Core Concepts Mendel’s Law of Segregation states that there are two alleles • for every gene determining a specific characteristic, and these alleles are segregated into separate gametes during reproduction. When the 2 different alleles occur together in one individual • (heterozygote), the dominant allele will be the one that is expressed in the phenotype of the individual. Mendel’s Law of Independent Assortment says that 2 or • more different genes, if found on separate chromosomes, are determined independently of each other. The Addition Rule is used to combine probabilities, when • there are 2 or more ways to arrive at a given outcome. The Multiplication Rule is used to combine probabilities of 2 • or more different events that need to occur in combination. • Many characteristics have been found to follow patterns of inheritance that are modifications of Mendel’s rules.
Keywords • gene • pure breeding • Punnett • allele • hybrid Square • dominant • homozygous • forked line • recessive • heterozygous • P (parental • character generation) • trait • F 1 (first filial • phenotype generation) • genotype • F 2 (second filial generation)
TRAITS Mendel’s CHARACTERS Discoveries • Pre-Mendel beliefs in genetics: blending theory • Augustinian monk and science teacher • Why Pisum sativum ? – Pea plants available in many varieties • character ( heritable feature) • trait (character variant) – Perfect flowers • cross-pollination and self- pollination – Short generation time – Many offspring
MONOHYBRID CROSS inheritance of a single trait P Generation x (true-breeding parents) Purple White flowers flowers F 1 Generation Cross- fertilization (hybrids) All plants had purple flowers self-pollination F 2 Generation 224 plants 705 plants
Results for other hybrid (F 1 ) characters
Blending inheritance Complete dominance
Mendelian • Alternative Inheritance versions of genes exist ( alleles ) Complete dominance – Dominant Allele for purple flowers ( P ) – Recessive • Organism inherits 2 alleles Homologous Locus for flower-color gene pair of – Dominant is fully chromosomes expressed – Recessive has no Allele for white flowers ( p p ) visible effect
Alternative • versions of PP PP pp pp genes exist ( alleles ) • Dominant Pp • Recessive • Organism inherits 2 alleles ( genotype ) • Dominant is PP PP or fully expressed Pp pp pp • Recessive has no visible effect
Other terms in genetics • Homozygous – identical alleles, true-breeding • Heterozygous – different alleles • Phenotype – physical appearance • Genotype – genetic make-up
Mendelian Inheritance Law of Segregation 2 alleles for one character separate and go to different gametes
Parental generation • Pure-breeding parents are homozygous for different traits for flower color • Each parent produces only one type of gamete: purple parent (P), white parent (p) F 1 generation • Hybrids are heterozygous for flower color • Can produce two kinds of gametes: P or p F 2 generation • Due to the law of segregation, eggs and sperm with different kinds of gametes combine to form a specific ratio • Genotypic ratio 1 PP : 2 Pp : 1 pp • Phenotypic ratio 3 purple : 1 white
An organism displays the Problem-solving dominant phenotype. It may be either homozygous or heterozygous for the trait. How can its genotype be determined? Perform a test-cross: Dominant phenotype x recessive phenotype
Dihybrid cross Inheritance of two characters • Produces four phenotypes in the F 2 generation
Dihybrid cross Inheritance of two characters • Characters are NOT transmitted as a package (YR and yr only) • Genes are packaged into gametes in all possible combinations of alleles (YR, Yr, yR, yr)
Law of independent assortment Allele pairs separate independently during the formation of gametes
Genetics and Probability Monohybrid cross Addition Rule Round seed shape is dominant combines probabilities over wrinkled seeds in pea for mutually exclusive plants. events What percentage of the F2 generation can be expected to: Multiplication Rule a. Be homozygous dominant combines probabilities of for seed shape? 2 or more independent events that need to b. Be heterozygous for seed occur together shape? c. Have round seeds?
Genetics and Probability Dihybrid cross Round seed shape is dominant Addition Rule over wrinkled seeds in pea combines probabilities plants. Green pod color is for mutually exclusive dominant over yellow pods. events What percentage of the F2 generation can be expected to: Multiplication Rule a. Be heterozygous for both combines probabilities of characters? 2 or more independent events that need to b. Have round seeds in green occur together pods? c. Have wrinkled seeds in green pods?
Trihybrid cross Genetics and Round seed shape is dominant Probability over wrinkled seeds in pea plants. Green pod color is dominant over yellow pods. Addition Rule Tall plants are dominant over combines probabilities short ones. for mutually exclusive What percentage of the F2 events generation can be expected to: a. Be heterozygous for all Multiplication Rule characters? combines probabilities of b. Be tall plants with round 2 or more independent events that need to seeds in yellow pods? occur together c. Be short plants with round seeds in green pods?
Monohybrid cross problems 1. In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds? 2. In pea plants, yellow seed color is dominant to green seed color. A. If a heterozygous pea plant is crossed with a plant that is homozygous recessive for seed color, what is the probability that the offspring will have green seeds? B. If all of the offspring of a particular cross have the genotype Yy, what must the genotype of the parents be? C. T o 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 genotype _______. 3. In rabbits, black fur (B) is dominant to white fur (b). Perform the following crosses. For each cross, give the phenotype and genotype of all offspring. A. BB x bb B. Bb x Bb C. BB x Bb 4. In summer squash, white fruit color (W) is dominant over yellow fruit color (w). If a squash plant homozygous for white is crossed with a plant homozygous for yellow, what will the phenotypic and genotypic ratios be for: A. the F1 generation? B. the F2 generation? C. What will the phenotypic and genotypic ratios of the offspring be if you perform a testcross with the F1 generation?
Dihybrid and trihybrid cross problems 1. The ability to roll one’s tongue is dominant over non -rolling. The ability to taste certain substances is also genetically controlled. For example, there is a substance called phenylthiocarbamate (PTC for short), which some people can taste (the dominant trait), while others cannot (the recessive trait). A woman who is both a homozygous tongue-roller and a non-PTC-taster marries a man who is a heterozygous tongue-roller and is a PTC taster, and they have three children: a homozygous tongue-roller who is also a PTC taster, a heterozygous tongue-roller who is also a taster, and a heterozygous tongue-roller who is a non-taster. A. If these parents have more children so that they had 12 in all, how many of those 12 would you expect to be non-tasters who are homozygous for tongue- rolling? B. If the first child (the homozygous tongue-roller who is also a PTC taster) marries someone who is heterozygous for both traits, draw the Punnett square that predicts what their children will be. 2. Using the forked-line method, determine the genotypic and phenotypic ratios of these trihybrid crosses: A. AaBbCc x AaBBCC B. AaBBCc x aaBBCc
Non-mendelian inheritance 1. Codominance – two dominant alleles affect phenotype in separate ways both alleles manifest – e.g. roan coloring in horses – – both red and white 2. Incomplete dominance – phenotype of offspring is between phenotypes of two parents e.g. red and white parents give – rise to pink offspring 3. Multiple alleles – genes with more than two alleles that control the phenotype e.g. ABO blood type system has – 3 alleles--A,B,i. A and B are codominant, i is recessive to both
Sample problems 1. A cross between a blue blahblah bird & a white blahblah bird produces silver blahblah birds. a) What are the genotypes of the parent blahblah birds in the original cross? b) What is the genotype of the silver offspring? c) What would be the phenotypic ratios of offspring produced by two silver blahblah birds? 2. The color of fruit for plant "X" is determined by two alleles. When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit. What are the genotypes of the parent orange-fruited plants? 3. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a red roan bull. 4. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?
Genetics Core Concepts Mendels Law of Segregation states that there are two
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