Mendels Genetics

Introduction to Mendelian Genetics

  • Review of Chromosomes

    • Human cells contain a total of 46 chromosomes.

    • These chromosomes are composed of DNA that gives each cell its unique identity.

    • Chromosomes are inherited as follows:

    • 23 chromosomes from the father.

    • 23 chromosomes from the mother.

    • This inheritance structure indicates that each person possesses a combination of genetic codes from both parents.

    • The phrase "23 pairs of chromosomes" emphasizes the maternal and paternal copies of each chromosome.

Alleles

  • Definition of Allele

    • An allele is defined as a small section on a chromosome that codes for a specific gene that contributes to an individual's uniqueness.

    • Since humans usually have two copies of each chromosome, they generally possess two alleles for every specific gene (one from each parent).

Example: Blood Type

  • Explanation of Blood Types

    • Blood types are traditionally identified by letters, such as A, B, and O.

    • Each blood type corresponds to specific alleles that determine the blood type's characteristics.

  • Individual Examples

    • Male Example:

    • Alleles code for blood type A (denoted as AA).

    • Since both alleles are the same (AA), his blood type is confirmed as A.

    • Female Example:

    • One allele codes for blood type A and the other for blood type O (denoted as AO).

    • The presence of one A allele means her blood type will also be A.

Terminology

  • Homozygous vs. Heterozygous

    • Homozygous

    • Definition: An individual with two identical alleles for a particular gene.

    • Example: The man with AA alleles is considered homozygous.

    • Etymology: "Homo" means same; "zygos" refers to the mix of DNA from parents.

    • Heterozygous

    • Definition: An individual with two different alleles for a particular gene.

    • Example: The woman with AO alleles is considered heterozygous.

    • Etymology: "Hetero" means different.

  • Dominant vs. Recessive Alleles

    • Dominant Allele: This allele will express its trait even if there is only one copy present.

    • In the example, the A allele is dominant over the O allele.

    • Recessive Allele: This allele will only express its trait if two copies are present.

Genotype vs. Phenotype

  • Definitions

    • Genotype

    • Refers to the specific alleles present in an individual.

    • Example: The man's genotype is AA.

    • Phenotype

    • Refers to the observable traits of an individual resulting from the genotype.

    • Example: The phenotype for both the man and the woman is blood type A.

  • Relationship Between Genotype and Phenotype

    • It is possible for different genotypes to lead to the same phenotype due to the dominance of certain alleles.

Gene Inheritance

  • Introduction to Punnett Squares

    • A Punnett square is a tool used to predict the genotype and phenotype ratios of offspring from two parents.

    • Each parent contributes one allele to the offspring, represented in the Punnett square.

  • Example Scenario: Offspring from a Homozygous Male and Heterozygous Female

    • Parent Alleles:

    • Father's alleles (AA) are placed vertically.

    • Mother's alleles (AO) are placed horizontally.

    • Resulting Genotypes

    • Square results in:

      • 2 squares with AA (representing homozygous children).

      • 2 squares with AO (representing heterozygous children).

    • Phenotype: All children will have blood type A phenotype since both AA and AO code for blood type A.

  • Scenario Change: Heterozygous Male and Female

    • If both parents are heterozygous (AO), the Punnett square indicates:

    • 1 square with AA (25% probability).

    • 2 squares with AO (50% probability).

    • 1 square with OO (25% probability).

    • Outcome:

    • 75% of children will exhibit blood type A phenotype.

    • 25% will exhibit blood type O phenotype.

Summary of Learning Outcomes

  • Key Concepts Covered

    • Definition of an allele and the difference between homozygous and heterozygous.

    • Understanding of dominant vs. recessive traits related to alleles.

    • Distinction between genotype (genetic makeup) and phenotype (observable traits).

    • Use of Punnett squares to analyze inheritance patterns and predict offspring traits.