Pea Plants and Genetics

Pea Plants and Genetics

• Gregor Mendel's experiments with garden peas in the 1860s laid the foundation for genetics, the study of heredity.
• Mendel used advanced math to interpret his experiments, building upon earlier work.
• Garden peas were ideal for genetic study because they exhibit seven distinct characteristics with two traits each, possess both male and female reproductive parts for self or cross-fertilization, and reproduce quickly with numerous offspring.
• A trait is a genetically determined variant of a character.
• Pollination, the transfer of pollen grains from anthers to the stigma, can occur through self-pollination or cross-pollination.
• Mendel prevented self-pollination by removing anthers and manually cross-pollinated plants to create hybrids.

Mendel's First Experiment

• Mendel's experiments followed a three-step method: establishing true-breeding P (parental) generation through self-pollination, cross-pollinating P generation plants with contrasting traits and recording traits in the F1 generation, and allowing F1 generation to self-pollinate, recording traits in the F2 generation.
• True-breeding plants consistently produce offspring with the same trait when self-pollinating.
• In a cross between true-breeding purple and white flowered plants, the F1 generation only displayed purple flowers.
• The white flower trait reappeared in the F2 generation, with a ratio of approximately 3 purple flowers to 1 white flower.
• Mendel hypothesized that traits are controlled by separate factors, with each trait inherited through a distinct factor.
• Due to the two forms of each character, he proposed a pair of factors controlled each trait.
• Mendel termed the factor that masks the other as dominant and the hidden factor as recessive.

The Law of Segregation

• The law of segregation states that chromosomes separate during gamete formation (meiosis), ensuring each gamete receives only one copy of each trait.

Mendel's Second Experiment

• Mendel also investigated if two traits could be inherited together, conducting dihybrid crosses to study two characteristics simultaneously.
• A dihybrid cross involves parents that are hybrids for both characters.
• The law of independent assortment states that factors separate independently during gamete formation.
• This holds true if factors are on different chromosomes or far apart on the same chromosome.

Mendel's Laws

• Mendel's work was initially overlooked but later rediscovered when other scientists reached similar conclusions.
• Characteristics of organisms are controlled by genes located on chromosomes.
• The position of a gene on a chromosome is called its locus.
• Sexually reproducing organisms have two copies of each gene due to inheriting two sets of chromosomes (homologous chromosomes), one from each parent.
• Homologous chromosomes are similar but not identical.

Genotype and Phenotype

• An organism's genotype is its genetic makeup, consisting of inherited alleles.
• An organism's phenotype is the physical expression of its genotype.
• Genotype determines phenotype.
• Homozygous individuals have identical alleles (RR or rr) for a trait.
• Heterozygous individuals have different alleles (Rr) for a trait.
• During meiosis, homologous chromosomes separate, leading to different gametes and alleles independently of each other.
• During fertilization, gametes (n) unite, forming a zygote (2n) and inheriting two alleles for each gene from each parent, constituting the individual's genotype.

Probability

• Probability is the likelihood of a specific event occurring, expressed as a fraction, decimal, or percentage.
• Probability calculated by the ratio of desired outcome to the number of possible outcomes.

Punnett Squares

• A Punnett square is a diagram used to predict the outcome of a genetic cross by considering all possible gamete combinations.
• Test crosses determine if an individual's genotype is heterozygous or homozygous by crossing an individual of unknown genotype with a homozygous recessive individual.
• A dihybrid cross tracks two characteristics.

Non-Mendelian Inheritance

• Non-Mendelian inheritance includes codominance and incomplete dominance.
• Codominance occurs when both alleles are expressed simultaneously.
• Heterozygous individuals express both proteins.
• Incomplete dominance occurs when one allele is not completely dominant over another, resulting in an intermediate phenotype.
• Red snapdragons crossed with white snapdragons produce pink snapdragon flowers.

Multiple Alleles and Polygenic Traits

• Some genes have multiple alleles.
• The A, B, O blood type system: A and B are codominant over the recessive O allele.
• Polygenic traits are determined by more than one gene.
• Hair, skin, and eye color, height, weight, and intellect are examples of polygenic traits.

Complex Characteristics

• Complex characteristics are influenced by multiple genes (polygenic) and the environment.
• Cancer, hair and skin color, height, weight, and intellect, are examples of complex characteristics.

Human Genome

• The Human Genome Project completed sequencing the 3.2 billion base pairs of human DNA in 2003.
• A genome is all the available genetic content for an individual.
• The human genome consists of an estimated 20,000 - 22,000 genes distributed throughout 22 autosomes and 2 sex chromosomes.
• Exons are coding regions of a gene; introns are interrupting regions.
• Non-coding DNA sequences regulate gene expression.

Human Chromosomes

• During meiosis, males pass either an X or a Y chromosome, while females pass an X chromosome.
• Offspring are female if the egg is fertilized with an X chromosome-containing sperm.
• Offspring are male if the egg is fertilized with a Y chromosome-containing sperm.
• The Sex-determining Region Y (SRY) gene on the Y chromosome triggers the development of the male reproductive system.

Genetic Linkage

• Independent assortment is true only when genes are on separate chromosomes or far enough apart on the same chromosome.
• Crossing over in Prophase 1 results in the swapping of genes between homologous chromosomes.
• Genes located close together on the same chromosome are referred to as "linked".
• The frequency of crossing over determines how close genes are to each other.
• Genes showing up in different gametes less than 50% of the time are likely located on the same homologous chromosome and tend to be inherited together.
• Sex-linked genes are located on sex chromosomes.
• The X chromosome is much larger than the Y chromosome, leading to more X-linked genes.

Mendelian Inheritance and Genetic Disorders

• Mendelian inheritance involves traits controlled by a single gene with two alleles.
• Mendelian traits are inherited based on whether the genes lie on autosomes or sex chromosomes.
• Red-green color blindness is an X-linked trait, more common in men due to having only one X chromosome.
• Examples of sex-linked disorders include hemophilia, color blindness, and Duchene Muscular Dystrophy.
• Autosomal traits are located on autosomes (1-22).
• Autosomal traits appear equally in males and females.
• Both sexes can be carriers for autosomal traits.
• Only females can be carriers of sex-linked traits.
• Carriers have the allele for a trait but do not express it.

X-Linked vs. Sex-Influenced Traits

• Sex-influenced traits are generally autosomal.
• Phenotypic variations in sex-influenced traits arise due to differing male and female sex hormones.

Inheritance of Traits

• Human genetic traits and diseases are traced through generations via pedigrees.
• Pedigrees are diagrams showing how traits are inherited across generations.
• Pedigrees reveal patterns indicating if a trait is dominant, recessive, autosomal, or sex-linked.
• Autosomal traits appear equally in both sexes.
• Sex-linked traits appear more often in males.
• Autosomal dominant traits require at least one parent with the trait.
• Autosomal recessive individuals may have one, two, or neither parent expressing the trait.
• Heterozygous carriers do not express the trait.

Detecting and Treating Genetic Diseases

• Genetic screening analyzes a person's genetic makeup and can be done on a fetus through amniocentesis or chorionic villi sampling (CVS).
• Amniocentesis involves sampling fetal cells from the amniotic fluid.
• Chorionic Villi Sampling (CVS) involves sampling cells from the area between the uterus and placenta.
• Genetic disorders are often managed by treating symptoms.
• Gene therapy replaces defective genes with healthy ones, often using viruses.