Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Mendel’s Experimental, Quantitative Approach Advantages of pea plants for genetic study: – They are available in many varieties. For example, one variety has purple flowers, while a contrasting variety has white flowers. – The use of peas also gave Mendel strict control over which plants mated with which. So he could always be sure of the parentage of the new seeds.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Pea Flowers – Each pea flower has pollen-producing organs (stamens) and egg-producing organs (carpels). – Cross-pollination ( The transfer of pollen from an anther of the flower of one plant to a stigma of the flower of another plant ) can be achieved by dusting one plant with pollen from another. Cross-pollination

LE 14-2 Removed stamens from purple flower Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower Carpel Stamens Parental generation (P) Pollinated carpel matured into pod Planted seeds from pod Examined offspring: all purple flowers First generation offspring (F 1 )

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some genetic vocabulary – Character: a heritable feature, such as flower color – Trait: a variant of a character, such as purple or white flowers

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel also made sure he started his experiments with varieties that were true-breeding. A true-breeding plant is one that, when self- fertilized, only produces offspring with the same traits. → → →

Copyright © 2005 Pearson Education, Inc. publishing as 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 F 1 generation When F 1 individuals self-pollinate, the F 2 generation is produced

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The Law of Segregation When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F 1 hybrids were purple When Mendel self-pollinated the F 1 hybrids, many of the F 2 plants had purple flowers, but some had white Mendel discovered a ratio of about three to one, purple to white flowers, in the F 2 generation

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel reasoned that only the purple flower gene was affecting flower color in the F 1 hybrids Mendel called the purple flower color a dominant trait and white flower color a recessive trait Mendel observed the same pattern of inheritance in other six pea plant characters.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel’s Model Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F 2 offspring. Four related concepts make up this model

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The first concept: The gene for a particular character such as flower color resides at a specific position on a certain chromosome known as locus. The gene for flower color in pea plants as an example exists in two versions, one for purple flowers and the other for white flowers. These alternative versions of a gene (gene for purple and gene for white) are now called alleles.

LE 14-4 Allele for purple flowers Homologous pair of chromosomes Allele for white flowers Locus for flower-color gene

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The second concept: for each character, an organism inherits two alleles, one from each parent These two alleles may be identical or different.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The third concept: If the two alleles at a locus differ – Then one, the dominant allele, is fully expressed in the organism’s appearance – The other allele, the recessive allele, has no noticeable effect on the organism’s appearance in the presence of the dominant allele.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The fourth concept: Now known as 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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel derived the law of segregation by performing monohybrid crosses i.e. breeding experiments using parental varieties that differ in a single character such as flower color. ?v=F3AKldl6JZghttp:// ?v=F3AKldl6JZg

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Punnett Square The combination resulting from a genetic cross may be predicted by using a Punnett Square. A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele PQ&feature=related

LE 14-5_2 Appearance: P Generation Genetic makeup: Gametes F 1 Generation Appearance: Genetic makeup: Gametes: F 2 Generation Purple flowers Pp P p P p F 1 sperm F 1 eggs PPPp pp P p 3: 1 Purple flowers PP White flowers pp P p

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Useful Genetic Vocabulary An organism that is homozygous for a particular gene, also called a homozygote – Has a pair of identical alleles for that gene, such as homozygous dominant (PP) and homozygous recessive (pp) – Exhibits true-breeding An organism that is heterozygous for a particular gene, also called a heterozygote – Has a pair of alleles that are different for that gene such as (Pp).

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An organism’s phenotype – Is its physical appearance An organism’s genotype – Is its genetic makeup In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes Useful Genetic Vocabulary PP Pp or

LE 14-6 Phenotype Purple 3 Genotype PP (homozygous Pp (heterozygous Pp (heterozygous pp (homozygous Ratio 1:2:1 White Ratio 3:1 1

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Law of Independent Assortment Mendel identified his second law of inheritance by following the inheritance of two characters at the same time A dihybrid cross is a mating of parental varieties differing in two characters such as seed color and seed shape in peas.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 other pairs of alleles during gamete formation. The Law of Independent Assortment The genotype GgYy will give four classes of gametes

LE 14-8 P Generation F 1 Generation YYRR Gametes YR yr yyrr YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Sperm Eggs YR Yr yrYR yr Eggs YYRRYyRr yyrr yR yr Phenotypic ratio 3:1 F 2 Generation (predicted offspring) YYRR YYRrYyRRYyRr YYRrYYrrYyRrYyrr YyRRYyRryyRRyyRr YyRrYyrryyRryyrr Phenotypic ratio 9:3:3:1 YRYryRyr Sperm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Law of Independent Assortment Dihybrid Cross:

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Spectrum of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, one allele is not completely dominant over the other, so the heterozygote has a phenotype that is intermediate between the phenotypes of the two homozygotes. In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

Copyright © 2005 Pearson Education, Inc. publishing as 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: I A, I B, and i. The enzyme encoded by the I A allele adds the A carbohydrate (antigen), whereas the enzyme encoded by the I B allele adds the B carbohydrate (antigen); the enzyme encoded by the i allele adds neither

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Multiple Alleles

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Symptoms of sickle-cell disease include physical weakness, pain, organ damage, and even paralysis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Extending Mendelian Genetics for Two or More Genes Some traits may be determined by two or more genes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Polygenic Inheritance polygenic inheritance: two or more genes affect a single phenotype that is expressed as a result of an additive effect of these genes. Skin color in humans is an example of polygenic inheritance

LE aabbccAabbccAaBbccAaBbCcAABbCcAABBCcAABBCC AaBbCc 20 / / 64 6 / 64 1 / 64 Fraction of progeny

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity Norms of reaction are generally broadest for polygenic characters Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 14.4: Many human traits follow Mendelian patterns of inheritance Humans are not good subjects for genetic research because: – generation time is too long; – parents produce relatively few offspring; – and breeding experiments are unacceptable However, basic Mendelian genetics endures as the foundation of human genetics

LE 14-14a Wwww Ww wwWwww Ww WWww or Ww No widow’s peak Third generation (two sisters) Widow’s peak Second generation (parents plus aunts and uncles) First generation (grandparents) Dominant trait (widow’s peak)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recessively Inherited Disorders Many genetic disorders are inherited in a recessive manner Recessively inherited disorders show up only in individuals homozygous for the allele Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sickle-Cell Disease Sickle-cell disease affects one out of 400 African-Americans The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells Symptoms include physical weakness, pain, organ damage, and even paralysis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dominantly Inherited Disorders Some human disorders are due to dominant alleles One example is achondroplasia, a form of dwarfism that is lethal when homozygous for the dominant allele