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.
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 Parental generation (P) Stamens Carpel Pollinated carpel matured into pod Planted seeds from pod Examined offspring: all purple flowers First generation offspring (F1)
Some genetic vocabulary • Some genetic vocabulary • Character: a heritable feature, such as flower color • Trait: a variant of a character, such as purple or white flowers
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. → → →
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
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 self-pollinated 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
Mendel reasoned that only the purple flower gene was affecting flower color in the F1 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.
Mendel’s Model • Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring. • Four related concepts make up this model
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 Locus for flower-color gene Allele for white flowers
The second concept: • for each character, an organism inherits two alleles, one from each parent • These two alleles may be identical or different.
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.
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
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. • http://www.youtube.com/watch?v=F3AKldl6JZg
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 http://www.youtube.com/watch?v=d4izVAkhMPQ&feature=related
LE 14-5_2 P Generation Purple flowers PP White flowers pp Appearance: Genetic makeup: p P Gametes F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: 1 1 p P 2 2 F1 sperm P p F2 Generation P PP Pp F1 eggs p Pp pp 3 : 1
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).
Useful Genetic Vocabulary • 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 PP or Pp
LE 14-6 Genotype Phenotype PP (homozygous Purple 1 Pp (heterozygous 3 Purple 2 Pp (heterozygous Purple pp (homozygous White 1 1 Ratio 1:2:1 Ratio 3:1
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.
The Law of Independent Assortment • 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 genotype GgYy will give four classes of gametes
LE 14-8 P Generation YYRR yyrr Gametes yr YR YyRr F1 Generation Hypothesis of dependent assortment Hypothesis of independent assortment Sperm YR Yr yR yr 1 1 1 1 4 4 4 4 Sperm Eggs YR yr 1 1 2 2 YR 1 4 Eggs YYRR YYRr YyRR YyRr YR 1 2 F2 Generation (predicted offspring) YYRR YyRr Yr 1 4 YYRr YYrr YyRr Yyrr yr 1 2 YyRr yyrr yR 1 4 YyRR YyRr yyRR yyRr 3 1 4 4 yr 1 4 Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9 3 3 3 16 16 16 16 Phenotypic ratio 9:3:3:1
The Law of Independent Assortment • Dihybrid Cross: http://www.youtube.com/watch?v=9kAFEbhzz94
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
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: IA, IB, and i. • The enzyme encoded by the IA allele adds the A carbohydrate (antigen), whereas the enzyme encoded by the IB allele adds the B carbohydrate (antigen); the enzyme encoded by the i allele adds neither
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
Extending Mendelian Genetics for Two or More Genes • Some traits may be determined by two or more genes
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 14-12 AaBbCc AaBbCc aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC 20/64 15/64 Fraction of progeny 6/64 1/64
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
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 First generation (grandparents) Ww ww ww Ww Second generation (parents plus aunts and uncles) Ww ww ww Ww Ww ww Third generation (two sisters) WW ww or Ww Widow’s peak No widow’s peak Dominant trait (widow’s peak)
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
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
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