Honors Genetics 1. Gregor Mendel (1822-1884) German monk at the Augustine Abbey of St. Thomas in Brno (today in the Czech Republic). He was a gardener, teacher and priest. Mendel conducted experiments with pea plants in the monastery's garden. He conducted his research for 7 years and ultimately using 29,000 pea plants. Published his research in an obscure journal. 2. Mendel's Work What Mendel did was breed pea plants and observe their patterns of inheritance. He exerted a great amount of control over the reproduction of the plants and kept extremely detailed records. He prevented the plants from self-pollinating. This means that he had to pollinate all of his plants by hand. He first created plants that when self-pollinated made identical plants. These types of plants are true breeding. True breeding: When a plant is self-pollinated, its offspring are identical it. 3. Mendel's Plant Traits Mendel looked at seven different traits. He was lucky because these seven traits are located on different chromosomes. Seed shape: Round or wrinkled Seed color: Yellow or green Flower color: Purple or white Seed pod color: Green or yellow Seed pod shape: Smooth or constricted Plant height: Tall or dwarf Flower position: Axial or terminal 4. Pea Plants Each pea plant that was crossed only differed in one trait (for example, if he wanted to see the result of crossing a true breeding purple flowered plant with a true breeding white flowered plant, they would be the exact same and true breeding for every other trait). He called the parent generation P; the first generation of offspring were called F 1 and the second generation of offspring were called F 2 and so on (the F stands for filial ). 5. Definitions Genetics: Study of heredity Heredity: Set of traits passed from parent to child Genotype: Makeup of the chromosome itself (the forms of genes) Phenotype: Physical characteristic or trait (example: tall) Dominant: Trait expressed, Capital Letter (example: B ) Recessive: Trait masked, lowercase letter (example: b ) Alleles: Different forms of the same gene (P codes for purple flowers; p codes for white flowers; they both code for flower color, but they code for different colors) Homozygous: 2 identical alleles, called true breeding (example: BB or bb ) Heterozygous: 1 dominant allele, 1 recessive allele, called Hybrids or Carriers (example: Bb ) Hybrids: Offspring that are the result of two genetically different parents Purebreds: Offspring that are the result of two genetically similar parents Test Cross: The crossing of an organism, with an unknown genotype, to a homozygous recessive organism (this organism is called the tester).
6. Mendel's Results When Mendel crossed two true breeding plants (let's say for flower color a purple flowered plant crossed with a white flowered plant), all of the F 1 plants had purple flowers. It is important to point out two things: 1. He got the same shade of purple; he didn't get light purple colored flowers there was no blending of the two colors. 2. None of the flowers were white. Mendel called the trait that showed up in the F 1 generation the Dominant Trait, and the trait that failed to show up in the F 1 generation was called the Recessive Trait. Mendel used symbols to represent the different forms of the traits. He used upper-case letters to represent the Dominant form and lower-case letters to represent the recessive form. For example, P for the purple trait and p for the white trait. Mendel wondered what happened to the recessive trait (the white flowers), so he allowed the F 1 generation to self-pollinate. Mendel found that in the F 2 generation, ¾ of the flowers were purple and ¼ of the flowers were white. Mendel realized that some plants with purple flowers only gave rise to plants with purple flowers when allowed to self-pollinate. Other plants with purple flowers gave rise to plants with purple flowers and plants with white flowers when allowed to self-pollinate. We now say that the true breeding plants are homozygous (only give rise to purple flowers or white flowers). The plants that give rise to both purple flowered and white flowered plants are said to be heterozygous. Mendel found that: ¼ of the F 2 generation had purple flowers and were homozygous (PP) ½ of the F 2 generation had purple flowers and were heterozygous (Pp) ¼ of the F 2 generation had white flowers and were homozygous (pp) 7. Mendel's Laws of Inheritance From these results, Mendel formed a number of conclusions, two of which we will look at. Mendel's First Law: Law of Segregation Only one form of a characteristic/trait can be represented in a gamete. Example: When a heterozygous purple-flowered plant (Pp) forms gametes, even though each diploid cell of the plant has both alleles [the purple allele (P) and the white allele (p)], each of the haploid gametes formed will only have one of the alleles. Using the previous example, half of the gametes, let's say sperm, have the gene for purple flowers (P) and the other half have the gene for white flowers (p). This same thing is true for the eggs that the plant produces: ½ will have the purple gene (P), ½ will have the white gene (p). If this plant is allowed to self-pollinate, there is a ½ probability that the egg will have the p gene and a ½ probability that the sperm will have the p gene. ½ x ½ = ¼ (Multiplicative Law states that the probability of two separate events occurring together is equal to the product of their individual probabilities) Therefore, there is a ¼ chance that the resulting offspring will have pp (white flowers). This matches with what Mendel found. 8. Punnett Squares Instead of doing all of this math for each of the possible outcomes, we can do Punnett Squares. 1. Draw a square. 2. Divide the square horizontally so that the number of rows corresponds to the number of unique sperm cells formed. 3. Divide the square vertically so that the number of columns corresponds to the number of unique egg cells formed.
9. Mendel's Second Law (Law of Independent Assortment) The genes for two different characteristics are inherited separately. This applies if we look at two traits together. Dihybrid Cross: A cross that tracks the inheritance of two different characteristics If we look at two traits, let's say seed shape and seed color [yellow (Y) is dominant over green (y) and round (R) is dominant over wrinkled (r)], and we self-pollinate a plant that is heterozygous for both traits (YyRr), what will be the expected offspring? 10. Second Law Practice Problem YyRr x YyRr (Sperm x Egg) The first thing we need to do is determine the possible gene combinations in the gametes You can use the foil method for this step. YyRr: YR Yr yr yr first outer inner last Because both gametes come from the same individual, they will have the same gene combinations. Set up your Punnett Square (because we have four possible combinations for each gamete, we need a 4 x 4 square) YR Yr yr yr YR Yr yr yr Possible Phenotypes Possible Genotypes 11. Molecular Genetics Remember, these letters stand for actual genes that produce actual proteins. So what is happening at the protein level with these crosses? Again, let's look at flower color. The P gene produces an enzyme that converts a white substance into a purple pigment. If the flower has this P gene (and therefore the enzyme), it is purple. The white gene (p) is actually a mutated purple gene that doesn't produce a functioning enzyme. In a pp plant, the functional enzyme is missing totally, the plant is unable to convert the white substance into a purple pigment and the flower stays white. The heterozygote (Pp) has enough functional enzyme to convert all white substance to purple so it is the same shade of purple. 12. Other types of Inheritance Not all traits are binary (purple or white; yellow or green; round or wrinkled). Many traits have multiple states and are not just dominant or recessive. Examples: Incomplete Dominance, Codominance, Multiple Alleles, Polygenic Inheritance, Epistasis, Lethal Recessive, Sex-Linked Traits
13. Incomplete Dominance Neither allele is dominant and the heterozygote is somewhere in between the phenotypes of the two homozygotes. Example: If a snapdragon with red flowers is crossed with a snapdragon with white flowers, the offspring will have pink flowers. This is because the heterozygote doesn't have enough enzyme to convert all of the white to red, so the flower possesses some red substance and some white substance, so it is a mix of red and white (pink). 14. Codominance Both alleles are dominant; that is, both alleles appear in the phenotype separately. In camelias (a type of flowering bush), one allele's protein makes a red pigment from a starting substance and the other allele's protein makes a white pigment from the same starting substance. This results in the presence of both white and red pigments which appear distinct from one another. Another example is blood type (this is discussed in the next section). 15. Multiple Alleles There are more than two alleles present in the population for a single trait, but each individual only has two of those alleles. Example: Blood type. There are three alleles present in the population (A, B and O), but each person only has two of those alleles (AA, AO, BB, BO, AB or OO). Blood Type Alleles The A allele (A) produces the A marker on blood cells. The B allele (B) produces the B marker on blood cells. The O allele (O) is a mutant, produces a non-functioning protein & therefore does not result in markers. Blood Types The different blood types are due to the presence of different types of markers on red blood cells. People with A type blood either have two copies of the A allele or one copy of the A allele and one copy of the O allele. People with B type blood either have two copies of the B allele or one copy of the B allele and one copy of the O allele. People with O type blood have two copies of the O allele. People with AB type blood have one copy of the A allele and one copy of the B allele. Charles Drew: African-American physician credited with the process of blood banking during WWII. 16. Polygenic Inheritance Instead of a trait being controlled by a single pair of genes, these types of traits are controlled by multiple genes. Examples: Skin color, height, hair color, etc In the old model, skin color is controlled by three genes (the new model suggests that there are at least 5 genes). The more dominant genes there are, the darker the skin color. The more recessive genes there are, the lighter the skin color. 17. Epistasis When the effects of one pair of genes is modified by another set of genes. Mice Coat Color In mice, what happens is that coat color is determined by two different sets of genes (polygenic). If a mouse has a dominant C allele and a dominant B allele, then it has Black fur. If a mouse has a dominant C allele and two recessive b alleles, then the mouse has brown fur. However, if a mouse has two recessive c alleles, it doesn't matter what the B alleles are, it will be an albino. This is because the C allele gene product makes an enzyme that makes a pigment precursor. This pigment precursor is then acted on by the B allele gene product, which makes it either brown or black. If the organism is homozygous recessive for the C gene, then no pigment precursor is made and therefore no colored pigment can be made.
Labrador Coat Color Similar to coat color in mice: Black lab is BxEx Yellow lab is xxee Chocolate lab is bbex Probable pathway: E B Mol 1 (Yellow) Mol 2 (Chocolate) Mol 3 (Black) 18. Lethal Recessive (Lethal Alleles) If an organism receives two mutated copies of an essential gene, then the organism cannot live. Examples: Cystic Fibrosis, Sickle-cell Anemia, Tay-Sachs, etc Heterozygotes are able to survive because they have one functional form of the gene. Heterozygotes are said to be carriers for the disorder because if two heterozygotes have offspring, one of the offspring could inherit two recessive copies of the gene and die. 19. Sex Determination and Sex-Linked Traits Out of the 23 pairs of chromosomes, one pair contains the sex chromosomes. The non-sex chromosomes are referred to as autosomes. Females are XX Males are XY Females can only pass on an X chromosome to their children. Males can pass on either an X or a Y chromosome to their children. Males determine the sex of the offspring. Besides sex information, there are approximately 1,000 genes on the X chromosome, and about 90 on the Y chromosome. Traits and genes linked to the X chromosome are referred to as X- linked. Traits and genes linked to the Y chromosome are referred to as Y-linked. 20. X-Linked Traits Genes linked to the X chromosome that produce disease or certain conditions, are mostly recessive (colorblindness, hemophilia, baldness). A A a a a a A A Illustration 2: Mendel's First Law of Segregation Illustration 1: One of Mendel's Crosses
Illustration 4: Codominance Illustration 3: Incomplete Dominance Illustration 5: Codominance & Multiple Alleles Illustration 6: Epistasis & Polygenic Inheritance Illustration 7: X-Linked Trait (Hemophilia)
Genetics Problems Practice Problem 1 (Dominance-Recessive) In pea plants, Round seeds (R) are dominant to wrinkled seeds (r). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have round seeds? Practice Problem 2 (Dominance-Recessive) In dogs, Wire hair (W) is dominant to smooth (w). In a cross of a homozygous wire-haired dog with a smooth-haired dog, what will be the phenotype of the F 1 generation? What would be the genotype? What would be the ratio of wire-haired to smooth-haired dogs in the F 2 generation? Practice Problem 3 (Dominance-Recessive) A genetic cross between two F 1 -heterozygous pea plants for height will yield what percentage of tall plants in the F 2 generation? (Recall, tall plants are dominant over dwarf plants). Practice Problem 4 (Test Cross) To identify the genotype of yellow-seeded pea plants as either homozygous dominant (YY) or heterozygous (Yy), you could do a test cross with plants of which genotype? What results in the offspring would you expect to get if your plant is homozygous dominant? What results in the offspring would you expect to get if your plant is heterozygous? Practice Problem 5 (No Punnett Square is needed for this problem) A pea plant is heterozygous for both seed shape and seed color. R is the allele for the dominant, round shape characteristic; r is the allele for the recessive, wrinkled shape characteristic. Y is the allele for the dominant, yellow color characteristic; y is the allele for the recessive, green color characteristic. What will be the distribution of each these alleles in this plant's gametes (expressed as percentages)? Practice Problem 6 (Dihybrid Cross) In a dihybrid cross, AaBb x AaBb, what fraction of the offspring will be homozygous for both recessive traits? Practice Problem 7 (Dihybrid Cross) Following a rryy x RrYy cross, what fraction of the offspring are predicted to have a genotype that is heterozygous for both characteristics? Practice Problem 8 (Incomplete Dominance) Feather color in Andalusian fowls follows an incomplete dominance pattern of inheritance. If a homozygous black fowl (C B C B ) mates with a homozygous white fowl (C W C W ), the F 1 hybrids will have blue feathers (C B C W ). What would be the phenotypic and genotypic ratios of offspring produced by two blue fowls? Practice Problem 9 (Blood Type) A man with homozygous A blood (I A I A ) has a child with a woman with heterozygous B (I B I O ) blood. What are the possible genotypes and phenotypes for their child? Practice Problem 10 (Epistasis) In sweet peas, purple flower color (P) is dominant over white (p), but there is also a control gene such that if the plant has a C, the purple is expressed. If the plant is cc, the purple is not expressed and the flower will be white anyway. If a plant that is homozygous dominant for both the purple and control genes is crossed with a plant that is homozygous recessive for both genes, diagram the Punnett square for the F 1 and F 2 generations and calculate the genotype and phenotype ratios. Practice Problem 11 (Extra Credit) Suppose a person with type A blood and a person with type B blood get married. What are the possible genotypes their children could have? You will need to draw your own Punnett Squares for this problem.
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