Gregor Mendel's Work

Gregor Mendel's Work

• Organisms inherit unique characteristics from their parents through genes.
• Different forms of a gene are distributed to offspring.

Mendel's Experiments

• Peas were used as a model system.
• Experiments began with true-breeding, homozygous plants.
• These plants consistently produced offspring with the same traits as themselves.
• A trait is a specific characteristic, such as seed color or plant height.
• Hybrids are created from true-breeding individuals, resulting in heterozygous offspring.

Pollination in Pea Plants

• Mendel's pea plants were self-pollinators, capable of reproducing both asexually and sexually.
• Self-pollination occurs when the plant fertilizes its eggs with its own pollen.
• Cross-pollination eliminates self-pollination by removing the male or female parts from the plant.

Cross-Pollination Technique

• Cross-pollination involves transferring pollen from the male organ of one plant to the female organ of another using a paintbrush.
• Mendel tested the prevalence of 7 observable traits in pea plants.

Pea Characteristics

• Each of the 7 observable traits had different expressions of those genes.
• Flower color had both purple and white expressions.

Genes and Alleles

• Genes are segments of DNA passed from one generation to the next, determining an individual's characteristics.
• Alleles are different forms of a gene, e.g., round or wrinkled for seed shape.
• Seed color characteristics are determined by genes.
• Yellow and green seed colors are determined by alleles.
• The allele for yellow seeds is dominant to the allele for green.

Principle of Dominance

• Some alleles are dominant, and some are recessive.
• In eukaryotes, alleles typically come in pairs, one inherited from the mother and one from the father.
• An organism with at least one dominant allele will exhibit that trait.
• An organism with a recessive allele will exhibit the trait only in the absence of a dominant allele.

Mendel's F1 Crosses

• When Mendel crossed plants with contrasting traits, the hybrid offspring showed traits of only one parent.

Segregation

• Segregation is the separation of alleles during gamete formation.

The F1 Generation

• The F1 generation is the offspring of the P generation.

The F2 Generation

• The F2 generation is the offspring of the F1 generation.
• In the F2 generation, 1/4 of the offspring were yellow, and 3/4 were green.

The Formation of Gametes

• Each allele independently assorts to a different gamete.

Applying Mendel's Principles

• Mendel's principles can be applied to other organisms, such as sheep.

Learning Objectives

• Use probability to predict inherited traits.
• Explain how alleles segregate when more than one gene is involved.
• Identify Mendel's contributions to our understanding of genetics.

Probability and Heredity

• Probability is the likelihood that an event will occur.
• The probability of either outcome is 50% or 1/2, for a coin flip.
• Each flip is an independent event.
• Probabilities do not determine the actual outcome, only the likelihood of each event.

Using Segregation to Predict Outcomes

• The probability of a gamete receiving a G is 50%, or 1/2.
• The probability of a gamete receiving a g is 50%, or 1/2.
• The probability of green offspring is 3/4.
• The probability of yellow offspring is 1/4.

Genotype and Phenotype

• Genotype refers to genetic makeup.
• Phenotype refers to physical traits.
• Two organisms may share the same phenotype but have different genotypes.

Heterozygous and Homozygous

• Homozygous means having two identical alleles for a gene.
• Heterozygous means having two different alleles for a gene.

Punnett Squares and Their Components

• Used to show possible combinations of alleles from two parents

Making a Punnett Square

• A Punnett square is used to predict the genotypes of offspring from a cross.

Two-Factor Cross

• Dominant alleles should be written first when there is more than one trait.

FOIL Method

• Used to determine the possible allele combinations.

Making a Punnett Square: Two Factors

• Used to analyze the results of the cross.

The Two-Factor Cross: F2

• Genes for different traits can segregate independently during gamete formation.
• Independent assortment results in a 9:3:3:1 ratio of phenotypes.

Summary of Mendel's Principles, Part I

• Inheritance is determined by genes, which are passed from parents to offspring.
• Where more than one form of a gene for a single trait exists, some alleles may be dominant, and others recessive.

Summary of Mendel's Principles, Part II

• Each adult has two copies of each gene—one from each parent.
• These genes segregate from each other when gametes are formed.
• Alleles for different genes usually segregate independently of each other.

Other Patterns of Inheritance

Incomplete Dominance

• Some alleles are neither dominant nor recessive.
• Incomplete dominance occurs when one allele is not completely dominant over another, resulting in a blending of traits.
• Hybrid individuals (Rr) exhibit a blending of the two homologous phenotypes to produce a hybrid (Pink).

Codominance

• Codominance occurs when the phenotypes for both alleles are clearly expressed.
• A red Camellia and a white Camellia can form a red and white Camellia.
• Examples include chicken feather color and human protein controlling blood cholesterol levels.

Multiple Alleles

• Many genes exist in more than two forms.
• Examples include human blood types and fur color in rabbits.

Polygenic Traits

• Many traits are produced by the interaction of several genes.
• Examples include eye color in fruit flies, coat color in dogs, and skin tone.
• Traits typically show a wide variety of phenotypes.

Sex Linked Traits

• Sex-linked traits are found on X and Y chromosomes.

Genes and the Environment

• Environmental conditions can affect gene expression and influence genetically determined traits.
• The phenotype of an organism is only partly determined by its genotype.

Meiosis

Chromosome Number

• Homologous chromosomes have the same genes, one originally from each of the organism's parents.
• Diploid cells contain both sets of homologous chromosomes (2N).
• Haploid cells contain only a single set of chromosomes (1N).

Reviewing Mitosis

Meiosis

• Meiosis happens in two cycles: Meiosis I and Meiosis II.
• Meiosis I forms 2 daughter cells that will undergo a second division.
• No interphase happens between cytokinesis I & II.
• The end result of Meiosis II is 4 genetically unique haploid cells (n).

Prophase I

• Major events of Prophase I include the nuclear envelope disappearing.
• The spindle apparatus forms, and homologous chromosomes form tetrads.
• Crossing over occurs here.

Crossing Over and Recombination

• Homologous chromosomes are sections of DNA on different chromosomes that code for the same genes.
• As homologous chromosome tetrads form, sections of DNA exchange places.
• Duplicated segments of DNA from both parents (male and female) are represented by different colors.
• The result of crossing over is that each daughter cell at the end of Meiosis II is genetically unique.
• When two genetically unique gametes (sex cells) fuse, the offspring are the genetically unique, increasing the overall genetic variability of the gene pool.

Metaphase I

• Homologous chromosomes pairs (tetrads) align on the midline of the cell.

Anaphase I

• Homologous chromosome pairs (tetrads) break apart and travel to opposite poles of the cell.

Telophase I and Cytokinesis

• Telophase I involves the nuclear envelope reforming, the spindle apparatus breaking down, and DNA relaxing or decondensing.
• Cytokinesis I involves the cell plate or cleavage furrow forming, and two new daughter cells are formed.

Prophase II

• Main events includes nuclear membranes disappearing, spindle apparatus forming, and DNA condensing into chromosomes.

Metaphase II and Anaphase II

• Sister chromatids align on the midline of the cell during Metaphase II.
• Sister chromatids break apart and travel to opposite poles of the cell during Anaphase II.

Telophase II and Cytokinesis

• The end product of meiosis is four daughter cells that have half the normal chromosome number.

Summary of Meiosis

Comparing Mitosis and Meiosis

Meiosis Overview

Gene Linkage

• Alleles of different genes tend to be inherited together when those genes are located on the same chromosome.

Gene Maps