Fig. 14-1 Mendel & the gene idea Overview:

Fig. 14-1 Mendel & the gene idea Overview: Drawing from the Deck of Genes 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) Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 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 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 Fig. 14-2a TECHNIQUE 1 2 Parental generation

(P) Stamens Carpel 3 4 Fig. 14-2b RESULTS First filial generation offspring (F1) 5 Mendel chose to track only those characters that

varied in an either-or manner He also used varieties that were true-breeding (pure strains) meaning that plants produce offspring of the same variety when they self-pollinate) Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 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 Fig. 14-3-1

EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers Fig. 14-3-2 EXPERIMENT P Generation (true-breeding

parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers Fig. 14-3-3 EXPERIMENT

P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation

705 purple-flowered plants 224 white-flowered plants 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 Different versions of the gene are called alleles Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Table 14-1 Mendels Model Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring Four related concepts make up this model These concepts can be related to what we now know about genes and chromosomes Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-4 Principle of Dominance Allele for purple flowers Locus for flower-color gene

Homologous pair of chromosomes Allele for white flowers Allele is an alternate version of a gene & ea. indiv. Inherits an allele from ea. parent Fig. 14-5-1 P Generation Purple flowers Appearance: PP Genetic makeup: Gametes:

P White flowers pp p Fig. 14-5-2 P Generation Purple flowers Appearance: PP Genetic makeup: Gametes: White flowers pp p

P F1 Generation Appearance: Genetic makeup: Gametes: Purple flowers Pp 1 /2 P 1 /2 p

Fig. 14-5-3 P Generation Law of Segregation Purple flowers Appearance: PP Genetic makeup: Gametes: White flowers pp p P

F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: 1 /2 P 1 /2 Sperm F2 Generation

P p PP Pp Pp pp P Eggs p 3

1 p Useful Genetic Vocabulary An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character Unlike homozygotes, heterozygotes are not truebreeding Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Because of the different effects of dominant and recessive alleles, an organisms traits do not always reveal its genetic composition

Therefore, we distinguish between an organisms phenotype, or physical appearance, and its 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 Fig. 14-6 3 Phenotype Genotype Purple

PP (homozygous) Purple Pp (heterozygous) 1 2 1 Purple Pp (heterozygous)

White pp (homozygous) Ratio 3:1 Ratio 1:2:1 1 The Testcross How can we 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 The answer is to carry out 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 Test cross Dominant phenotype, Recessive phenotype, unknown genotype: known genotype: PP or Pp? pp Predictions

If PP Sperm p p P Pp Eggs P Pp Eggs P Pp

If Pp Sperm p p or p Pp Pp Pp pp pp

RESULTS or All offspring purple /2 offspring purple and 1/ offspring white 2 1 Fig. 14-7a TECHNIQUE Test cross Dominant phenotype, Recessive phenotype, known genotype:

unknown genotype: pp PP or Pp? Predictions If PP Sperm p p P Eggs Pp Pp P Pp Pp

If Pp Sperm p p or P Eggs p Pp Pp pp pp

Fig. 14-7b Test cross RESULTS or All offspring purple /2 offspring purple and 1/ offspring white 2 1 The Law of Independent Assortment Mendel derived the law of segregation by following a single character

The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character A cross between such heterozygotes is called a monohybrid cross But what happened when 2 traits were observed? Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-8b RESULTS 315 108 101

32 Phenotypic ratio approximately 9:3:3:1 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 Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes Genes located near each other on the same chromosome tend to be inherited together Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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