) American geneticist Common fruit fly Drosophiliamelanogaster Produced offspring very quickly Single pair of flies=100 offspring Mendel’s principle’s were tested with Drosophilia and many other organisms and they applied to all of them as well
34. Would genes on the same chromosome be inherited together? Thomas noticed that almost every single time he crossed two flies that each had red eyes and mini wings, the offspring almost always inherited BOTH red eyes and mini wings This went against Mendel’s Principle of Independent Assortment…(review!)
35. Gene Linkage Thomas Hunt Morgan gave us the answer 1910 PRINCIPLE of LINKAGE 50 Drosophilia genes Seemed to contradict Principle of Independent Assortment b/c certain genes were always inherited together He grouped the fly’s genes into linkage groups
36. Linkage groups are made up of genes that seem to be inherited together Linkage groups assort independently but all genes in one group are inherited together Drosophilia 4 linkage groups 4 chromosomes What can be concluded?
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38. Conclusions Each chromosome is actually a group of linked genes Mendel’s Principle of Independent Assortment holds true but an adjustment needs to be made… IT IS THE CHROMOSOMES THAT ASSORT INDEPENDENTLY, NOT THE INDIVIDUAL GENES
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41. How Mendel missed it… What 7 genes did he study? 6 of the 7 genes were on different chromosomes Two genes were on the same chromosome but they were so far apart on the same chromosome that they assorted independently
42. If two genes are on the same chromosome, does that mean they are linked for ever? No, chromosomes cross over during meiosis so they can separate Crossing over produces new combinations of alleles Important for genetic diversity
43. Lucky Student Alfred Sturtevant 1911 Columbia University Worked in Morgan’s Lab Hypothesis If two genes are farther apart on a chromosome, the they are more likely to be separated during meiosis Experiment Measured the rate at which linked genes were separated and recombined to make a “map” of distances between genes Conclusion Recombination rates could be used to make gene maps Gene maps showed the location of a gene on a chromosome
44. Gene Linkage and Crossing Over The farther apart 2 genes are on a chromosome…. The more likely they are to “cross-over” The closer two genes are on the same chromosome… The less likely they are to be separated
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49. Steps for Dihybrid cross Make a Key Trait 1: height Dominant phenotype: Tall TT or Tt (ways to get it) Recessive phenotype: Short tt Trait2: color Dominant phenotype: Purple PP or Pp Recessive phenotype: White pp Write out genotypes for each parent _ _ _ _ x _ _ _ _ Write Out Gametes for each Parent (use arrows) 4 gametes for each parent (_ _) Make Punnett Square (16) boxes Label parent one and write gametes along top Label Parent 2 and write their gametes on side Fill in each box (should have 4 letters) Tally genotypes Write out 4 possible phenotypic combinations Dominant trait 1 and dominant for trait 2:______ Dominant trait 1 and recessive trait 2:_______ Recessive trait 1 and Dominant trait 2:_______ Recessive trait 1 and recessive trait 2:________ Tally Phenotypes (should =16) Write phenotypic ratio __dd__:__dr__:__rd__:__rr__ Dominant- capital letter D Recessive- lower case d Homozygous- 2 of the same size letter If its two little letters recessive dd If its 2 big letters it is dominant DD Heterozygous- 2 different size letters (capital and lowercase) Dd ALWAYS dominant