Mendel and the Gene Idea Chapter 14 Mendel and the Gene Idea

What genetic principles account for the passing of traits from parents to offspring? The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green) The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes) Mendel documented a particulate mechanism through his experiments with garden peas. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 14-1 Figure 14.1 What principles of inheritance did Gregor Mendel discover by breeding garden pea plants?

Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Mendel’s Experimental, Quantitative Approach Advantages of pea plants for genetic study: There are many varieties with distinct heritable features Mating of plants can be controlled Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

TECHNIQUE Parental generation (P) Stamens Carpel RESULTS First filial Fig. 14-2 TECHNIQUE 1 2 Parental generation (P) Stamens Figure14.2 Crossing pea plants Carpel 3 4 RESULTS First filial gener- ation offspring (F1) 5

He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The true-breeding parents are the P generation In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are the P generation The hybrid offspring of the P generation are called the F1 generation When F1 individuals self-pollinate, the F2 generation is produced. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The Law of Segregation When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

EXPERIMENT P Generation (true-breeding parents) Purple flowers White Fig. 14-3-1 EXPERIMENT P Generation (true-breeding parents)  Purple flowers White flowers Figure 14.3 When F1 hybrid pea plants are allowed to self-pollinate, which traits appear in the F2 generation?

F1 Generation (hybrids) Fig. 14-3-2 EXPERIMENT P Generation (true-breeding parents)  Purple flowers White flowers F1 Generation (hybrids) Figure 14.3 When F1 hybrid pea plants are allowed to self-pollinate, which traits appear in the F2 generation? All plants had purple flowers

EXPERIMENT P Generation (true-breeding parents) Purple flowers White Fig. 14-3-3 EXPERIMENT P Generation (true-breeding parents)  Purple flowers White flowers F1 Generation (hybrids) Figure 14.3 When F1 hybrid pea plants are allowed to self-pollinate, which traits appear in the F2 generation? All plants had purple flowers F2 Generation 705 purple-flowered plants 224 white-flowered plants

What Mendel called a “heritable factor” is what we now call a gene Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids Mendel called the purple flower color a dominant trait and the white flower color a recessive trait Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits What Mendel called a “heritable factor” is what we now call a gene Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Table 14-1

Mendel’s Model Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring: 1. Alternative versions of a gene exist - are now called alleles. The position of the gene is called the locus. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Allele for purple flowers Fig. 14-4 Allele for purple flowers Homologous pair of chromosomes Figure 14.4 Alleles, alternative versions of a gene Locus for flower-color gene Allele for white flowers

Alternatively, the two alleles at a locus may differ – heterozygous. 2. For each character an organism inherits two alleles, one from each parent. Mendel made this deduction without knowing about the role of chromosomes! The two alleles at a locus on a chromosome may be identical – homozygous. Alternatively, the two alleles at a locus may differ – heterozygous. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

3.The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

4. The law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes. Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Punnett Squares P Generation Appearance: Purple flowers White flowers Fig. 14-5-1 Punnett Squares P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p Figure 14.5 Mendel’s law of segregation

P Generation Appearance: Purple flowers White flowers Genetic makeup: Fig. 14-5-2 P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F1 Generation Appearance: Purple flowers Figure 14.5 Mendel’s law of segregation Genetic makeup: Pp Gametes: 1/2 P 1/2 p

P Generation Appearance: Purple flowers White flowers Genetic makeup: Fig. 14-5-3 P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F1 Generation Appearance: Purple flowers Figure 14.5 Mendel’s law of segregation Genetic makeup: Pp Gametes: 1/2 P 1/2 p Sperm F2 Generation P p P PP Pp Eggs p Pp pp 3 1

phenotype, or physical appearance An organism’s phenotype also includes internal anatomy, physiology, molecular pathways, and behavior genotype, or genetic makeup In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Phenotype Genotype PP Purple 1 (homozygous) 3 Purple Pp (heterozygous) Fig. 14-6 Phenotype Genotype PP Purple 1 (homozygous) 3 Purple Pp (heterozygous) Figure 14.6 Phenotype versus genotype 2 Purple Pp (heterozygous) pp 1 White 1 (homozygous) Ratio 3:1 Ratio 1:2:1

Important ratio: for monohybrid cross, Aa x Aa 3:1 dominant to recessives

The Law of Independent Assortment A monohybrid cross follows one trait. A dihybrid cross follows two traits. Using a dihybrid cross, Mendel developed the law of independent assortment The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

What does that mean in terms of meiosis? Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes. So a AaBb parent can produce gametes: AB, aB, Ab, ab

Predicted outcomes of heterozygous dihybrid cross: AaBb x AaBb 9 both dominant traits 3 one dominant, one recessive trait 3 other dominant, other recessive trait 1 both recessive traits

EXPERIMENT RESULTS Fig. 14-8 P Generation F1 Generation Hypothesis of YYRR yyrr Gametes YR  yr F1 Generation YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Predictions Sperm or Predicted offspring of F2 generation 1/4 YR 1/4 Yr 1/4 yR 1/4 yr Sperm Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? 1/2 YR 1/2 yr 1/4 YR YYRR YYRr YyRR YyRr 1/2 YR YYRR YyRr 1/4 Yr Eggs YYRr YYrr YyRr Yyrr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/16 3/16 3/16 1/16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1

Fig. 14-UN11 REMEMBER! Each gamete has to have one of each allele!

Fig. 14-UN6

The laws of probability govern Mendelian inheritance Probability of an event occurring = expected over possibilities EX: Probability of throwing a tail with a coin is? 1/2 Segregation in a heterozygous plant is like flipping a coin: Each gamete has a 1/2 chance of carrying the dominant allele and a 1/2 chance of carrying the recessive allele

1/2 1/2 1/2 1/4 1/4 1/2 1/4 1/4 Rr Rr  Segregation of Segregation of Fig. 14-9 Rr Rr  Segregation of alleles into eggs Segregation of alleles into sperm Sperm 1/2 R 1/2 r Figure 14.9 Segregation of alleles and fertilization as chance events R R 1/2 R R r 1/4 1/4 Eggs r r 1/2 R r r 1/4 1/4

Mendel’s laws of segregation and independent assortment reflect the rules of probability The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities

What is the probability of getting an F2 (Aa x Aa) offspring that is homozygous (aa) for both traits? ½ x ½ = 1/4

The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Using the addition rule: In the cross of Rr x Rr, what is the probability of Rr in offspring? If sperm is R and egg is r = ½ x ½ = ¼ If sperm is r and egg is R = ½ x ½ = ¼ So adding those together is ¼ + ¼ = 1/2

Solving Complex Genetics Problems with the Rules of Probability We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together

Examples AaBb X Aabb What is probability of obtaining a Aabb offspring? Aa = ½ bb = ½ Answer = 1/4

Probability of obtaining AabbCc? AaBbCc x aabbCc Probability of obtaining AabbCc? For Aa from Aa x aa = ½ For bb from Bb x bb = ½ For Cc from Cc x Cc = ½ The answer is ½ x ½ x ½ = 1/8

AaBbcc x AabbCC a) Probability of offspring that are A_B_C_? A_ = ¾, B = ½, C = 1/1 answer 3/8 b) A_bbC_ aaB_C_ A_B_C_ c) aabbC_

Used to tell the genotype of an individual with the dominant phenotype The Testcross Used to tell the genotype of an individual with the dominant phenotype Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous A testcross: breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype, the mystery parent must be heterozygous. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 14-7 TECHNIQUE RESULTS Dominant phenotype, unknown genotype:  Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If PP If Pp or Sperm Sperm Figure 14.7 The testcross p p p p P P Pp Pp Pp Pp Eggs Eggs P p Pp Pp pp pp RESULTS or All offspring purple 1/2 offspring purple and 1/2 offspring white

Extending Mendelian Genetics for a Single Gene Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: When alleles are not completely dominant or recessive When a gene has more than two alleles When a gene produces multiple phenotypes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

In codominance, both alleles are expressed fully Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance, both alleles are expressed fully Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Important ratio Aa x Aa IF INCOMPLETE DOMINANCE 1:2:1 1 dominant 2 mix 1 recessive

The Relation Between Dominance and Phenotype A dominant allele does not subdue a recessive allele; alleles don’t interact Alleles are simply variations in a gene’s nucleotide sequence For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain At the organismal level, the allele is recessive At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant At the molecular level, the alleles are codominant Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Inheritance of Tay-Sachs Organism level Imperfect enzymes The disease is caused from mutations on chromosome 15 in the HEXA gene. This gene defect would be codominant with a normal gene.

Frequency of Dominant Alleles Dominant alleles are not necessarily more common in populations than recessive alleles For example, one baby out of 400 in the United States is born with extra fingers or toes, the result of a dominant allele Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Multiple Alleles Most genes exist in populations in more than two allelic forms For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: A, B, AB, O. O is recessive. The enzyme encoded by the A allele adds the A carbohydrate, whereas the enzyme encoded by the B allele adds the B carbohydrate; the enzyme encoded by the O allele adds neither

You may see IA , IB or i for the alleles. Fig. 14-11 Allele Carbohydrate You may see IA , IB or i for the alleles. A A B B O none (a) The three alleles for the ABO blood groups and their associated carbohydrates Red blood cell appearance Phenotype (blood group) Genotype Figure 14.11 Multiple alleles for the ABO blood groups AA or AO A BB or BO B AB AB OO O (b) Blood group genotypes and phenotypes

If Mary has A blood and Ben has B blood, can they have a child with O blood?

Pleiotropy Most genes have multiple phenotypic effects, a property called pleiotropy For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Epistasis In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus For example, in mice and many other mammals, coat color depends on two genes One gene determines the pigment color (with alleles B for black and b for brown) The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4  BbCc BbCc Sperm Eggs BBCC BbCC BBCc Fig. 14-12  BbCc BbCc Sperm 1/4 1/4 1/4 1/4 BC bC Bc bc Eggs 1/4 BC BBCC BbCC BBCc BbCc Figure 14.12 An example of epistasis 1/4 bC BbCC bbCC BbCc bbCc 1/4 Bc BBCc BbCc BBcc Bbcc 1/4 bc BbCc bbCc Bbcc bbcc 9 : 3 : 4

Interactive Question 14.7 MmBb x MmBb Phenotype Genotype Ratio Black

Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum Polygenic inheritance results in an additive effect of two or more genes on a single phenotype which results in quantitative variation. Skin color in humans is an example of polygenic inheritance Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 14-13  AaBbCc AaBbCc Sperm 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 Figure 14.13 A simplified model for polygenic inheritance of skin color 1/8 Eggs 1/8 1/8 1/8 1/8 Phenotypes: 1/64 6/64 15/64 20/64 15/64 6/64 1/64 Number of dark-skin alleles: 1 2 3 4 5 6

Nature and Nurture: The Environmental Impact on Phenotype Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype The norm of reaction is the phenotypic range of a genotype influenced by the environment Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 14-14 For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity Figure 14.14 The effect of environment on phenotype

Norms of reaction are generally broadest for polygenic characters Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The risk of heart disease follows a continuum The risk of heart disease follows a continuum. It is multifactorial due to both genetic and environmental influences.

Role of environment? An organism’s phenotype reflects its overall genotype and unique environmental history Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

What is the dominant trait? ____________________________ EXPERIMENTAL DATA In a cross of true-breeding green seed coat peas with true-breeding yellow seed coat peas, all of the F1 peas had green-seed coats. In the F2, you counted 122 green ones and 33 yellow seeded peas. The total number of individuals you counted, N, is 155. What is the dominant trait? ____________________________ What is your hypothesis for the F2 population? Show a Punnett square to show the proposed results.  Fill in the tables below with your observed data, calculate the expected result according to your hypothesis. Determine the Χ2 value for each experiment, and use the table of probabilities to accept or reject the null hypotheses. Monohybrid cross Phenotypes Observed Expected d = o –e d2 d2/e   Total (X2 )_____  X2 = _____     degrees of freedom: _____ Range of probability: ______________ Accept or reject null hypothesis? __________  

Experimental Data for Chi Square Determination In a cross of true-breeding rough seed coat peas with true-breeding smooth seed coat peas, all of the F1 peas had rough-seed coats. In the F2, you counted 79 rough coats and 33 smooth coats. The total number of individuals you counted is 112. What is the dominant trait? ____________________________ What is your hypothesis for the F2 population? Show a Punnett square to show the proposed results.

Fill in the tables below with your observed data, calculate the expected result according to your hypothesis. Determine the Χ2 value for each experiment, and use the table of probabilities to accept or reject the null hypotheses. Monohybrid cross Phenotypes Observed (o) Expected (e) d = o – e d2 d2/e Rough 79 Smooth 33 Total _____ Χ2 = _____     degrees of freedom: _____ Range of probability: ______________ Accept or reject null hypothesis? __________