CHR6 notes

Traits Controlled by Single Genes

• Heredity: The passing of characteristics from parent to offspring through the parent's genes.

  • Offspring resemble their parents more than they resemble unrelated individuals in the population.

Selective Breeding

• Selective Breeding: Process that became possible once breeders recognized the existence of heredity.

  • Example: Breeding for reduced size in horses.

Single-gene Traits

• Single-gene Traits: Traits determined by the instructions carried on one gene.

  • Most human characteristics are influenced by multiple genes and the environment.

  • Examples of traits in cats:

  • FUR LENGTH IN CATS:

    • Long-haired cat

    • Short-haired cat

  • COAT COLOR IN CATS:

    • White-haired cat

    • Colored-haired cat

Take Home Message on Single-gene Traits

• Some traits are determined by the instructions an individual carries on a single gene, exhibiting straightforward patterns of inheritance.

Mendel's Experiments and Findings

• Background: Gregor Mendel learned about heredity through methodical experiments.

  • Question he explored: What do parents “give” their offspring that confers similarity?

  • Preformationism was a mistaken belief proposing that a tiny, pre-made human existed in every sperm cell (originating in the 1600s).

Critical Features of Mendel’s Research

• Gregor Mendel (1822-1884): His methodical research established critical findings in the study of heredity.

• Three critical features of Mendel’s methodology included:

  1. Focusing on easily observed and categorized traits in garden peas.

  2. Application of methodical experimentation.

  3. Rigorous hypothesis testing.

Key Take Home Message from Mendel's Work

• Mendel's work in the mid-1800s was pivotal in understanding heredity.

Mendel's Law of Segregation

• Segregation: Each individual has two copies of each gene but only one copy is placed into each sperm or egg.

• Dominant vs. Recessive Traits:

  • A dominant trait masks the effect of a recessive trait when an individual carries both versions.

  • Example: Purple flower (dominant) vs. white flower (recessive).

• Mendel's observations:

  • Crossing true-breeding plants: Crossing purple-flowered with white-flowered plants resulted in all purple flowers.

  • F1 Generation: When crossing two purple-flowered offspring, most were purple, some were white.

Three Important Ideas from Mendel's Research

1. Each parent contributes a single set of genes into each sperm or egg.

2. Offspring receive two copies of the instructions for any trait.

3. The observed trait in an individual depends on the inherited copies from its parents.

Mendel’s Law of Segregation Explained

• Only one of two alleles for a gene is placed into a gamete.

• Offspring receive one allele from each parent at fertilization.

• Examples:

  • Heterozygous pea plant (two different alleles for flower color).

  • Illustrates the processes of meiosis (one copy of each gene in gametes) and fertilization (two copies in fertilized egg).

Take Home Message on Genes and Traits

• Each parent contributes a single set of instructions (genes) into every reproductive cell.

• The trait observed (phenotype) depends on the alleles inherited from parents.

Genotype vs. Phenotype

• Phenotype: The outward appearance of an individual.

• Genotype: The genetic composition for a particular trait.

  • Example: Homozygous recessive for albinism showcases a phenotype with little or no pigmentation.

Punnett Squares in Genetic Outcomes

• Punnett Square: A tool for determining potential outcomes of a genetic cross.

  • Cross 1 Scenario:

  • MOTHER: Albino homozygous ($mm$)

  • FATHER: Pigmented homozygous ($MM$)

  • Offspring phenotype/genotype outcomes show all heterozygous and pigmented individuals.

  • Cross 2 Scenario:

  • A 50% chance that offspring from two heterozygous parents will be albino (1/4 homozygous recessive $mm$).

  • Phenotypic ratio from Cross 2 approximates 3:1.

Take Home Messages on Genotypes

• Observing phenotype cannot always determine genotype.

• Individuals may carry recessive alleles not seen phenotypically if a dominant allele is present.

• Genetic analysis often employs Punnett squares for accurate determination of genotypes.

Role of Chance in Genetics

• Chance influences segregation and fertilization events, leading to probabilistic outcomes.

  • Example of probability calculation:

  • 100% of the albino mother’s eggs have the $m$ allele.

  • 50% chance the father's sperm carries the recessive $m$ allele results in 50% chance of albino offspring.

Tay-Sachs Disease Probability

• From two heterozygous parents for Tay-Sachs:

• 25% chance that a child will inherit Tay-Sachs disease due to probability calculations.

Take Home Message on Probability in Genetics

• Probability is fundamental in genetics, with segregation and gamete production being chance events.

Test-Cross Usage

• Test-Cross: A method to reveal unknown genotype of an individual with a dominant trait.

  • Problem presented: Uncertain genotype of alligators for a white trait.

Test-Cross Scenarios

1. If the mother's genotype is known (white homozygous $mm$) and the father's is unknown (M_), crossing outcomes help determine the genotype.

   • If $M$ is homozygous dominant, all offspring will carry the dominant trait.

   • If $M$ is heterozygous, half offspring will show the dominant trait, and half will show recessive traits.

Take Home Message from Test-Crosses

• A test-cross helps establish the genotype of an individual exhibiting a dominant trait.

Antigens, Antibodies, and Blood Type

• Antigens: Chemicals on the surface of cells acting as signposts for the immune system.

• Antibodies: Molecules in the bloodstream attacking foreign invaders.

  • Individuals produce antibodies for antigens not present on their cells.

• Blood Types and their characteristics:

  • Type A: A antigens, produces B antibodies.

  • Type B: B antigens, produces A antibodies.

  • Type AB: A and B antigens, produces neither A nor B antibodies (universal recipient).

  • Type O: No antigens, produces A and B antibodies (universal donor).

Blood Compatibility

• Type O individuals are universal donors due to lack of antigens.

• Type AB individuals are universal recipients due to the presence of both A and B antigens without producing antibodies against them.

Rh Markers Explained

• Rh Markers: A single gene with two alleles determining the presence of Rh markers.

• Individuals with one or more copies of the dominant Rh marker are termed Rh-positive.

• Individuals with two copies of the recessive allele are termed Rh-negative.

Take Home Message on Multiple Allelism

• In multiple allelism, a single gene can have more than two alleles.

• Each individual carries only two alleles, but multiple alleles can exist in a population (e.g., ABO blood groups).

Pleiotropy

• Pleiotropy: When one gene influences multiple, unrelated traits.

  • Example: The allele causing sickle-cell disease is pleiotropic, influencing:

  • Red blood cell shape—sickle shape leads to sickle-cell disease.

  • Malaria resistance in carriers.

Take Home Message on Pleiotropy

• One gene can influence multiple traits and this phenomenon is common in genetics.

Mendel’s Law of Independent Assortment

• States traits are inherited independently from one another.

• This law implies that the relation of each pair of different characters in hybrid unions is unaffected by other differences in parent stocks provided traits segregate independently.

Independent Assortment of Genes Demonstrated

• Focus on one trait at a time during dihybrid crosses will show standard genotypic and phenotypic ratios (1:2:1 genotypes, 3:1 phenotypes).

• When both parents are heterozygous for two traits, various gametes are produced leading to predictable offspring traits without influence from the other traits.

Take Home Message on Independent Assortment

• Genes typically behave independently, meaning the inheritance pattern of one trait does not influence the inheritance pattern of other traits.