Introduction to Genetics

The Work of Gregor Mendel

• Genetics: The scientific study of heredity. (Is now at the core of a revolution in understanding biology.)

• Gregor Mendel: Was an Austrian Monk, who was in charge of the monastery garden. He was the first person to trace the characteristics of successive generations of a living thing (pea plants).

• Mendel noticed while doing his work in the gardens, that part of each flower produces pollen, which contains the plant’s male reproductive cells, or sperm. Similarly, the female portion of the flower produces egg cells.

• During sexual reproduction, male and female reproductive cells join, in a process known as fertilization. Fertilization produces a new cell, which develops into a tiny embryo encased within a seed.

Mendel’s Genetics

• Mendel’s pea plants were true-breeding, meaning that if they were allowed to self pollinate, they would produce offspring identical to themselves.

• Mendel wanted to form different pea plants other than his true-breeding plants so he cross pollinated his pea plants by joining male and female reproductive cells from

different plants.

Cross Pollination

Mendel’s Genetics

• Mendel studied 7 different pea plant traits

• Trait: a specific characteristic, such as seed color or plant height, that varies from one individual to another.

• Hybrids: are the offspring of crosses between parents with different traits

• P= parental generation• F1= 1st son or daughter

generation (1st offspring)

Seed Shape

Flower Position

Seed CoatColor

Seed Color

Pod Color

Plant Height

PodShape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth Green Axial Tall

Mendel’s Seven F1 Crosses on Pea Plants

Mendel’s Conclusions From His Experiments1st: was that biological inheritance is determined by factors that are passed from one generation to the next. (Scientist, call the chemical factors that determine traits genes.)

2nd: Principal of dominance: states that some alleles are dominant and others are recessive. (Alleles: different forms of a gene)3rd: During gamete formation, alleles segregate from each other so that each gamete (sex cells) carries only a single copy of each gene. Each F1 plant produces two types of gametes- those with the allele for tallness and those with the allele for shortness

Genetic Material

Alleles

P Generation F1 Generation F2 Generation

Tall Short Tall TallTall Tall Tall Short

Principles of Dominance

Probability and Punnett Squares• Probability: The likelihood that a particular event will

occur.-Ex. Flipping a coin

• The principals of probability can be used to predict the outcomes of genetic crosses.

• Punnett Squares: used to predict and compare the genetic variations that will result from a cross– Homozygous: Organisms that have 2 identical alleles

for a particular trait “TT or tt” (true-breeding)– Heterozygous: Organisms that have 2 different alleles

for the same trait “Tt” (hybrid)– Phenotype: physical characteristics (Ex. tall plants)– Genotype: genetic makeup (Ex. TT or Tt)

Punnett Square

Exploring Mendel’s Genetics• The Two-Factor Cross:F1 Mendel crossed plants that were

homozygous dominant for round yellow peas with plants that were homozygous recessive for wrinkled green peas. (All of the F1 offspring were heterozygous dominant for round yellow peas.) p.270

• The Two-Factor Cross:F2 When Mendel crossed plants that were heterozygous dominant for round yellow peas, he found that the alleles segregated independently to produce the F2 generation.

• Independent Assortment: independent segregation of genes during the formation of gametes (The Principal of

Independent Assortment)– Accounts for the many genetic variations observed in plants,

animals, and other organisms.

Independent Assortment in Peas

Shared Traits• Some alleles are neither dominant nor recessive, and many traits are

controlled by multiple alleles or multiple genes.• Incomplete Dominance: Cases in which one allele is not

completely dominant over another (Ex. crosses between red flowers and white flowers are pink flowers)

• Co-dominance: Cases in which both alleles contribute to the phenotype. (Ex. Feathers that are speckled with black and white)

• Multiple Alleles: Many genes have more than two alleles. (Ex. A rabbit’s coat color is determined by a single gene that has at least 4 different alleles.)

• Polygenic Traits: Traits controlled by 2 or more genes. (Ex. At least 3 genes are involved in making the reddish-brown pigment in eyes of fruit flies.)

Incomplete Dominance

Co-dominance

Multiple Alleles

Polygenic Traits

Meiosis

Meiosis• Human cells contain 46 chromosomes• Homologous: chromosomes that each have a corresponding

chromosome from opposite-sex parent. (23 from dad and 23 from mom)

• Diploid (2N): used to refer to a cell that contains both sets of homologous chromosomes. (Human Body Cell “46”)

• Haploid (N): used to refer to a cell that contains only a single set of chromosomes and therefore only a single set of genes. (Gametes “23”)

Homologous

Diploid

Haploid

Meiosis• How are haploid (N) gamete cells produced from diploid (2N) cells?• Meiosis: is the process of reduction division in which the number of

chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

• Meiosis I and Meiosis II– Tetrad: When each chromosome pairs with its corresponding

homologous chromosome.– Crossing-over: When homologous chromosomes pair up and form

tetrads during meiosis I, and exchange portions of their chromatids.

Crossing-Over

Meiosis

Meiosis I

Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

Meiosis

Meiosis Animation

Meiosis Results

• Gamete formation takes places at the end of meiosis.– Male Animals: Sperm– Plants: Pollens grains containing haploid sperm cells– Female Animals: Egg– Plants: Egg Cell

Comparing Mitosis To Meiosis• Mitosis results in the production of two genetically

identical diploid cells (daughter cells).• Somatic Cells: Body Cells

• Meiosis produces four genetically different haploid cells.• Germ Cells: Sperm and Egg

Gene Linkage

• A chromosome is actually a group of linked genes.• It is the chromosomes, however, that assort

independently, not individual genes.

Gene Mapping• Gene Map: Shows were the relative locations of each known

gene is located on a particular chromosome. • Human Genome Project: Completed in 2006, mapped all the

genes on the 23 pairs of human chromosomes.

Exact location on chromosomes Chromosome 2

Gene Map of the Fruit Fly