Classical Mendelian Genetics BiologyUnit 13 Definition of Genetics

Classical Mendelian Genetics Biology-Unit 13

Definition of Genetics Branch of Biology that studies the ways in which hereditary information is passed on from parents to offspring. Genetics as a science was first practiced by Gregor Mendel was an Austrian monk who experimented for many years with pea plants

Mendel (cont. ) Before his experiments, people believed that “factors” were inherited as a blend of Mom and Dad’s “factors”

Mendel (cont. ) Why Peas? Mendel used peas to study inheritance because: True breeding strains were available Peas are easy to grow and maintain They have a short generation time

Mendel (cont. ) Peas have many easy to observe traits including: n Seed color - Green or yellow n Seed shape - Round or wrinkled n Pod color - Green or yellow n Pod shape - Smooth or constricted n Flower color - White or purple n Flower position - Axial or terminal n Plant size - Tall or dwarf

Mendel (cont. ) Pea flowers are constructed in such a way that they typically self fertilize Because of this, it is relatively easy to control crosses in peas By removing the anthers of one flower and artificially pollinating using a brush, crosses can be easily controlled in peas.

The seven pea characteristics studied by Mendel Dominant Flower Color Flower Position Seed Color Seed Shape Pod Color Plant Height Recessive

Mendel’s First Experiment • P generation – (parental) truebreeding • F 1 - (first filial) offspring of P generation • F 2 – (second filial) offspring from F 1 cross

Mendel’s First Experiment In this breeding experiment, Mendel cross-pollinated (hybridized) two contrasting, truebreeding pea varieties. Mendel would then allow the F 1 hybrids to self-pollinate to produce an F 2 generation.

Mendel’s First Experiment When Mendel allowed the F 1 plants to selffertilize, the F 2 generation included both purple-flowered and white-flowered plants. n The white trait, absent in the F 1, reappeared in the F 2.

Mendel’s Results When crossing purple flowered peas with white flowered peas, Mendel got the following results: In the first filial (F 1) generation all offspring produced purple flowers In the second generation (second filial or F 2) approximately a 3: 1 ratio of purple to white flowers was seen

Mendel’s Results Because the F 1 generation did not produce light purple flowers and because white flowers showed up in the F 2 generation, Mendel disproved blended inheritance. Mendel said that the parents had two sets of factors (genes), thus two copies of the flower color gene Each gene has two varieties-called alleles In the case of the flower color gene, the two alleles are white and purple

Mendel’s Results In the F 1 generation, the white allele was hidden by the purple “dominant” allele In the F 2 generation, 1/4 of the offspring wound up with two copies of the white allele, thus they were white Mendel termed this discovery “The Law of Dominance”

Mendel’s First Law: The Law of Dominance If only one of the genes in a pair is expressed, it is called the dominant allele The gene which is present but not expressed is called the recessive allele Recessive alleles may be hidden and not shown for many generations When two copies of a recessive allele are present then the recessive trait will show

Mendel’s 2 nd Law-The Law of Segregation The two alleles for each character segregate (separate) during gamete production

Mendel’s 2 nd Law-The Law of Segregation

Mendel’s 3 rd Law-The Law of Independent Assortment When Mendel crossed peas and looked at two different traits, he discovered that the traits assorted independently In other words, if he was looking at the height of the plants and the color of the flowers, all four possible combinations of height and flower color were produced: Tall Purple Tall white Dwarf Purple Dwarf white

Mendel’s 3 rd Law-The Law of Independent Assortment • As long as genes are on different chromosomes, they will assort independently • This meant that a seed’s color did not depend on its shape, nor did height of the plant affect the flower color

Mendelian Genetics Vocabulary Character –heritable feature Trait – each variant for a character Dominant Allele: a gene that exerts it’s full effect no matter what effect it’s allelic partner may have. The dominant allele is represented by a capitalized letter (ex. T=tall)

Mendelian Genetics Vocabulary Recessive Allele: a gene that is masked by the dominant allele The recessive allele is represented by the same letter but lower case (ex. t=short) Remember: Diploid organisms have 2 copies of every gene, therefore they have 2 alleles for a specific trait

Mendelian Genetics Vocabulary Homozygous: an organism that contains identical alleles (ex. TT or tt) Also called pure-bred R R Homozygous parents can only pass one form of an allele to their offspring. R R

Mendelian Genetics Vocabulary Heterozygous: an organism that contains two different alleles (ex. Tt) Also called hybrid Heterozygous parents can pass either of two forms of an allele to their offspring.

Mendelian Genetics Vocabulary Genotype: genetic make-up of an organism or the specific alleles n represented by letters n ex. tt, TT, Tt Phenotype: the observable appearance of an individual resulting from it’s genotype n ex. Tall, short, yellow

Solving Genetic Problems Involving Different Types of Inheritance The Key Steps n 1. ) decide the type of inheritance involved n 2. ) choose a capital letter for the dominant trait – it’s lower case for the recessive counterpart n 3. ) decide your mating (cross) n 4. ) construct a “Punnett” Square to describe results of mating n 5. ) be sure to answer the question(s) posed in the problem

Making A Punnett Square White Flower Parent (pp) p Purple Flower Parent (PP) P Gametes P p

Basic Types of Inheritance + Genetic Crosses A. ) Simple Dominance – One trait dominates over a second n 1. ) Test Cross – Used to determine whether an organism is homozygous or heterozygous. Test organism is crossed to a pure recessive.

Basic Types of Inheritance + Genetic Crosses Double cross-Looking at the inheritance of two separate traits. If both organisms are hybrid for both traits this becomes a “dihybrid-cross” Mendel noticed that whenever two dihybrids are crossed, the ratio of the genotypes is 9: 3: 3: 1

Incomplete Dominance When traits are crossed and a blending of the dominant and recessive phenotypes takes place.

Basic Types of Inheritance + Genetic Crosses C. ) Co-Dominance (Multiple Alleles) – Two or more alleles both show over a third (human blood types) D. ) Sex-Linkage – Traits are carried only on the sex chromosomes Phenotype (Blood Genotype Type) A IAIA or IB i B IBIB i AB IAIB O ii

Genetics and People Genetic Diseases 1. sickle cell anemia Production of abnormal, crescent shaped red blood cells. It is autosomal (not in the sex pair) and recessive (you can be normal, a carrier, or have the disease) caused by malformed hemoglobin blood loses the ability to carry oxygen red blood cells can stick to sides of blood vessels and cause a clot

Genetics and People 2. Tay-Sachs disease: It destroys the nervous system of children. They usually do not live beyond the age of four. It is autosomal and recessive (you need 2 copies to have the disease) common in Jews of eastern European descent

Genetics and People 3. PKU (phenylketonuria): It is an inherited condition of severe mental retardation due to a failure to breakdown phenylalanine, an amino acid. It is autosomal and recessive. Brain fails to develop during infancy Can be prevented by eating a diet free of phenylalanine All newborns are screened for this disease

Genetics and People 4. Cystic fibrosis A respiratory disorder caused by excessive production of thick mucus in the lungs and pancreas. Autosomal and recessive.

Genetics and People 5. Thalassemia: A severe form of anemia (loss of red blood cells). It is autosomal and recessive. 6. Huntington’s Disease: An incurable form of insanity that starts at age 40. It is autosomal and dominant.

Genetics and People 7. Hemophilia- a sex-linked recessive disease Blood is missing a protein which allows it to clot properly Genetic engineering has led to manufacture of this protein and better treatment

Genetics and People B. genetic screening 1. Amniocentesis: a sample of amniotic fluid is genetically analyzed.

Genetics and People 2. Karyotype: an enlarged photograph of arranged human chromosomes.

Genetics and People 3. Genetic Counseling: when high risk groups are tested for and advised on possible genetic disorders of their offspring. Examples: genealogies or family pedigrees

Genetic Applications A. Plant and animal breeding: breeders apply the principles of genetics to improve plants and animals. Breeding Methods: 1. Artificial Selection: plants or animals having desired qualities are selected to reproduce their kind. Examples: horse and dog breeders

Genetic Applications 2. Hybridization Crosses are made between organisms having desirable qualities in the attempt to produce some offspring that incorporate these qualities. Examples: American cattle and Brahman cattle

Genetic Applications 3. Inbreeding Is the mating of organisms of the same strain or type Example: German shepherds will only be bred with German shepherds Problem: offspring are more susceptible to diseases and have shorter life spans.

Genetic Applications 4. Vegetative Propagation Desirable traits in plants can be easily produced by asexual means. Example: seedless fruits

Heredity and Environment Both the genetics and the environment affect the phenotypes and development of organisms.

Heredity and Environment 1. Effect of Temperature on hair color in the Himalayan rabbits are white except for its black nose, ears, tail, and feet. In experiments, fur was removed from the rabbits back, an ice pack was applied while the fur grew back.

Heredity and Environment This low temperature produced black fur. Conclusion: low temperature prevents the action of the white fur gene and enzyme.