%-1. Chapter 12 Mendel and Genetics ASA MENDEL S HISTORY

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1 Mendel inheritance 1 ASA I hapter 12 Mendel and Genetics ALABAMA 7TH GRA ScINc STANARS VR IN THIS HAPTR INLU: 11 Identify Mendel s laws of genetics? ui ) -J z ) ) t ) ) MNL S HISTORY In 1 853, Gregor Mendel ( ) began his work at an Austrian monastery Of particular interest to Mendel were natural phenomena Mendel loved nature and often took walks in the gardens of the monastery uring one walk, he happened to notice an unusual color of an ornamental plant The plant was a ianthüs flower, like the one pictured on the cover of this book Mendel later used several types of plants in his famous experiments, including ianthus and Pisum (common peas) In case you are unfamiliar with Mendel s story, a quick description is given below i %-1 and his assistants worked with over 28, pea plants during his studies The pea plant was an excellent model for plant for several reasons first, the pea plant has a very simple genetic make-up; it only has seven chromosomes In addition, the physical characteristics of the pea plant can be easily seen, and the shape of the pea flower allows humans to easily Figure 121 Mendel control the pollination of the flowers Remember, when flowers are pollinated, seeds are formed ontrolling the source of pollen allows humans to control the seed s genes Mendel did not know about genes or, for that matter, chromosomes In his time, humans did not fully understand how the traits of living things were passed to offspring This was one of the things Mendel was exploring in his experiments We now know that chromosomes carry genes and they are passed to offspring To begin his exj,eriments, Mendel used plants that were true breeders for one trait True breeders were plants that would selfpollinate to produce offspring identical to themselves When a plant self pollinates, the pollen from its own flowers is used to make seeds; in effect, the new offspring has only one parent In this way, Mendel ensured that traits under experimentation only changed when he wanted them to 91

2 ;,, Mendel and Genetics Mendel carefully examined each plant characteristic by cross-pollinating two true breeders The characteristics Mendel studied are shown in Table 121 After many plant generations and a lot of statistical analysis, Mendel developed three basic laws about plant inheritance These laws form the foundation of modem heredity Table 121 Possible Traits ofpea Plants Seed Shape Seed olor Round* Wnnkled Yellow* Green J 7 Pod olor Flower Position Green* Yellow A 1* Terminal I Grav* Seed oat olor, White Pod Shape Smooth* onstricted \b Plant Height T II4 a Short 4, :_/k *ominant Axial: along the length ofthe stem Terminal: at the end ofthe stem TRMS Before we discuss Mendel s laws, we need to review some terminology Organism characteristics are determined by specific parts of chromosomes called genes (: Genes are roughly defined as a pailicular sequence of biological chemicals located on a chromosome Genes can come in different INKS L1kL ww;f forms We call each form of a gene an allele Alleles code for ABOJ characteristics of organisms In pea plants, each plant ; characteristic has two alleles For example, a pea plant can be tall or short Tallness is one allele for height; we use an uppercase T to refer to the tall allele Shortness is another allele for height; we use the lowercase t to refer to the short allele ach organism has at least two alleles for each trait The physical expression of alleles is called the phenotype, or what the organism looks like The combination of alleles inherited from the parents is the genotype S : MNL S LAW OF SGRGATION uring meiosis, alleles separate The result ofmeiosis is a gamete (sperm or egg cell) with only half the original amount of chromosomes ach parent donates one of its gametes, and in turn, one allele to the offspring The resulting offspring then has two alleles for each trait 92

3 to explain why some alleles are expressed in organisms, and some alleles are hidden hapter 12 ci give PUNNTT SQUAR parental gametes are easily charted using a Punneft square Before you can make a Punnett square, you must identify the genotypes of the parents dominant Homozygous means both alleles are the same Please note, an individual could also be homozygous recessive What would a homozygous rcessive genotype look like? That s right called heterozygous Heterozygous means both alleles are different A heterozygous genotype might look like this: Tt How many different gamete s can a heterozygous individual make? Two U! either T or t The Punnett square is a handy tool geneticists use to identify thepossible genotypes of offspring ach parent can only donate one allele for each trait to offspring The possible In most cases, you will be told the parental genotypes If a true breeder for tall is used offspring It can only make T gametes Geneticists say this plant is homozygous in one mating, his genotype will be TT This parent can only donate a T allele to the to offspring? Right t What is the only allele a homozygous recessive parent can If an individual has two c4fferent alleles, that individual is trait! plant would have a genotype of TT, while a true breeding short pea plant would have a example, the tall allele is represented with a capital T and the short allele with a lower genotype ofif Remember, there are two T s because organisms have two alleles for each As you know, geneticists use letters from the alphabet to represent the genotype of each allele apital letters distinguish dominant alleles from recessive alleles In our case t Notice, for this one trait a single letter is used, the letter t A true breeding tall pea RPRSNTING GNOTYPS The allele that is hidden is called recessive For recessive alleles to be expressed, the ominant means the allele will always be expressed ifthe individual carries the allele Mendel noticed that when he crossed two different true breeding pea plants, one trait be tall The tall allele (T) hides the short allele (t) We say, tall is dominant over short individual must carry both recessive alleles would always appear while the other remained hidden For example, ifhe crossed a true breeding tall pea plant with a true breeding short pea plant, all the offspring plants would One allele is expressed while the alternative allele remains hidden This law attempts MNL S LAW OF OMINAN

4 Mendel and Genetics Let s say that you are investigating how height in pea plants is inherited You are told that two heterozygous pea plants are cross-pollinated What would their genotypes be? orrect, Tt We start to work the problem by writing the parental genotypes at the top of the page separated by a X The X means the individuals are crossed Next draw a box under the parental genotypes and separate it into four quadrants It should look like figure 122 Tt x Tt figure 122 Starting the Problem This next step might be a little tricky for some, so pay close attention Now, we must figure out all the possible gametes each parent can make In the example above, each parent can make two different kinds of gametes, one containing a T and one containing a t Write the gamete possibilities for one parent along one top of the box Then write the gamete possibilities for the other parent along a the side See Figure 123 to help you better understand this step 6 figure 123 etermine the Gametes Finally we note the possible offspring We fill in the offspring alleles in the interior of the box by fertilizing our possible gametes Remember each parent gives one allele to an offspring, so each possible offspring ends up with two alleles xamine Figure 124 to see this step o In In LU -I a- I- z >% ( T :: : z Figure 124 Make Possible Offspring ( - a) 94

5 hapter 12 The best way to learn to use Punnett squares is to practice doing them Return to Table 12 1 and work some Punnett squares using traits of pea plants ach dominant trait is marked with an asterisk Remember to always place dominant alleles before recessive ones, as in Tt not tt Activity Look at each crossing below and determine the genotypes and phenotypes of the offspring pea plants 1 Seed shape 2 Seed color 3 round x round yellow x green RrxRr Yy x yy Flower position axial x terminal Aa x aa 4 Pod shape smooth x constricted Ss x ss 5 Pod color green x green Gg x Gg 6 Plant height short x short tt x tt o In o U -I a hallenge: Pod color and pod shape green smooth x green smooth Gg $sx Gg Ss I- z > (U a (U ) ȧ) In each question, determine the percentage of offspring that are genetically identical to the parent plant Also, determine the offspring that are phenotypically identical to the parent a 95

6 Mendel and Genetics HAPTR 1 2 RVIW 1 Use a Punnett square to predict the cross of a homozygous green parent with a homozygous yellow parent ifyellow is dominant over green The phenotype of the offspring will be A B yellow green P neither yellow nor green some yellow and some green 2 If an individual inherits one dominant allele and one recessive allele, the genotype is A B homozygous recessive heterozygous phenotype 3 Identify the homozygous genotype below A It B Rr Gg B pp 4 What percentage of the offspring in the Punnett square below will show the dominant phenotype? P d p o o d A 25% B 5% 75% B 1% 5 What do the letters along the edges of the Punnett square represent? A possible offspring B possible gametes definite offspring P phenotypes o? LI -j I- z > u a u (3 u a 96