Genetics and Inheritance Patterns

Allele Variations and Phenotypes

  • Heterozygous Condition

    • A person who is heterozygous at a given locus (gene pair) exhibits a wavy hair phenotype.

  • Genotype and Phenotype Definitions

    • Homozygous: When an individual has two identical alleles at a locus.

    • Homozygous Dominant: Two dominant alleles (e.g., curly hair).

    • Phenotype: Curly hair.

    • Heterozygous: Two different alleles at a locus.

    • Phenotype: Wavy hair.

    • Heterozygous Recessive: When an individual carries one recessive allele.

    • Phenotype: Straight hair.

  • Dominance Hierarchy

    • Dominant Phenotype: Curly hair (homozygous dominant).

    • Recessive Phenotype: Straight hair.

    • Intermediate Phenotype: Wavy hair in heterozygotes indicates incomplete dominance.

Incomplete Dominance

  • Definition: A genetic situation where one allele does not completely dominate another, resulting in a phenotype that is a blend of both.

  • In heterozygotes, they produce less of a protein than homozygous dominants, leading to intermediate traits (e.g., wavy hair).

    • Heterozygotes for that gene produce an intermediate phenotype (wavy hair).

Codominance

  • Definition: A genetic scenario where two dominant alleles are expressed simultaneously.

    • The term "co" means together—implying both traits are seen.

Example of Codominance: Hemoglobin

  • Hemoglobin consists of four protein chains (globins).

  • Different alleles for globin chains lead to two types of hemoglobin:

    • Adult Hemoglobin (dominant): Normal globin, normal hemoglobin carrying oxygen effectively.

    • Sickle Allele: A mutation that alters one nucleotide in the instructions, producing sickle hemoglobin leading to sickle-shaped red blood cells.

Function of Hemoglobin

  • Hemoglobin binds oxygen in red blood cells and transports it through the blood.

  • When it binds to oxygen, it releases carbon dioxide, a byproduct of cellular respiration.

  • Sickle hemoglobin aggregates, resulting in deformation of red blood cells, which can lead to blockage in blood vessels and organ damage.

Phenotypes Associated with Hemoglobin Genetics

  • Normal Hemoglobin:

    • Genotype: Two normal alleles.

    • Phenotype: Normal shape and function of red blood cells.

  • Sickle Cell Disease (SCD):

    • Genotype: Two sickle alleles.

    • Phenotype: Produces only sickle hemoglobin, resulting in sickled red blood cells.

  • Sickle Cell Trait (SCT):

    • Genotype: One normal allele and one sickle allele.

    • Phenotype: Produces both normal and sickle hemoglobin, typically exhibits no major health problems but can have anatomical variations under stress.

Medical Implications of Sickle Cell Disease

  • Individuals may suffer from severe pain episodes (sickle cell crisis) during low oxygen availability.

  • Regular blood transfusions are often required due to the risks of organ damage and other complications.

  • Gene editing research is currently underway aiming for potential cures.

Evolutionary Advantage of Sickle Cell Trait

  • Sickle cell trait provides resistance to malaria, which can be more deadly than sickle cell disease in certain populations (e.g., areas with high malaria rates).

  • This is why the allele remains prevalent within some populations despite the severe implications of having two sickle alleles.

Polygenic Inheritance

  • Definition: Involves multiple genes contributing to a single trait.

  • Examples include height, skin pigmentation, and eye color.

  • Polygenic traits demonstrate continuous variation — for instance, height is influenced by many genes controlling various aspects of growth.

Example of Polygenic Trait: Height

  • Traits like height are determined by contributions from at least 100 genes plus environmental factors (e.g., nutrition).

  • Average height may vary widely across individuals in a population, indicative of polygenic inheritance.

Skin and Hair Color Genetics

  • Skin pigmentation is mainly controlled by the pigment melanin, produced by melanocytes.

  • Melanin Types:

    • Eumelanin: Produces brownish colors.

    • Pheomelanin: Related to reddish hues.

Lethal Genes

  • Definition: Some genotypes result in embryonic death, leading to missing phenotypes in offspring.

  • Example: Certain genes involved in spinal cord development affecting tail length in cats may lead to varying developmental outcomes.

Mink Cats

  • Result from a gene leading to shortened spinal cord development (M gene).

    • Heterozygous individuals might develop normally, while homozygous individuals (two copies of M) may not survive pregnancy.

Summary of Dominance Types

  • Complete Dominance: One allele completely masks the other (example: eye shape).

  • Incomplete Dominance: Blending of traits (example: hair texture).

  • Codominance: Expression of both alleles (example: hemoglobin).

Review and Application

  • Understanding these concepts is crucial for practical applications within laboratory settings, especially in genetics.

  • Situations that illustrate these principles may be encountered during genetic studies, making mastery of the material essential for successful outcomes in the course.

Y Chromosome and Sex Determination

  • Y Chromosome Overview

    • Contains up to 80 functional genes that influence male development.

    • The SRY gene (Sex-determining Region Y) acts as a master control gene.

    • The SRY gene is initially nonfunctional; it is turned on during a critical period of embryonic development, stimulating the production of testosterone, which leads to the development of male characteristics.

    • Development of male sexual characteristics occurs in two stages:

    • First: Early in embryo development.

    • Second: During puberty, when another burst of testosterone promotes secondary male characteristics.

    • If SRY is not activated, the fetus will develop female characteristics.

Hormonal Production in Males and Females

  • All humans produce both testosterone and estrogen.

  • The difference lies in the amount produced, not the type of hormones.

Sex Determination Systems in Various Organisms

  • Humans and fruit flies:

    • Males (XY) and females (XX) have different sex chromosomes.

  • Birds:

    • Males have the same type of sex chromosomes, and the females have different types (ZW system).

  • Grasshoppers:

    • Females have two of the same sex chromosomes (XX), while males possess only one (X).

  • Bees and ants:

    • Females develop from fertilized eggs (diploid), while males develop from unfertilized eggs (haploid).

Experiments with Fruit Flies

  • Background:

    • Early genetic experiments utilized fruit flies (Drosophila) due to their simple genetics.

  • Mutant fruit flies produced interesting results, including white-eyed variants (lacking red pigment).

  • Genotypes:

    • Female fruit flies can be homozygous for red eyes (dominant allele) or heterozygous (one allele for red, one for white).

    • Male fruit flies have only one X chromosome, leading to hemizygosity.

  • Results of Crossbreeding:

    • Crossing red-eyed females with white-eyed males produced all red-eyed offspring due to the dominant allele's expression in females.

    • Heterozygous females pass on red-eyed alleles to male offspring, leading to different ratios of red-eyed to white-eyed males (indicates X-linked inheritance).

Human Eye Color Genetics

  • Similar patterns observed in humans:

    • Eye color traits are often X-linked, revealing ratios in males and females.

    • Retina Composition:

    • Comprised of rod cells (light sensing) and cone cells (color sensing).

    • Color blindness is often tied to mutations in the cone cells.

    • Red-Green Color Blindness:

    • Condition where individuals cannot perceive red or green hues due to mutated alleles on the X chromosome.

  • Punnett Square for Color Blindness:

    • Using symbols (e.g., L for normal vision, l for colorblindness):

    • Mother: Heterozygous (L/l)

    • Father: Homozygous dominant (L/—; normal vision)

  • Offspring Analysis:

    • Both daughters can see normally.

    • Sons have a probability of being colorblind (one son may be colorblind if he inherits l).

  • Hemizygous Definition:

    • Term for males regarding X-linked traits since they possess one allele (e.g., hemizygous dominant or hemizygous recessive).

Female Color Blindness

  • A female can only be color blind if she is homozygous recessive (l/l) for the colorblind allele.

  • If her father is colorblind, she will inherit the recessive allele from him.

  • Discussion of complications in a male’s life due to color blindness (e.g., military concerns).

Nondisjunction in Meiosis

  • Definition:

    • Nondisjunction occurs when homologous chromosomes do not separate properly during meiosis, leading to gametes with abnormal chromosome numbers.

  • Mechanisms of Nondisjunction:

    • Can happen during Meiosis I (both chromosomes move to one side of the division) or Meiosis II (sister chromatids do not separate).

  • Consequences:

    • Human gametes: Normal = 23 chromosomes.

    • Nondisjunction can lead to gametes with 22 (missing one chromosome) or 24 (extra chromosome) chromosomes.

  • Age Factor in Females:

    • Eggs are arrested in Meiosis I at birth; nondisjunction more common after age 35 due to the aging of eggs.

Trisomy Conditions

  • Down Syndrome (Trisomy 21):

    • Caused by an extra copy of chromosome 21.

    • Survival rates relatively high due to smaller chromosome size and fewer critical genes.

    • Associated health challenges like developmental delays and organ enlargement.

  • Trisomy 13 and Other Autosomal Trisomies:

    • Trisomy 13 usually leads to non-survivable conditions shortly after birth.

    • Other autosomal trisomies are often fatal early in development.

Conclusion and Upcoming Topics

  • Next discussions will cover nondisjunction involving sex chromosomes, which tend to be more viable than those relating to autosomes.

Chromosomal Disorders

  • Y Chromosome

    • Associated disorders by chromosomal abnormalities

    • Turner syndrome (monosomy X)

    • Characterized by the presence of only one X chromosome

    • Klinefelter syndrome

    • Characterized by two X chromosomes and one Y chromosome (XXY)

    • Jacob's syndrome

    • Characterized by one X and two Y chromosomes (XYY)

    • Importance of distinguishing between these syndromes for proper diagnosis and management

Dosage Compensation

  • Definition: Dosage compensation

    • Mechanism to equalize gene expression between sexes with different sex chromosome numbers

    • X chromosome dosage in females (two Xs) is compensated by inactivating one X chromosome

  • Overview of X chromosome inactivation

    • In females, only one X chromosome remains active; the other is randomly deactivated

Incident of Gas Leak

  • A message was received indicating that there is a gas leak at the building.

  • Important Points:

    • The gas leak is not affecting anyone in any way.

    • Authorities have the situation under control.

    • Everyone is instructed to shelter in place to avoid exposure to gas clouds.

    • Communication is encouraged; notify if you receive any updates via text.

Understanding Nondisjunction

  • Definition:

    • Nondisjunction refers to an error that occurs during meiosis, where improper separation of chromosomes takes place.

  • Types of Nondisjunction:

    • Meiosis I: Homologous chromosomes do not separate properly.

    • Meiosis II: Sister chromatids do not separate properly.

  • Consequences:

    • Resulting gametes contain an abnormal number of chromosomes, leading to chromosomal abnormalities.

Chromosomal Abnormalities in GametesAutosomes

  • Important Concepts:

    • Autosomes are the non-sex chromosomes.

    • Chromosomal abnormalities result in serious developmental issues for the fetus.

    • Common Examples:

    • Trisomy 21:

      • The most common form of autosomal trisomy.

      • Characterized by having an extra copy of chromosome 21.

      • Relatively fewer genes on chromosome 21 due to its size.

    • Trisomy 13:

      • Involves an extra copy of chromosome 13.

      • Infants with this condition typically do not survive beyond a few weeks or months.

    • Implications:

    • Both excess and inadequate autosomes lead to unviable outcomes; survivability decreases with aberrations in chromosomal count.

Sex Chromosomes

  • Differing Mechanisms:

    • The effects of nondisjunction can be quite different between sex chromosomes compared to autosomes.

  • Sperm and Egg Chromosomal Contributions:

    • Cases presented include sperm with nonnormal chromosomes fertilizing a normal egg, and vice versa.

    • Example 1:

    • A sperm lacking any sex chromosome (nondisjunction) would fertilize a normal egg.

    • Resulting zygote:

      • X chromosome from the egg and no other sex chromosome causes the development of Turner Syndrome (monosomy X).

      • Symptoms commonly include developmental delays in early development stages.

      • Shift in X chromosome activation: Extra X chromosomes in females typically become deactivated at a certain developmental stage.

    • Example 2:

    • A normal sperm fertilizing an egg with irregular sex chromosomes (not detailed, indicating reverse nondisjunction cases).

    • Effects would depend on the type of irregularity in the egg's sex chromosome count.