Incomplete Dominance

Transcription

1 Biology 3201 Genetics Unit #2: Mendelian Genetics #2 Mendelian Genetics (part 2) and Beyond Incomplete Dominance O Incomplete dominance: a situation where NEITHER of the two alleles for a trait are dominant O Ex. Snapdragon flowers (red, white, heterozygous is pink) O Ex. Four o'clock flowers (red, white, heterozygous is pink) O We represent the alleles with the same capital letter with a to show the difference O Red flowers = RR O White flowers = R R O Pink flowers = RR 1

2 Incomplete Dominance O Ex. What would be the genotypic and phenotypic ratios of a cross between a red flower and white flower? O Parents: Red = RR; White = R R R R RR RR R RR RR Genotypic ratio: 100% RR Phenotypic ratio: all pink R Incomplete Dominance O Ex. 2 What would be the expected ratios of a cross between two pink flowers? O Parents : RR x RR R R R RR RR R RR R R Genotypic ratio: ¼ RR: ½ RR : ¼ R R Phenotypic ratio: ¼ red: ½ pink: ¼ white 2

3 Co-dominance O Co-dominance: a situation where both alleles for a trait may be dominant. Both alleles are expressed in a heterozygous individual. O Ex. O Feather colour in chickens (black and white expressed at the same time is barred plummage) O Roan horses (red and white hair expressed at the same time O Blood type (AB type A and B blood expressed at the same time Co-dominance crosses O In chickens, the gene for black feather colour is co-dominant to the gene for white fur colour. What are the expected genotypic and phenotypic ratios of a cross between a black rooster and a white hen? O Parents: BB x WW O Note: in co-dominant situations, each allele is given a different capital letter. 3

4 Co-dominance crosses B B W BW BW W BW BW Genotypic ratio: 100 % BW Phenotypic Ratio: all barred plummage (black and white) Multiple Alleles O Some genetic traits are expressed by multiple (many) alleles. Such a trait is blood type in humans. The blood type in humans is controlled by 3 alleles resulting in 4 distinct blood types: O Type A O Type B O Type AB O Type O 4

5 Blood types Genotype Phenotype I A I A or I A i Type A I B I B or I B i Type B I A I B ii Type AB (A and B are codominant). Most common blood type O (recessive) Least common blood type Blood Types 5

6 Mixing Blood O Only certain types of blood can be mixed. If you mix the wrong types of blood agglutination (clumping) will occur. This will cause the blood cells in the person being affected to clump together and the person would die from a blood clot in the brain, heart, etc. Red blood cell compatibility Blood group AB individuals have both A and B antigens on the surface of their RBCs, and their blood plasma does not contain any antibodies against either A or B antigen. Therefore, an individual with type AB blood can receive blood from any group (with AB being preferable), but cannot donate blood to any group other than AB. They are known as universal recipients. Blood group A individuals have the A antigen on the surface of their RBCs, and blood serum containing IgM antibodies against the B antigen. Therefore, a group A individual can receive blood only from individuals of groups A or O (with A being preferable), and can donate blood to individuals with type A or AB. 6

7 Red blood cell compatibility O O Blood group B individuals have the B antigen on the surface of their RBCs, and blood serum containing IgM antibodies against the A antigen. Therefore, a group B individual can receive blood only from individuals of groups B or O (with B being preferable), and can donate blood to individuals with type B or AB. Blood group O (or blood group zero in some countries) individuals do not have either A or B antigens on the surface of their RBCs, and their blood serum contains IgM anti-a and anti- B antibodies against the A and B blood group antigens. Therefore, a group O individual can receive blood only from a group O individual, but can donate blood to individuals of any ABO blood group (i.e., A, B, O or AB). (Universal donors) Crosses involving Multiple Alleles O Ex. A woman with blood type A marries a man with blood type AB. What are the possible blood types for the offspring? O Recall: Type A has 2 genotypes; I A I A or I A i, so two separate crosses need to be done 7

8 Crosses involving Multiple Alleles O Cross 1: I A I A x I A I B I A I A I A I A I A I A I A I B I A I B I A I B Genotypic ratio: ½ I A I A : ½ I A I B Phenotypic ratio: ½ type A: ½ Type AB Crosses involving Multiple Alleles O Cross 2: I A i x I A I B I A I A I A I A I A i I B I A I B I B i i Genotypic ratio: ¼ I A I A : ¼ I A i : ¼ I A I B : ¼ I B i Phenotypic ratio: ½ Type A: ¼ Type AB: ¼ Type B 8

9 Chromosome Theory of Inheritance O Proposed by Walter Sutton and Theodor Boveri (1902) O Studied chromosomes during the various phases of meiosis O Findings: O Chromosomes occur in pairs O Chromosome pairs segregate (separate) during anaphase I (this backs up Mendel s claims of the Law of Segregation O Chromosomes align themselves independently along the equator (this backs up Mendel s idea of independent assortment Chromosome Theory of Inheritance O What the theory says O Mendel s factors or genes are carried on chromosomes O The segregation and independent assortment of chromosomes during meiosis is what accounts for inheritance O Note: The work of Sutton and Boveri confirmed Mendel s ideas about genes (factors) and how they are inherited (segregation and independent assortment) 9

10 Thomas Morgan and Gene Linkage O Studied fruit flies (Drosophila melanogaster) O Did crosses involving eye colour O Results: O Found that chromosomes in fruit flies are the same except one pair O He called the dissimilar pair sex chromosomes because he believed they determined the sex of the fly O found that certain traits such as eye colour in fruit flies are found on the X chromosome. This is what he called sex-linkage. Today we called this sex-linked traits or genes Sex-linked traits O Sex-linked trait: a trait that is carried on one of the sex chromosomes (X or Y) O Morgan also found that certain genes on the same chromosome are called linked genes O He said that linked genes get inherited together and not separately as Mendel had proposed (They do not obey Mendel s Law of Independent Assortment). This would account for some differences in ratios of crosses. 10

11 Law of Independent Assortment in Modern Terms O Morgan also found that genes on the same chromosome that are separated by a great distance will separate as a result of crossing over O If crossing over does not occur, genes that are located on the same chromosome will be inherited together while those on separate chromosomes will sort independently. Sex linked traits in humans O Sex-linked traits are traits that are carried on sex chromosomes (X and Y) O Most often the traits are carried on the X chromosome (because of the size difference) O Most of the traits are recessive and sometimes may be lethal O Sex-linked traits affect males more often than female. Since they are recessive, females would have to have 2 defective X chromosomes. Males only have 1. 11

12 Sex linked traits in humans O Examples O Red-green colour blindness O Male pattern baldness O Hemophilia O Duchenne Muscular Dystrophy Sex-linked crosses O Ex. A woman with normal colour vision and with no instances of colour-blindness in her family history marries a man who is colourblind. What are the possible ratios for their offspring? O Parents: X N X N x X n Y O Note: for sex-linked crosses, X and Y chromosomes must be involved to distinguish males and female offspring 12

13 Sex-linked crosses X N X N X n X N X n X N X n Y X N Y X N Y Genotypic ratio: ½ X N X n : ½ X N Y Phenotypic ratio: ½ carrier female: ½ normal male Note: Carriers for recessive sex-linked disorders do not have the disorder, but do have the ability to pass it on to their offspring. Only females can be unaffected carriers Sex-linked crosses O Ex. 2 A woman who is a carrier for male pattern baldness marries a male who is not bald. What are the possible genotypic and phenotypic ratios of their offspring? O Parents: X B X b x X B Y 13

14 Sex-linked crosses X B X b X B X B X B X B X b Y X B Y X b Y Genotypic ratio: ¼ X B X B : ¼ X B X b : ¼ X B Y: ¼ X b Y Phenotypic ratio: ¼ normal female: ¼ carrier female: ¼ normal male: ¼ bald male Note: out of the females, ½ were normal and ½ carriers but physically they would look the same. Sex-linked traits: important points O 1. sex-linked traits are recessive (small letters) O 2. sex-linked traits are carried on the X chromosome, normally O 3. if a person has a big allele and a small allele, they are a carrier of the trait. Only females can be carriers. Males cannot be carriers (only one X chromosome). 14

15 Polygenic Inheritance O Polygenic inheritance: where traits are affected by more than one gene. O Because of this, you get a range of phenotypes instead of just two or three (ex. Not just tall or short but some in the middle). O Continuous variation: variation among individuals in a population where there is a gradient of phenotypes O Ex. Length of an ear of corn O Human skin colour O Human height Polygenic Inheritance O How does this work? O Two or more genes work at the same time for the same trait. This leads to variations in the expression of the trait. O Ex. In corn, the shortest lengths occur when the alleles for both genes (four alleles in total) are homozygous recessive. The longest lengths occur when all the alleles are homozygous dominant. All other lengths occur by other allele combinations. O CUT OFF FOR GENETICS #1 TEST 15