Mendelian Genetics Chapter 6 Father of Genetics: Gregor Johann Mendel Austrian monk Studied the inheritance of traits in pea plants Developed the laws of inheritance Mendels work was not recognized until the turn of the 20 th century Gregor Johann Mendel continued.. Between 1856 and 1863 Mendel cultivated and tested some 28,000 pea plants He found that the plants offspring retained traits of the parents Mendel was called the Father of Genetics Particulate Inheritance -Mendel stated that physical traits are inherited as particles -Mendel did not know that these particles were actually genes on chromosomes. -Remember genes are segments of DNA that code for a particular protein! Genetic Terminology Trait- any characteristic that can be passed from parent to offspring Heredity- passing of traits from parent to offspring Genetics- study of heredity Punnett Squares Used to help solve genetics problems They show all the possible genotypes & phenotypes of the offspring from a test cross of two specific parents Types of Genetic Test Crosses Monohybrid cross- cross involving a single trait Example: Flower color Dihybrid cross- cross involving two traits Example: flower color and plant height Designer Genes Alleles- two forms of a gene (dominant and recessive) Dominant- most common of the two genes and expressed in a hybrid (represented by a capital letter) Recessive- shows up less often in a cross and is hidden when a dominant gene is present (represented by a lower case letter) More Terminology Genotype- gene combination for a trait (Example: RR, Rr, or rr) Phenotype- the physical feature resulting from a genotype (Example: Red or white) Genotype and Phenotype in Flowers Genotypes of alleles: R = red flower r = yellow flower All genes occur in pairs, so 2 alleles affect a characteristic Possible combinations are: Genotypes:RRRrrr Phenotypes:REDREDYELLOW Genotypes Homozygous genotype: gene combination involving 2 dominant or 2 recessive alleles (RR or rr); also called purebred Heterozygous genotype: gene combination of one dominant and one recessive allele (Rr); also called hybrid Mendels Experiments: Why peas, Pisum sativum? Peas can be grown in a small area Produce lots of offspring Produce pure plants when allowed to self-pollinate Can be artificially cross-pollinated Reproduction in Flowering Plants Pollen contains sperm produced by the stamen Ovary contains eggs found inside the flower Mendels Experimental Methods -Mendel hand-pollinated flowers using a paintbrush -He snipped the stamens (male parts) to prevent self-pollination -Covered each flower with a cloth bag Generation Gap P 1 Generation- the parental generation in a breeding experiment F 1 Generation- the first generation of offspring in a breeding experiment F 2 Generation- the second generation of offspring in a breeding experiment (from breeding individuals from the F 1 generation) Following the Generations Cross 2 pure plants TT and ss Results in all hybrids (Ts) Cross two hybrids and get 3 tall and one short (TT, Ts, and ss) Law of Dominance In a cross of parents that are pure for different traits, only one form of the trait will appear in the next generation All the offspring will be heterozygous and express only the dominant trait. GG x gg = all Gg Mendels Laws Law of Segregation- during the formation of gametes (sex cells; aka eggs or sperm), the two alleles responsible for a trait separate from each other Alleles for a trait are then recombined at fertilization to produce the genotype of the offspring bb b b 2. A pure female white flower (rr) rr Yy Male red flower (Rr) Female white flower Rr Sperm 1Sperm 2 rr Egg 1Egg 2 = = = = GenotypePhenotype Law of Independent Assortment Alleles for different traits are distributed to sex cells independently of one another. This law can be illustrated using dihybrid crosses. Bellringers Week of 12/14/2015 Jenny has genotype tt. Her genotype would best be described as: a.heterozygous dominant b.homozygous recessive c.homozygous dominant d.heterozygous recessive The idea that for any particular trait, the pair of alleles of each parent separate and only one allele from each parent passes to an offspring is Mendel's principle of: a) independent assortment b) hybridization c) segregation d) dependent assortment A.25% B.50% C.75% D.100% BB When crossing a homozygous recessive with a heterozygote, what is the chance of getting an offspring with the homozygous recessive phenotype? A.100% B.75% C.50% D.25% If you had two guinea pigs of opposite sex, both homozygous, one black and one brown, but you didn't know which was the dominant characteristic, how could you be certain that the guinea pigs are truly homozygous? A) The guinea pigs would be homozygous for black (or brown) coat color if each strain could be bred for many generations and only black (or brown) colored offspring were produced. B)If the immediate parents of the black (or brown) guinea pigs were both of that color, it proves they are homozygous. C)If a cross between the black and brown guinea pig produced four all black offspring, the black guinea pig would have to be homozygous for black coat color. D)Any of the results would prove the black guinea pig was homozygous. E)Only microscopic examination of the guinea pig's genes could absolutely confirm homozygosity.