2 Mendel’s experiments (2015)

Genetics is a biological discipline that studies: the transmission of traits from one generation to the next gene distribution, variation and change in populations evolution of genes Genetics

Character: observable physical feature (e.g., flower colour). Trait: form of a character (e.g., purple flowers or white flowers). A heritable trait is passed from parent to offspring. Genetics: Mendel and Beyond - What are the mendelian laws of inheritance?

Mendel’s experiments Genetics: Mendel and Beyond - What are the mendelian laws of inheritance?

Genetics: Mendel and Beyond Gregor Mendel and his garden Mendel carried out the first study on heredity in the second half of the 19th century using garden pea plants (Pisum sativum).

6 Plant reproduction

In Mendel's time, inheritance was thought to be the result of a blending of two parents' characters

Mendel looked for well-defined, true- breeding traits (the observed trait is the only one present for many generations). True-breeding strains (pure line) were isolated by inbreeding and selection. He concentrated on seven characters, each showing two alternate traits. Genetics: Mendel and Beyond - What are the mendelian laws of inheritance?

Controlling pollination and plant generations: pollen from one parent was transferred to the carpel of the other parent parental generation = P; resulting offspring = first filial generation or F 1 ; if F 1 plants self pollinate, produce second filial generation or F 2. Genetics: Mendel and Beyond - What are the mendelian laws of inheritance? P

Mendel’s first experiment: 1.crossed plants differing in just one trait (P); 2.F 1 generation are monohybrids; 3.the monohybrids were then allowed to self pollinate to form the F 2 generation - a monohybrid cross; 4.Mendel repeated this for all seven traits. Genetics: Mendel and Beyond - What are the mendelian laws of inheritance?

One trait of each pair disappeared in the F 1 generation and reappeared in the F 2 — these traits are recessive. The trait that appears in the F 1 is the dominant trait. Genetics: Mendel and Beyond - What are the mendelian laws of inheritance? P F1F1

The ratio of dominant to recessive in the F 2 was about 3:1. Genetics: Mendel and Beyond - What are the mendelian laws of inheritance?

Mendel concluded that each plant possesses two factors controlling a given characteristic; these could be identical (as in true-breeding plants) or different (as in hybrid plant) and in this case one dominates the other; these factors separate and go to different gametes when a parent reproduces These “factors” are now called genes. The different forms of a single gene are called alleles. Genetics: Mendel and Beyond - What are the mendelian laws of inheritance?

p p PP 18 Try yourself P P P p p

Mendel’s first and second law The Law of Dominance: in a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. The Law of Segregation: each characteristic is controlled by two factors, which separate and go to different gametes when an organism reproduces. Genetics: Mendel and Beyond - What are the mendelian laws of inheritance?

Mendel’s second experiment Mendel wondered whether different characteristics are inherited together. For example, are purple flowers and tall stems always inherited together? Or do these two characteristics show up in different combinations in offspring? To answer these questions, Mendel next investigated two characteristics at a time. Genetics: Mendel and Beyond - What are the mendelian laws of inheritance?

Independent assortment

Mendel’s third law The Law of Independent Assortment Factors controlling different characteristics are inherited independently of each other Genetics: Mendel and Beyond - What are the mendelian laws of inheritance?

How modern genetics explain Mendel’s law of segregation? Today, we known that characteristics of organisms are controlled by genes (Mendel’s “factors”) located on chromosomes. A gene is a sequence on a DNA molecule that resides at a particular site on a chromosome—the locus—and encodes a particular character. Genes are expressed as proteins with particular functions. Different alleles of a gene separate during meiosis. Genetics: Mendel and Beyond - What are the mendelian laws of inheritance?

Meiosis accounts for the segregation of alleles (part 1)

Genetics: Mendel and Beyond - What are the mendelian laws of inheritance? Meiosis accounts for the segregation of alleles (part 2)

Genetics: Mendel and Beyond - What are the mendelian laws of inheritance? Meiosis accounts for independent assortment of alleles

When gametes unite during fertilization, the resulting zygote inherits two alleles for each gene. One allele comes from each parent. The alleles an individual inherits make up the individual’s genotype, i.e. The genetic makeup ov an organism. The expression of an organism’s genotype produces its phenotype, i.e.the the physical appearance of an organism (e.g., spherical seeds). Genetics: Mendel and Beyond - What are the mendelian laws of inheritance?

An individual that inherits two alleles of the same type is said to be homozygous for the allele (e.g., AA or aa). An organism with two different alleles is called heterozygous (e.g., Aa). Genetics: Mendel and Beyond - What are the mendelian laws of inheritance? homozygous heterozygous

Genetics: Mendel and Beyond - What are the mendelian laws of inheritance? Problem samples 1.Classify the following as heterozygous or homozygous: RR, Rr, yy, YyRR 2.What is the phenotype of the following: Yy, Ry, yy, YyRr 3.What genotypes could have a purple flowered plant? PP, pp, Pp, Yp 4.Which cross would produce both green and yellow seed? YY x yy, Yy x Yy, Yy x yy, yy x yy