Genetics  Page 262

Genetics  Every living thing has a set of characteristics inherited from its parents (sexual reproduction) or parent (asexual reproduction).  Genetics is the study of heredity.

Gregor Mendel’s Peas  Gregor Mendel  born 1822  Polish monk  lived in a monastery  taught  looked after the monastery’s garden. In this garden, he did the work that would change biology forever.

Observing his pea plants, Mendel knew that: -the anthers of each flower produced pollen which contained the sperm. -the female portion of the flower produces the egg cells.

 During sexual reproduction, male and female gametes join and a nucleus containing both sets of genetic information is formed, a process known as fertilization.  Fertilization produces a new cell, a zygote, which develops into a tiny embryo. With flowering plants, this embryo is encased in a seed.

 Pea plants usually self-pollinate, meaning that they have a single parent. The pollen fertilizes the egg of the same plant.  True breeding plants are self-pollinating and therefore produce offspring identical to themselves.

Mendel’s Pea Plants  Mendel observed that some of his pea plants were tall, others short;  some had smooth pods, some wrinkled;  some had yellow pods, some green;  some had round peas, some wrinkled;  some had green peas, some white;  some had flowers in an axial position, some terminal;  some had yellow cotelydons, some green. Each offspring always had the same traits as its parent.

Mendel’s Experiments  Mendel wanted to see what would happen if he produced seeds by fertilizing the seeds from one plant with the pollen from another plant with different traits.  He removed the pollen-producing parts of one plant and dusted pollen from another plant on the stigma (check the diagram in slide 4).  Mendel cross-pollinated the pea plants.

 Mendel studied 7 different pea plant traits. A trait is a specific characteristic that varies from one individual to another.

 Genes are the molecular factors (pieces of code along DNA)that determine traits.  The different forms of a gene are called alleles. (In peas, genes can occur in two contrasting forms. i.e. yellow pods/ green pods)

Terminology  P : Each original pair of plants (the parental generation)  F1 : the offspring (first filial) of the P o F2: the offspring of the F1

 Hybrids : The offspring of crosses between parents with different traits. Ex. A yellow-podded pea plant that is crossed with a green-podded pea plant.

Gregor Mendel concluded 2 things from his experiments:  Heredity is determined by the elements that are contained in the cell contributed by each parent of the organism.  When two characteristics are crossed (as in cross pollination, one of the characteristics is dominant over the other. (this theory turned out not to be applicable in all cases)

Dominance Mendel’s principle of dominance states that :  some alleles are dominant and some are recessive.  An organism with a dominant allele for a specific trait will always exhibit that form of the trait.  An organism with a recessive allele for a specific trait will only exhibit that trait if the dominant allele is not present. (2 recessive alleles)

Take the alleles for plant height. T = tall t = short First cross the pure-breeding P: - all of the plants that are tall with all of the plants that are short.

1 st cross producing the F 1

2 nd cross producing the F2

Vocabulary:  Homozygous : Having the same alleles for a gene. (TT, tt)  Heterozygous : Having different alleles for a gene. (Tt)

 Locus : Location of a particular gene on a chromosome.

 Genotype : Actual genetic makeup. What kinds of alleles the organism has (Tt, TT, tt).  Phenotype : The trait displayed. (tall, short)

 Genome : Totality of genes. (human genome)  Diploid : 2n (2 sets of chromosomes)  Haploid : n (1 set of chromosomes)

Problem A homozygous black-coated pig (B) is crossed with a heterozygous pig. What are the phenotypes of the F1 generation? What are the possible phenotypes of the F2 generation?

Problem Mendel crossed pea plants that were homozygous dominant for round yellow peas with plants that were homozygous recessive for wrinkled green peas. What did he get in the F 1 generation?

Segregation is the separation of alleles during gamete formation.

Dihybrid cross F

Dihybrid cross F+

Segregation

The Principle of Independent Assortment States: genes for different traits can segregate independently during the formation of gametes. Independent assortment helps to account for the many variations observed in plants, animals, and other organisms.

 A cross of an individual with a dominant phenotype to a homozygous recessive individual in order to find out its genotype is called a testcross.

Genetic variation "reshuffling the genes" comes from several sources with each meiosis and fertilization like a 'roll of the dice’- so no two individuals are alike!

Sources of genetic variation: 1. Crossing Over at prophase I 2.Independent assortment : Homologues line up or "shuffle" randomly on the metaphase plate in Meiosis I. With 23 chromosomes assorting independently, there are 2^23, or 8 million, possible assortments of chromosomes inherited for every cell!! 3. Random fertilization : The ovum has 8 million possible chromosome combinations, so does the sperm cell. 8 million x 8 million = 64 trillion possible diploid combinations in EACH AND EVERY zygote! In other words, you're unique...(...just like everyone else.....).