Genetics Lecture Notes chapter 23
Updates
Pre-lab quiz due by 11-6 at 5:45 PM.
Chapters 23 and 24 homework recommended due date by 11-11.
Today's lecture schedule: 5:40-6:40 PM lecture, 6:40-7:10 PM break, 7:10-8:10 PM finish slides. Everyone present will receive a 100 on the quiz this week.
Instructor will be absent on 11-18 but will distribute a study guide, which can be completed during class.
Lecture test 3 scheduled for 11-25 at 5:30 PM.
Chapter 23
23.1 Mendelian Patterns of Inheritance
Genetics: Study of inheritance and variability between generations.
Importance: Understanding inheritance is crucial in agriculture, animal husbandry, and medicine.
Gregor Mendel
Key figure in genetics, investigated plant inheritance in the 1860s.
Conclusions:
Plants transmit distinct factors (now termed genes) to offspring, which are located on chromosomes.
Homologous Chromosomes
Definition: Chromosomes that occur in pairs in both plants and humans.
Inheritance: One chromosome from the female parent, one from the male parent.
Characteristics of homologous chromosomes:
Same length and centromere position.
Similar gene types present on both members of the pair.
Alleles: Different forms of a gene for a trait, indicated by letters (e.g., G and g).
Alleles for each gene are located at specific loci.
The Law of Segregation
Mendel's principles based on observations:
Each organism possesses two factors for each trait, one dominant and one recessive.
These factors segregate during gamete formation, resulting in each gamete containing one factor from the pair.
Fertilization restores the pair of factors in the offspring.
The Inheritance of a Single Trait
Phenotype: Physical appearance or characteristic of an individual.
Genotype: Genetic makeup, specifically the alleles present for a trait.
Details of Inheritance of a Single Trait
In diploid organisms, homologous chromosomes contain two alleles for each trait.
Notation:
Capital letters for dominant alleles, lowercase for recessive alleles.
Dominant allele: Masks expression of a recessive allele if both are present.
Example: Freckles - F (dominant) and f (recessive).
Examples and Definitions
Homozygous: Individual with identical alleles (e.g., FF or ff).
Heterozygous: Individual with different alleles (Ff).
Dominance of Human Traits
An organism with at least one F allele shows the dominant phenotype (freckles) regardless of the second allele.
Both FF and Ff result in freckles phenotype.
Genotype Related to Phenotype (Table 23.1)
HOMOZYGOUS DOMINANT (FF): Freckles
HETERZYGOUS (Ff): Freckles
HOMOZYGOUS RECESSIVE (ff): No freckles
Gamete Formation
Each gamete contains one allele for each trait from the diploid genotype.
During meiosis, homologous chromosomes separate leading to haploid gametes.
Gamete formation rules: No two letters in a gamete can be identical (e.g., from fF genotypes, gametes are F or f).
One-Trait Crosses
Steps to resolve a genetics cross:
Identify the genotype of each parent.
List possible gametes for each parent.
Combine all possible gametes.
Determine genotypes and phenotypes of the offspring.
Sample Cross Example
Homologous male with freckles (FF) mated with female without freckles (ff).
Offspring genotype will consistently be heterozygous (Ff), showing freckled phenotype.
Monohybrid Cross
Example: fF × fF
Use of Punnett square to solve the monohybrid cross provides:
Genotypic ratio: 1 FF : 2 Ff : 1 ff (1:2:1).
Phenotypic ratio: 3 freckles : 1 without freckles (3:1).
Probabilities calculated based on offspring production not guaranteeing traits.
One-Trait Crosses and Probability
Product Rule of Probability: Probability of two independent events is the product of their individual probabilities.
Calculations:
Probabilities for fF (F, f) offspring calculated as 1/4, 1/4 for F or f respectively.
One-Trait Testcross
To ascertain whether an individual with a dominant phenotype is homozygous or heterozygous: Cross a dominant phenotype with a recessive homozygote.
Two-Trait Crosses
During meiosis, each gamete receives one member of each homologous chromosome. Genes assort independently unless they are linked.
Dihybrid Cross Example
Two-trait (dihybrid cross) example: Cross of homozygous freckles and short fingers (FFSS) with no freckles and long fingers (ffss).
All offspring will be heterozygous and exhibit both dominant traits (freckles and short fingers).
Phenotypic Ratios in Dihybrid Crosses
If a dihybrid reproduces with another dihybrid, possible offspring gametes reflect the law of segregation and independent assortment.
Expected phenotypic ratio for 2 traits: 9 freckles/short fingers : 3 freckles/long fingers : 3 no freckles/short fingers : 1 no freckles/long fingers (9:3:3:1).
Sex-Linked Inheritance
Patters observed due to traits residing on sex chromosomes, particularly X-linked (common in males) traits.
X-Linked Recessive Disorders
Examples include color blindness, hemophilia, and muscular dystrophy. More prevalent in males due to single X dominance.
Changes in Chromosome Number
Nondisjunction during meiosis can lead to conditions such as trisomy and monosomy, affecting the resulting offspring's phenotype.
Updates
Pre-lab quiz due by 11-6 at 5:45 PM. This quiz is online (where) and is essential (why) to prepare you for the upcoming lab material. You can complete it via the course's online platform (how).
Chapters 23 and 24 homework recommended due date by 11-11. This self-paced assignment (how) helps reinforce lecture concepts (why) and can be accessed through the course website (where).
Today's lecture schedule: 5:40-6:40 PM lecture, 6:40-7:10 PM break, 7:10-8:10 PM finish slides. Everyone present will receive a 100 on the quiz this week. This attendance incentive (why) is applied automatically (how) for those physically present in the lecture hall (where).
Instructor will be absent on 11-18 but will distribute a study guide, which can be completed during class. This guide (how) is a preparatory resource (why) for the upcoming test and will be distributed in class (where) before the absence (when).
Lecture test 3 scheduled for 11-25 at 5:30 PM. This comprehensive assessment (why) will take place in the designated lecture room (where) at the specified time (when).
Chapter 23
23.1 Mendelian Patterns of Inheritance
Genetics: Study of inheritance and variability between generations.
When: The foundational principles were established in the mid-19th century by Gregor Mendel, evolving with molecular discoveries since the early 20th century.
Where: Research is conducted globally in universities, medical centers, and agricultural institutions.
Why: Understanding genetics is crucial in agriculture (crop improvement), animal husbandry (breeding), and medicine (diagnosing and treating genetic diseases, personalizing therapies).
How: Through observation of inheritance patterns, pedigree analysis, genetic crosses, and molecular techniques like DNA sequencing.
Gregor Mendel
Key figure in genetics, investigated plant inheritance in the 1860s.
When: His pioneering work was conducted primarily between 1856 and 1863.
Where: At the Augustinian St. Thomas's Abbey in Brünn (now Brno, Czech Republic), within the monastery's garden.
Why: He sought to understand how traits are passed from parents to offspring, aiming to refute the prevailing theory of blending inheritance.
How: By performing meticulous cross-breeding experiments with thousands of pea plants (), observing seven distinct traits, and applying quantitative analysis to his results.
Conclusions:
Plants transmit distinct factors (now termed genes) to offspring, which are located on chromosomes.
Homologous Chromosomes
Definition: Chromosomes that occur in pairs in both plants and humans.
When: Present within the nucleus of diploid somatic cells throughout an organism's life cycle.
Where: Found inside the nucleus of eukaryotic cells.
Why: They carry the same genes at the same loci, ensuring that an organism receives one genetic copy for each trait from each parent, allowing for genetic variation.
How: One chromosome of each homologous pair is inherited from the female parent and the other from the male parent via their gametes during fertilization.
Characteristics of homologous chromosomes:
Same length and centromere position.
Similar gene types present on both members of the pair.
Alleles: Different forms of a gene for a trait, indicated by letters (e.g., G and g).
When: Alleles exert their effects during gene expression, influencing the phenotype.
Where: Located at specific positions (loci) on homologous chromosomes.
Why: Different alleles arise due to mutations and lead to variations in traits within a population.
How: By coding for slightly different versions of a protein or by influencing gene regulation, resulting in observable differences in traits.
The Law of Segregation
Mendel's principles based on observations:
Each organism possesses two factors for each trait, one dominant and one recessive.
These factors segregate during gamete formation, resulting in each gamete containing one factor from the pair.
When: This segregation occurs during Anaphase I of meiosis.
Where: In the germline cells (e.g., ovaries and testes) where gametes are produced.
Why: To ensure that each gamete is haploid and contains only one allele for each gene, maintaining the correct chromosome number upon fertilization and contributing to genetic diversity.
How: Homologous chromosomes, carrying the two alleles for each gene, separate from each other and move into different daughter cells during meiosis.
Fertilization restores the pair of factors in the offspring.
The Inheritance of a Single Trait
Phenotype: Physical appearance or characteristic of an individual.
When: Observable throughout an organism's life, potentially changing with development or environmental factors.
Where: The observable expression of genetic traits (e.g., hair color, height).
Why: The visible outcome of the interaction between an organism's genotype and its environment.
How: Developed through the expression of genes, influenced by environmental factors.
Genotype: Genetic makeup, specifically the alleles present for a trait.
When: Established at conception and remains constant throughout an organism's life for somatic cells.
Where: Resides within the DNA sequence of an organism's cells (in the nucleus).
Why: It provides the genetic blueprint that dictates the potential range of an organism's traits.
How: Inherited from parents through their gametes.
Details of Inheritance of a Single Trait
In diploid organisms, homologous chromosomes contain two alleles for each trait.
Notation:
Capital letters for dominant alleles, lowercase for recessive alleles.
Dominant allele: Masks expression of a recessive allele if both are present.
When: The masking effect occurs during gene expression, from transcription to protein synthesis.
Where: At the molecular level within the cell (nucleus for transcription, cytoplasm for translation).
Why: Often, the dominant allele produces a functional protein (or sufficient quantity of it), whereas the recessive allele might produce a non-functional or absent protein.
How: One copy of the dominant allele is usually sufficient to produce the trait, effectively overriding the presence of a recessive allele.
Example: Freckles - F (dominant) and f (recessive).
Examples and Definitions
Homozygous: Individual with identical alleles (e.g., FF or ff).
Heterozygous: Individual with different alleles (Ff).
Dominance of Human Traits
An organism with at least one F allele shows the dominant phenotype (freckles) regardless of the second allele.
Both FF and Ff result in freckles phenotype.
Genotype Related to Phenotype (Table 23.1)
HOMOZYGOUS DOMINANT (FF): Freckles
HETERZYGOUS (Ff): Freckles
HOMOZYGOUS RECESSIVE (ff): No freckles
Gamete Formation
Each gamete contains one allele for each trait from the diploid genotype.
When: Occurs during meiosis in sexually mature organisms.
Where: In the gonads (testes in males, ovaries in females).
Why: To produce haploid cells (gametes) that can combine during fertilization to restore the diploid state, ensuring genetic continuity across generations and promoting genetic diversity.
How: Through the process of meiosis, which involves two rounds of cell division to reduce the chromosome number by half.
During meiosis, homologous chromosomes separate leading to haploid gametes.
Gamete formation rules: No two letters in a gamete can be identical (e.g., from fF genotypes, gametes are F or f).
One-Trait Crosses
Steps to resolve a genetics cross:
Identify the genotype of each parent.
List possible gametes for each parent.
Combine all possible gametes.
Determine genotypes and phenotypes of the offspring.
Sample Cross Example
Homologous male with freckles (FF) mated with female without freckles (ff).
Offspring genotype will consistently be heterozygous (Ff), showing freckled phenotype.
Monohybrid Cross
Example: fF × fF
Use of Punnett square to solve the monohybrid cross provides:
Genotypic ratio: 1 FF : 2 Ff : 1 ff (1:2:1).
Phenotypic ratio: 3 freckles : 1 without freckles (3:1).
Probabilities calculated based on offspring production not guaranteeing traits.
One-Trait Crosses and Probability
Product Rule of Probability: Probability of two independent events is the product of their individual probabilities.
Calculations:
Probabilities for fF (F, f) offspring calculated as for F or f respectively.
One-Trait Testcross
To ascertain whether an individual with a dominant phenotype is homozygous or heterozygous: Cross a dominant phenotype with a recessive homozygote.
When: Typically performed after observing an individual with a dominant phenotype whose ancestry is unknown.
Where: In a controlled breeding environment, such as a laboratory or agricultural setting.
Why: To determine the exact genotype (homozygous dominant or heterozygous) of an organism expressing a dominant trait.
How: By mating the individual in question with a known homozygous recessive individual. The phenotypes of the offspring will reveal the genotype of the unknown parent.
Two-Trait Crosses
During meiosis, each gamete receives one member of each homologous chromosome. Genes assort independently unless they are linked.
When: Independent assortment occurs during Metaphase I of meiosis.
Where: In the germline cells of sexually reproducing organisms during gamete formation.
Why: To generate a wide variety of allele combinations in gametes, increasing genetic diversity in the offspring and enhancing a species' ability to adapt.
How: Non-homologous chromosomes (and thus the genes located on them) align and separate independently of one another into gametes.
Dihybrid Cross Example
Two-trait (dihybrid cross) example: Cross of homozygous freckles and short fingers (FFSS) with no freckles and long fingers (ffss).
All offspring will be heterozygous and exhibit both dominant traits (freckles and short fingers).
Phenotypic Ratios in Dihybrid Crosses
If a dihybrid reproduces with another dihybrid, possible offspring gametes reflect the law of segregation and independent assortment.
Expected phenotypic ratio for 2 traits: 9 freckles/short fingers : 3 freckles/long fingers : 3 no freckles/short fingers : 1 no freckles/long fingers (9:3:3:1).
Sex-Linked Inheritance
Patters observed due to traits residing on sex chromosomes, particularly X-linked (common in males) traits.
When: These traits are expressed throughout an individual's life as the genes are active.
Where: Genes are located specifically on the sex chromosomes (X or Y).
Why: Males (XY) have only one X chromosome, so they express all alleles on it, even if recessive. Females (XX) have two X chromosomes, allowing one X to potentially mask a recessive allele on the other.
How: Males inherit their X from their mother and their Y from their father. Females inherit one X from each parent. The unique inheritance pattern leads to a higher prevalence of X-linked recessive disorders in males.
X-Linked Recessive Disorders
Examples include color blindness, hemophilia, and muscular dystrophy. More prevalent in males due to single X dominance.
Changes in Chromosome Number
Nondisjunction during meiosis can lead to conditions such as trisomy and monosomy, affecting the resulting offspring's phenotype.
When: Occurs during anaphase I or anaphase II of meiosis.
Where: In the germline cells during the formation of gametes.
Why: It is the failure of homologous chromosomes (meiosis I) or sister chromatids (meiosis II) to separate properly, leading to an unequal distribution of chromosomes into daughter cells.
How: This results in gametes having an abnormal number of chromosomes (e.g., or ). If such a gamete is involved in fertilization, the resulting zygote will have an abnormal chromosome number (e.g., trisomy like Down syndrome with 3 copies of chromosome 21, or monosomy, lacking one chromosome).