- - AGRICULTURE CORE CURRICULUM - - (CLF3000) Advanced Core Cluster: ANIMAL SCIENCE (CLF3250) Unit Title: LIVESTOCK BREEDING AND GENETICS ____________________________________________________________________________ (CLF3253) Topic: DOMINANT AND Time Year(s) RECESSIVE GENES 3 hours 3 / 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson, the student will be able to: Learning Outcome #: (D-4) - Diagram the phenotypic and genotypic results of a cross, using traits common to modern livestock, which exhibit classic dominant and recessive characteristics. (D-5) - Diagram a dihybrid cross (e.g. using two heterozygous gene pairs) and determine the genotypes of the offspring. Special Materials and Equipment: Plenty of scratch paper and Supplemental Worksheet #2. References: Ayala, Francisco, and Kiger, John A. (1980). MODERN GENETICS. Roediger, Roger D. (1977). LIVESTOCK BREEDING. Evaluation: Quiz by instructor and completion of Supplemental Worksheet #2. TOPIC PRESENTATION: DOMINANT AND RECESSIVE GENES REVIEW: Basic Core Curriculum CLF254. A. Mendelian genetics 1. Dominant and recessive traits: a. These are traits controlled by 1 pair of genes, and are referred to as QUALITATIVE TRAITS. Some examples include: 1) coat color in Angus cattle, 2) polled or horned trait in cattle, 3) white color in swine, and 4) white wool in sheep. b. The term DOMINANT refers to the situation when one gene completely masks the effect of the other. For example: 1) The black coat color is dominant over red coat color in Angus cattle. 2) The dominant variation of the trait is indicated by a capital letter, such as "B" for the black coat color. 3) The degree of dominance depends upon the animal's entire genetic makeup together with the environment to which it is exposed. c. The term RECESSIVE refers to the situation when a gene will not show its effects if there is a dominant gene present. For example: 1) The red coat color is recessive in Angus cattle. 2) The recessive variation of the trait is indicated by a lower case letter, such as "b" for red coat color. 2. Determining the genotype of an individual: a. HOMOZYGOUS means that the gene on each chromosome in the pair codes for the same variation of the trait. For example: 1) A cow that is homozygous for black coat color could be indicated by a genotype of "BB". 2) If she is red, her genotype must be "bb", since red color is recessive (this only occurs when there are no dominant genes present). b. HETEROZYGOUS means that the two alleles at a gene locus are different; each codes for one of the variations of the trait. For example: 1) A cow that is heterozygous for black coat color could be indicated by a genotype of "Bb". 2) The black gene is the dominant one and is displayed phenotypically. c. The Punnett Square method: 1) This may be used to determine the percentage of offspring that will show each variation of the trait. For example: a) A black cow with the genotype "Bb" is bred to a black bull with the genotype "Bb". b) What will be the percentage of calves having each possible genotype? c) The bull produces sperm carrying 50 percent "B" genes and 50 percent "b" genes, the cow has the same percent of eggs - 50 percent "b" and 50 percent "B" genes. d) We will draw a square and divide it into 4 equal boxes, placing each possible gene above or beside each box, by combining these genes. We find: | B | b | _|_______|________| B | BB | Bb | _|_______|________| b | Bb | bb | _|_______|________| 25 percent "BB" 25 percent "bb" 50 percent "Bb". e) The phenotypes will be 25 percent red and 75 percent black since "Bb" and "BB" have the same phenotype. __________________________________________________________ ACTIVITY: 1. Practice using the Punnett Square method and the following traits: "P" polled cattle, "p" horned cattle, "W" white hogs, "w" other colors. __________________________________________________________ B. Dihybrid Crosses 1. This refers to crosses using two separate traits. For example: a. The polled trait "P" and black coat color "B" will be used to demonstrate the dihybrid cross. b. Remember that GENES DETERMINING THESE DIFFERENT TRAITS ARE TRANSMITTED INDEPENDENTLY. 2. Punnett Square method will be used to determine the percentages of the various outcomes: a. Let's say a cow has the genotype "BbPp" ("B" = black, "b" = red, "P" = polled, and "p" = horned). b. The bull has the genotype "BbPp" also. 1) If we breed these two, what are the genotypic percentages of the offspring? a) First, determine which genotypes would be present in the eggs and sperm as they will be the same for both the cow and the bull. b) "Bp", "BP", "bp", and "bP" are the possible genotypes. c) Next, draw a square and put the genotypes for the cow down the side of the square and the genotypes for the bull across the top of the square. See below, then fill in the rest of the diagram: bull | Bp | BP | bp | bP | _|_______|_______|_______|______| Bp | | | | | _|_______|_______|_______|______| BP | | | | | cow _|_______|_______|_______|______| bp | | | | | _|_______|_______|_______|______| bP | | | | | _|_______|_______|_______|______| d) We can determine the percentages of the various genotypes by adding up similar genotypes. What are all the possible genotypes? "BBpp" = 1/16 "bbPp" = 2/16 (1/8) "BBPp" = 2/16 (1/8) "BBPP" = 1/16 "Bbpp" = 2/16 (1/8) "BbPP" = 2/16 (1/8) "BbPp" = 4/16 (1/4) "bbpp" = 1/16 "bbPP" = 1/16 **NOTE; Quick check, if the totals add up to 16/16, you have done it right! e) What are the possible phenotypes? Black and polled = 9/16 Red and polled = 3/16 Red and horned = 1/16 Black and horned = 3/16 __________________________________________________________ ACTIVITY: 1. Use Supplemental Worksheet #2 to practice genetic segregation problems. __________________________________________________________ Supplemental Worksheet #2 MENDELIAN GENETICS Name____________________________________ Date____________________________________ Per.____________________________________ 1. Mule foot is a dominant trait in swine where the toes are fused together. What could be the letter indicating this trait? _________ What would be the letter for the normal condition?________ If we breed a heterozygous boar to a homozygous recessive (normal) sow, how many pigs would show the mule foot trait? (what percentage?) Show your work! 2. Black coat color in horses is dominant over chestnut. If we make this cross: "BB" x "bb", what will the phenotype of the foals be? 3. Black and white coat color patterns in Holstein cattle are dominant over red and white patterns. If we breed a red and white bull to a heterozygous cow, what are the heritability percentages of the possible genotypes? Show your work! 4. The trotting gait ("T") is dominant over the pacing gait ("t") in horses. Suppose we breed a black pacer ("BBtt") to a chestnut trotter ("bbTT"). What would be the possible genotypes and their frequency? Show your work! 5. If we intermated offspring resulting from the above mating (BBtt x bbTT), what are all the possible phenotypes and their expected frequencies? 6. The white-face trait is dominant ("W") over solid colored faces ("w") in cattle. Suppose we breed a white-face, red-bodied bull ("Wwbb") to a black-faced, black bodied cow ("wwBb"). What are all of the possible genotypes? 7. What are all of the possible phenotypes for the above cross of white and solid face colored cattle? 5/2/90sg #%&C