HH

mendelian genetics

Monohybrid Cross and the Principle of Segregation

Overview

  • Course: BS1001: Introduction to Biological Processes

  • Focus: Mendel’s laws of heredity including the Principle of Segregation and the Principle of Independent Assortment.

Monohybrid Cross

Definition

  • A monohybrid cross is a genetic cross between two parents that are heterozygous for a single trait (e.g., F1 x F1).

Parent Generation

  • Starting with a pure breeding P generation is essential for producing hybrids.

Purpose

  • Monohybrid crosses are used primarily to determine the dominance relationship between alleles.

Genetic Predictions

  • A 2 x 2 Punnett square can be utilized to predict the potential genotypes and phenotypes of progeny.

Law of Segregation Illustrated

  • Each square in the Punnett square represents potential gamete combinations, illustrating that only one allele from each parent is transmitted, thereby demonstrating the law of segregation.

Visual Representation

  • Phenotypes and Genotypes of the offspring can be represented in the Punnett square.

Law of Segregation

Mendel’s Law

  • Mendel's law of segregation states that a parent will pass one randomly selected copy of any given gene to its offspring (example: a Yy parent can pass on either Y or y).

Process During Meiosis

  • The halving of genetic information occurs during the separation of homologous chromosomes in meiosis I.

Gamete Formation

  • For example, from a parent with genotype Yy, the gametes formed can be Y or y; and from a yy parent, the gametes can only be y.

Intended Learning Outcomes

  • Understand Mendel’s principles of segregation and independent assortment.

  • Solve genetics problems involving monohybrid, dihybrid, and test crosses.

  • Apply the product rule and sum rule to predict outcomes of genetic crosses.

Introduction to Drosophila melanogaster

Overview

  • Commonly known as the fruit fly or vinegar fly.

  • Scientific name translates to ‘black-bellied dew lover.’

Historical Context

  • Thomas Hunt Morgan established Drosophila as a model organism for genetic study.

Genetic Characteristics

  • Drosophila has only 4 pairs of chromosomes.

  • Exhibits easily recognizable mutations and is simple to care for.

  • Rapid reproduction cycle and cost-effectiveness enable extensive genetic studies.

  • Confirmed the chromosomal theory of inheritance: genes reside on chromosomes and some genes are linked.

  • The genome was sequenced in 2000.

Worked Example: Monohybrid Cross Using D. melanogaster

P Generation

  • Wild type (++) crossed with ebony (ee).

  • Phenotypes: Wild type (dominant) vs. Ebony (recessive).

  • Genotypes: ++ (wild type) and ee (ebony).

  • The + symbol denotes the dominant wild-type allele (can also represent it as E).

F1 Generation

  • All offspring from this cross will exhibit the wild type phenotype (genotype: +e).

F2 Generation Outcomes

  • Phenotypic ratio: 3 wild type : 1 ebony (3:1).

  • Genotypic ratio: 1 ++ : 2 +e : 1 ee (1:2:1).

  • Reaffirmation on the dominance of + over e in the context of body color for this trait.

Test Cross

Definition and Utility

  • A test cross is a method used to determine the genotype of an individual with a dominant phenotype.

Determining Unknown Genotype

  • If the genotype of the purple flowered plant is unknown (PP or Pp), a test cross can clarify this.

  • Cross the individual with a known genotype of homozygous recessive (pp).

Outcome Predictions

  • If the unknown parent is homozygous dominant (PP), all offspring will express the purple phenotype.

  • If the unknown genotype is heterozygous (Pp), then the resulting offspring will display a phenotypic ratio of 1:1 (purple:white).

Application

  • This method is a standard approach in genetics for confirming genotypes based on observed phenotypes.

Dihybrid Cross and the Principle of Independent Assortment

Definition

  • A dihybrid cross is conducted between two double heterozygotes, examining two different genes (e.g., F1 x F1).

Purpose

  • Dihybrid crosses are used to explore the linkage relationship between genes.

Genetic Predictions

  • A 4 x 4 Punnett square can be utilized to predict potential genotypes and phenotypes among progeny.

Law of Independent Assortment

  • The principle states that the segregation of alleles for one gene occurs independently of alleles for another gene (e.g., yellow/green and round/wrinkled)

Gamete Formation Examples

  • Each double heterozygote (e.g., RrYy) can produce 4 types of gametes in equal proportions: RY, Ry, rY, ry.

Practical Probabilities in Genetics

Probability Definitions

  • All genetic ratios can be expressed as probabilities.

Monohybrid Cross Probabilities

  • Phenotypic ratio of 3:1 and genotypic ratio of 1:2:1 for monohybrid crosses.

  • Dihybrid crosses produce a phenotypic ratio of 9:3:3:1.

Rules of Probability Applied

  • Product Rule: Used for determining the probability of simultaneous events (e.g., both parents having a certain genotype).

  • Sum Rule: Used for mutually exclusive events (e.g., the outcome where either A or B occurs, but not both).

Product Rule Example

Application in Genetics

  • If both parents are heterozygous (Bb), the probability that a child will be bb (homozygous recessive) can be calculated.

  • Calculation: The probability of a ‘b’ egg = $ rac{1}{2}$ and a ‘b’ sperm = $ rac{1}{2}$.

  • Therefore, the probability of bb = $ rac{1}{2} imes rac{1}{2} = rac{1}{4}$.

Sum Rule Example

Application in Genetics

  • To find the probability of obtaining specific genotypes, such as Bb.

  • We calculate individual probabilities and use “OR” to sum them up:

  • Probability of Bb = probability of getting B egg ($ rac{1}{2}$) AND b sperm ($ rac{1}{2}$), or vice versa.

  • Calculations ultimately yield a genetic probability output.

Sum of Different Probability Outcomes

  • For a couple wanting two children, their chance of having two boys can be calculated as:
    Probability = $ rac{1}{2} imes rac{1}{2} = rac{1}{4}$.

  • Total probability of obtaining either two boys or two girls across multiple scenarios can also be expressed accurately.

Complex Genetic Outcomes

Multiple Genes Understanding

  • Punnett squares are practical for 2-3 loci (maximum of 6 alleles), but for more complex relationships, apply product and sum rules accordingly.

Example of Cystic Fibrosis

  • Concerning a recessive disorder where both parents are carriers, probability calculations demonstrate how to predict affected outcomes effectively.

Implications of Probabilities

  • Understanding heredity through probabilities enriches the knowledge in genetics, predicting familial outcomes based on gametic combinations.