Patterns of Inheritance

Inheritance and Genetics Overview

  • Inheritance: The transmittance of inborn characteristics from parent to offspring.

  • Heritable: This adjective describes traits that are genetically based and can be passed on from parent to offspring.

  • Genetics: A branch of biology that focuses on heredity.

  • True Breeding: This term describes an individual that will, when self-pollinated, produce offspring with the same form of a trait as the parent.

Generational Terminology

  • P (Parental generation): The first two individuals crossed during a breeding experiment.

  • F1 (First Filial): The generation of offspring produced from breeding parents.

  • F2 (Second Filial): The generation of offspring produced by the F1 generation.

Pollination Definitions

  • Self-Pollinate: The process where pollen is transferred to the female gamete of the same flower.

  • Self-Fertilized: When pollen reaches the egg and it becomes a zygote.

  • Cross-Pollinate: The process in which pollen from one plant is used in reproduction with another plant.

Chapter 12: Patterns of Inheritance

Gregor Mendel (1822-1884)

  • An Austrian monk, considered the father of modern genetics.

  • Had a unique combination of agricultural background and mathematical education.

  • First to quantitatively study heritable traits focusing on peas.

  • Conducted research at his monastery leading to the foundational principles of genetics.

Advantages of Peas for Mendel’s Experiments

  • Farmers were already crossing peas, so some information was available.

  • Many true-breeding varieties are commercially available.

  • Small size and easy to cultivate with a short generation time.

  • Both male and female gametes are enclosed within the flower allowing for controlled breeding through self and cross-pollination.

Mendelian Traits of Inheritance

  • Traits can be classified as recessive or dominant.

Mendel's Experiments

Single Trait Cross

  • Mendel’s first experiment focused on one trait, using a Monohybrid Cross, which involves crossing a single pair of contrasting traits.

  • He established that there’s no blending between traits, as seen in the dominant and recessive traits.

Findings from the Monohybrid Cross:

  • Traits expressed in the F1 generation were identified as dominant.

  • Traits not visible in the F1 generation were termed recessive.

Seven Characteristics of Pea Plants Studied by Mendel

  • Assumed each trait is controlled by two factors (alleles).

  • Postulated that one factor could be dominant over the other, with factors separating during gamete formation.

Key Laws of Inheritance

  • Law of Segregation: States that alleles for a trait separate when gametes are formed.

  • Law of Independent Assortment: States that alleles of different genes separate independently of one another during gamete formation.

  • Locus: The location of a gene on a chromosome.

Alleles Representation

  • Dominant alleles are represented by capital letters (e.g., P).

  • Recessive alleles are represented by lowercase letters (e.g., p).

Predictive Power of Mendel's Work

  • Mendel was able to use mathematical concepts to predict the outcomes of crosses.

  • Recognized patterns of heredity in many organisms, leading to principles that are applicable across species.

Genetic Terminology

Homozygous vs. Heterozygous

  • Homozygous: An individual with two identical alleles for a trait (e.g., PP or pp).

  • Heterozygous: An individual with two different alleles for a trait (e.g., Pp).

Genotype and Phenotype

  • Genotype: The set of alleles that an individual has for a specific trait.

  • Phenotype: The physical expression of a trait.

Punnett Square

  • A diagram that predicts the outcome of a gamete cross, illustrating the potential genotypes of the offspring.

  • Individual outcomes can be classified as PP (homozygous dominant), Pp (heterozygous), or pp (homozygous recessive).

Probability in Genetics

  • The probability of two independent events occurring is the product of their individual probabilities.

Complex Inheritance Patterns

Polygenic Inheritance

  • Traits determined by many genes, leading to continuous variation in phenotypes.

Environmental Influence

  • The expression of some genes can be influenced by environmental conditions, such as temperature affecting color in some species.

Exceptions to Simple Inheritance Patterns

  • Traits that do not follow simple Mendelian inheritance include:

    • Polygenic inheritance

    • Environmental influences

    • Incomplete dominance

    • Codominance

    • Sex-linked traits

    • Linked genes

    • Multiple alleles

Examples of Complex Traits

  • Hydrangea Color: Influenced by soil pH.

  • Himalayan Rabbit Fur Color: Darker fur at cooler temperatures.

Incomplete Dominance

  • Example: Crossing red and white snapdragons results in pink offspring.

Codominance

  • Example: Human blood types where both A and B alleles are expressed in type AB blood.

Phenotype

Genotype

A

IAIA, IAi

B

IBIB, IBi

AB

IAIB

O

ii

Multiple Alleles and Linked Genes

Multiple Alleles

  • Some traits are influenced by more than two allelic forms, such as ABO blood types.

Sex-linked Traits

  • Traits controlled by genes on the sex chromosomes (e.g., color blindness).

Linked Genes

  • Genes that are located on the same chromosome are often inherited together unless recombination occurs.

Genetic Disorders and Inheritance

Types of Genetic Disorders

  • Genetic disorders can be categorized as autosomal or sex-linked, as well as dominant or recessive.

  • Autosomal Recessive Disorders: Common examples include Tay-Sachs Disease and Cystic Fibrosis.

  • Autosomal Dominant Disorders: Example includes Huntington's Disease.

  • Sex-linked Disorders: Example includes Hemophilia which affects clotting ability.

Genetic Counseling and Pedigree Analysis

  • Genetic Counseling: The field that studies inheritance patterns and predicts the chances of offspring with certain traits.

  • Pedigree: A diagram representing relationships among family members used to track traits across generations.

    • Circles represent females, squares represent males. Colored symbols represent affected individuals.

Karyotype and Chromosomal Abnormalities

Karyotype

  • A visual display of an individual's chromosomes used to identify abnormalities in chromosome number, shape, or size.

Genetic Counseling Procedures

  • Amniocentesis: A procedure involving needle insertion to collect amniotic fluid for analysis.

  • Chorionic Villi Sampling (CVS): A less invasive technique that collects fetal cells via a suction tube.

Genetic Syndromes

  • Abnormal chromosome numbers or structures can lead to syndromes such as Down syndrome caused by Trisomy 21.

Chromosomal Disorders

Example Syndromes

  • Turner Syndrome (XO): Characterized by short stature, lack of secondary sexual characteristics, and infertility.

  • Klinefelter Syndrome (XXY): Males with both male and some female characteristics, often fertile but with abnormal testes.

  • XYY Male: Typically normal male fertility.

Miscarriage Risks and Timing

  • Risks vary with procedures; amniocentesis has a 0.3% miscarriage risk while CVS has a higher rate of risk (0.8%).

Genetic Test Interpretation

Example Blood Type Inheritance Pattern

  • An understanding of blood type inheritance exemplifies the interactions of dominant and recessive alleles alongside multiple alleles.

Reading Pedigrees

  • Understanding how to read and analyze pedigrees is crucial for assessing the likelihood of genetic conditions in offspring based on parental traits.