Patterns of Inheritance

Transcription

1 atterns of Inheritance Introduction Dogs are one of man s longest genetic exeriments. Over thousands of years, humans have chosen and mated dogs with secific traits. The results : an incredibly diversity distinct body tyes and behavioral traits. 9.1 The science of genetics has ancient roots This breeding of animals is actually accelerated evolution, where man instead of nature redetermined what traits he/she wanted to see in the next offsring. This idea that hereditary materials can be assed on and mixed in forming offsring, was termed the blending hyothesis, suggested in the 19th century by scientists studying lants but later rejected because it did not exlain how traits that disaear in one generation can reaear in later generations. 1

2 9.2 Exerimental genetics began in an abbey garden Gregor Johann Mendel ( ) Was an ustrian friar (monk) when he began the field of in the 1860s He deduced the rinciles of genetics by breeding garden eas relied uon a background of mathematics, hysics, and chemistry ( and obviously biology as well). genetics 9.2 Exerimental genetics began in an abbey garden In 1866, Mendel correctly argued that arents ass on to their offsring discrete heritable factors and stressed that the heritable factors (today called genes), retain their individuality generation after generation. heritable feature that varies among individuals, such as eye color or flower color, is called a character. Each variant for a character, such as urle or white flowers, is a trait. 9.2 Exerimental genetics began in an abbey garden lthough the significance of Mendel's work was not recognized until the turn of the 20th century, the indeendent rediscovery of these laws formed the foundation of the modern science of genetics 2

3 9.2 Exerimental genetics began in an abbey garden Heredity is the transmission of traits from one generation to the next. Genetics is the scientific study of heredity. In the following, we will discuss the exeriments that rovided the basis for Mendel s discoveries. Thus we need to first look at some simle lant anatomy and sexuality. 9.2 Exerimental genetics began in an abbey garden The reroductive arts of a flowering lant are located within the flowers ollen is located on the anther arts of the stamen. ollen contains the lant serm. ollination is the movement of ollen from the anthers to the stigma; fertilization occurs in the ovule The fertilized ovules roduce seeds that are the next generation 9.2 Exerimental genetics began in an abbey garden Many flowers are caable of self-fertilization. Thus, the ollen of a flower ollinates the stigma of the same flower and results in seed formation. One would exect that the resulting seeds roduce offsring all identical to the arent. This is however not always the case as we will see later. 3

4 9.2 Exerimental genetics began in an abbey garden Science often requires a good dose of scientific design and luck. Mendle was fortunate that these ea lants he worked with were easy to maniulate caable of self- fertilization and cross fertilization. Showed many traits that occurred in an either/or format (blue/white; tall/short). etal Carel Stamen Exerimental design 1 Removal of stamens White ollination in nature occurs by different mechanisms ( wind, insects, birds). But lant breeders early on knew they could do this manually with simle tools such as fine aint brushes. Carel arents () Stamens 2 Transfer urle of ollen To revent self fertilization, the stamen of the flowers would be removed for examle. Some Imortant definitions True-breeding varieties are defined as lants that, when selffertilized, roduces offsring all identical to the arent. True-breeding arental lants are called the generation. If we look for examle at the trait of flower color: the outcome of self-fertilization of a true-breeding ea lant with urle flowers will always be ea lants with urle flowers (because there is only one genetic variety of that gene resent in the lant s genome). Or if we cross-fertilize two true-breeding ea lants with urle flowers, the result will always be 100 % lants with urle flowers. 4

5 Some Imortant definitions True-breeding lants X Self-fertilization Cross fertilization Offsring BUT a true breeding lant for flower tye may not be true breeding for another trait such as lant height, or seed color,! Some Imortant definitions The offsring of two different varieties are hybrids. The cross-fertilization is a hybridization, or genetic cross. Hybrid offsring from a generation cross are the F 1 generation. In this examle, we have two arental true breeders : one roducing urle flowers, the other roducing white flowers. In this examle, the F1 generation is also 100% urle flowered. 1 Removal of stamens White Carel arents () Stamens 2 Transfer urle of ollen 3 Carel matures into ea od 4 Seeds from od lanted Offsring (F 1 ) 5

6 Some Imortant definitions cross of F 1 lants or self-fertilization among F1 lants roduces a new generation of lants referred to as the F 2 generation. cross between two individuals differing in a single character is a monohybrid cross. So, crossing lants and looking only for the differences in flower color, where the flower colors have only two ossibilities, refers to a mono-hybrid cross Mendel erformed several monohybrid crosses between ea lants and initially only looked at each trait as individual events. The following two slides shows the traits he was crossing for Each of them nicely showing u as discrete characters (thus there were no in-between states) Character Dominant Recessive Traits Flower color urle White Flower osition xial Terminal Seed color Yellow Green Seed shae Round Wrinkled 6

7 Figure 9.2D_2 Character Dominant Traits Recessive od shae Inflated Constricted od color Green Yellow Stem length Tall Dwarf Mendel erformed a monohybrid cross between a lant with urle flowers and a lant with white flowers. Doing hunderds of crosses, lanting the resulting seeds and observing the flowers of over a thousand lants he observed that The F 1 generation roduced all lants with urle flowers. cross of F 1 lants with each other roduced an F 2 generation with ¾ urle and ¼ white flowers. The Exeriment generation (true-breeding arents) urle flowers White flowers F 1 generation F 2 generation ll lants have urle flowers Fertilization among F 1 lants (F 1 F 1 ) 3 of lants 4 have urle flowers 1 of lants 4 have white flowers 7

8 Thus the keen eye of Mendel was the he observed that none of the F1 generation lants were light urled ND the offsring of the all-urle F 1 generation did not roduce any light urle flowers either. This refuted the old standing hyothesis (the blending hyothesis) that assumed traits become blended when individuals reroduce and mix their gametes ( a mix of urle and white would have generated light urled flowers, but it didn t ). Mendel observed the same atterns of inheritance for six other ea lant characters. Trait Dominant Recessive Ratio Stem length Seed color od color Seed shae od shae ll characteristics showed u as either/or (no mixing). ll ratios between dominant and recessive trait in the F2 generation aroached 3:1 Mendel needed to exlain why white color seemed to disaear in the F 1 generation and white color reaeared in one quarter of the F 2 offsring. Mendel develoed four hyotheses, described below using modern terminology. Kee in mind that in Mendel s time, nothing was known about DN and genes. He used terms as heritable factors, that retain their individuality from generation to generation 8

9 1. Those heritable factors can exist in alternative versions that account for variations in inherited characters. These days we call these alternative versions of genes alleles 2. For each characteristic, an organism inherits two alleles, one from each arent. The alleles can be the same or different. homozygous genotye has two identical alleles. heterozygous genotye has two different alleles. 3. If the alleles of an inherited air differ, then one determines the organism s aearance and is called the dominant allele. The other has no noticeable effect on the organism s aearance and is called the recessive allele. The henotye is the aearance or exression of a trait. The genotye is the genetic makeu of a trait. The same henotye may be determined by more than one genotye. 4. serm or egg carries only one allele for each inherited character because allele airs searate (segregate) from each other during the roduction of gametes. This statement is called the law of segregation. 9

10 Thus Mendel viewed cells as having 2 heritable factors for each trait, and they segregate indeendently into the gametes Homozygous cell ossible gametes Dominant trait (allele) Heterozygous cell a ossible gametes Recessive trait (allele) a In terms of what we have seen with resect to chromosomes and meiosis, the homologous chromosomes carry the alleles. Meiosis would look like this for a heterozygous cell. a Meiosis I a Meiosis I a a Meiosis II a Segregation of the alleles into gametes a Often, to simlify things, the first letter of a dominant trait is used to designate that secific trait. For examle : urle flowers are dominant, white flowers are recessive. Thus the letter is used to designate the flower color trait Tall lants are dominant over small lants. The letter T is used to designate the lant length trait. 10

11 To differentiate between the dominant allele and recessive allele of a trait, a caital letter is used for the dominant allele, a small letter for the recessive allele With resect to flower color, since urle is dominant is for the dominant allele, for the recessive allele ( so even though recessive color is white, we do not use the letter w). With resect to length, since tall is dominant T is for the dominant allele, t for the recessive allele ( so even though recessive length is small, we do not use the letter s here). Remembering now that a diloid cell has airs of homologous chromosomes, thus each trait is reresented by 2 alleles In simle terminology, we write these alleles within the cells out as follows Trait Homozygous Dominant Heterozygous Homozygous Recessive Flower Color lant length TT Tt tt od color GG Gg gg Seed shae RR Rr rr Looking back at Mendel s exeriment, he initially crossed two true-breeders: one urle flowered ea lant with a white flowered ea lant. Thus these are the generation lants. Truebreeders indicating that they are homozygous for their trait. The urle homozygous flowered lants are thus, the white homozygous flowered lants are. What ossible gametes can each of them roduce? 11

12 Since they are homozygous, they only have one kind of allele (resent twice), and thus can only roduce one kind of gamete with resect to this trait. 100% generation gametes 100% Crossing these two arent lants rovides us the F1 generation. Only one ossible genotye can result from this cross: all offsring will be, 100% heterozygous 100% F1 generation: 100% generation gametes 100% Genotye of F1 : henotye : urle flower If we now cross the F1 generation with each other, we create the F2 generation. So we first have to find out what ossible gametes the F1 generation can roduce? ossible gametes are and. F1 generation crossing ossible gametes ossibilities for F2 generation? 12

13 The ossibilities for the F2 generation are the ossible combination of gametes between the 2 individuals being crossed. ossibilities for F2 generation? F1 generation crossing ossible gametes Mendel s hyotheses also exlain the 3:1 ratio in the F 2 generation. The F 1 hybrids all have a genotye and urle henotye Their gametes are a 50:50 % of and The F2 generation ends u having the genotyes 1, 2 and 1 (1:2:1 ratio, see revious slide) The henotyes are 3 urle to 1 white flower (3:1 ratio) since every genotye with at least one allele will dislay the urle flower trait 9.3 Mendel s law of segregation and unnet Squares unnett squares make it easier to show the four ossible combinations of alleles that could occur when these gametes combine. They also show the % ossibilities of these outcomes, very similar as what one would exect when fliing coins. 13

14 Question : what are the ossible outcomes when fliing a coin? Question : does the outcome of fliing of a coin influence the outcome fliing the coin again? The answer to the first question is of course heads or tails ( only two ossibilities) The answer to the second question is NO. These are called indeendent events. Similar, fliing two coins are indeendent events as well. The outcome of fliing one coin has no effect on fliing a second coin. What are the robabilities of heads vs tails? We call the outcome usually 50% or half and half. If the two ossible events have to add u to 1, the outcome of each would be ½ (= 0.5) ( heads would be ½ and tails would be ½; together ½ + ½ = 1) The other way of asking, what is the % chance of each. In this case, since the sum of the two events has to add u to 100%, each event has a 50% chance of occurring (50 % + 50% = 100%) So what are the ossible outcomes of fliing two coins in a row? Heads = H, tails = t HH Ht th tt 14

15 We can ut the ossible outcome of each fli into a unnet square and that visualizes the 4 ossibilities easier. HH HT TH ossible outcome H t TT H HH Ht t Ht tt This also rovides us with the robability of each event. 1/2 1/2 ossible outcome H t 1/2 1/2 H HH 25% Ht 25% t Ht 25% tt 25% The outcome of two indeendent events, is multilying their individual ossibility. H = ½, t= ½, thus for examle the outcome HH = (1/2)x(1/2)=1/4 (or 25%) nd since there are are 4 squares, each one has a 25 % ossibility This also rovides us with the robability of each event. 1/2 ossible outcome H 1/2 1/2 H HH 25% t Ht 25% The outcome of all events in this case : HH 25 % HT 25+25= 50% tt 25 % 1/2 t Ht 25% tt 25% 15

16 unnett squares are used to show the four ossible combinations of alleles that could occur when gametes combine during fertilization. The squares show the combinations between the gametes. gametes Male Female Figure 9.3B_s3 The Exlanation generation Genetic makeu (alleles) urle flowers White flowers Gametes ll ll F 1 generation (hybrids) F 2 generation Gametes Fertilization 1 2 ll lleles segregate Serm from F 1 lant 1 2 henotyic ratio 3 urle : 1 white Genotyic ratio 1 : 2 : 1 Eggs from F 1 lant F 2 generation Serm from F 1 lant henotyic ratio 3 urle : 1 white Genotyic ratio 1 : 2 : 1 Eggs from F 1 lant 16

17 ractice Examles ssume in rabbits, white coat is recessive, brown is dominant. How would you write out the following genotyes? Homozygous white : Homozygous dominant : Heterozygous : What color would the heterozygous rabbit have? ractice Examles Using a unnet Square, what are the ossible genotyes and henotyes when crossing a homozygous brown bunny with a white bunny? Using a unnet Square, what are the ossible genotyes and henotyes when crossing a homozygous brown bunny with a heterozygous brown bunny? ractice Examles Straight hair is dominant in humans and curly hair is recessive. How would you write out the following genotyes? erson with straight hair : erson with curly hair : 17