Genetics
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?
Reminders for the Perio 1. Last exam on Thursday (PM sked), after Math 2. Coverage: Cell Cycle – Non-Mendelian inheritance; guide questions available online 3. NBs (4% of grade) + any resubmissions due at 2:30 PM on the day of the Bio Perio 4. EoQRs available online due Aug 11 (Sun), 12 NN
Find the genotypic and phenotypic ratios of the offspring from the following crosses using the forked line method: 1. AaBb x aaBb 2. AABbCc x AaBbcc www.youtube.com
Non-mendelian inheritance Alleles with different degrees of dominance and recessiveness Codominance Multiple alleles Incomplete dominance - two dominant - genes with more alleles affect than two alleles that - phenotype of phenotype in control the offspring is between separate ways phenotype phenotypes of two - e.g. ABO blood type - both alleles manifest parents system has 3 alleles-- - e.g. roan coloring in - e.g. red and white I A ,I B ,i. I A ,I B are horses – both red and parents give rise to codominant, i is white pink offspring recessive to both
www.flickr.com
www.bio.georgiasouthern.edu
www.buzzle.com
http://nevinerk.weebly.c om/uploads/5/2/9/9/529 9320/236178.png?402
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?
Sample problems 5. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). a) What pattern of inheritance does this illustrate? b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat? 6. A woman with Type O blood and a man who is Type AB are expecting a child. What are the possible blood types of the kid? 7. What are the possible blood types of a child whose parents are both heterozygous for "B" blood type? 8. What are the chances of a woman with Type AB and a man with Type A having a child with Type O? 9. Determine the possible genotypes & phenotypes with respect to blood type of the offspring of a couple whose blood types are A and B. Both sets of grandparents have the AB blood type.
Recommend
More recommend
