Mendel's Cross for 7 Characters

This page delves deeper into Mendel's experimental crosses and introduces key genetic concepts.

The page presents a table showing Mendel's crosses for seven different pea plant characteristics, illustrating how he paired contrasting traits in the parental generation (P1) and observed the resulting offspring in the first filial generation (F1).

Example: For seed color, Mendel crossed yellow seeds with green seeds, and the F1 generation produced yellow seeds.

The page then introduces several important genetic terms:

Vocabulary:

• Genes: Units of heredity, sections of DNA sequence encoding a single protein.
• Alleles: Alternative forms of a gene.
• Locus: Fixed location on a DNA strand where a gene or one of its alleles is located.

The concept of using letter names to represent genes controlling hereditary characters is explained, using plant height as an example:

Example: For plant height, T represents the tall allele (dominant), while t represents the short allele (recessive).

Types of Alleles and Genetic Concepts

This page introduces the concepts of dominant and recessive alleles, as well as genotypes and phenotypes.

Definition:

• Dominant allele: An allele that masks or suppresses the expression of an alternate allele, represented by an uppercase letter.
• Recessive allele: An allele that is masked by a dominant allele, represented by a lowercase letter.

The page then explains the concepts of genotype and phenotype:

Vocabulary:

• Genotype: The genetic makeup of an organism, written in letter form (e.g., TT).
• Phenotype: The physical appearance of an organism as a result of its genotype (e.g., Tall).

An illustration of chromosomes in a non-dividing cell is provided, showing the locus and alleles on maternal and paternal chromosomes.

The page concludes by introducing two types of genotypes:

1. Homozygous: Pure-bred; having identical alleles for a particular trait.
2. Heterozygous: Hybrid; having two different alleles for a particular trait.

Example: In the case of plant height, TT and tt are homozygous genotypes, while Tt is a heterozygous genotype.

Genetic Crosses and Mendel's Conclusions

This page covers various types of genetic crosses, Mendel's conclusions, and introduces the Punnett square as a tool for predicting genetic outcomes.

The page begins by explaining different types of genetic crosses:

Vocabulary:

• Monohybrid cross: A genetic cross involving a single pair of genes (one trait).
• Dihybrid cross: A genetic cross involving two pairs of genes (two traits).

Mendel's conclusion from his experiments is presented:

Quote: "When true-breeding plants with contrasting traits are crossed, all offspring (100%) will express only one of the two traits."

The Punnett square is introduced as a useful tool for predicting the genotypes and phenotypes of offspring in genetic crosses.

Definition: Punnett square: A diagram used to predict the possible genotypes and phenotypes of offspring resulting from a genetic cross.

The concept of probability in genetics is also explained:

Definition: Probability: The chance of getting a particular outcome over all possible outcomes.

Example: Examples of probability include tossing a coin, rolling a dice, or picking a card from a deck of cards.

Monohybrid Cross Examples

This page provides detailed examples of monohybrid crosses, demonstrating how to use Punnett squares and calculate probabilities.

The first example involves a cross between a homozygous man with free earlobes and a woman with attached earlobes:

Example: Using the legend F for free earlobes (dominant) and f for attached earlobes (recessive), the cross is represented as FF x ff.

The Punnett square for this cross is shown, and the probabilities are calculated:

• Probability of a child with attached earlobes: 0%
• Probability of a child with free earlobes: 100%

A second example involves a couple who are both heterozygous for tongue rolling:

Example: Using the legend R for roller $dominant$ and r for non-roller (recessive), the cross is represented as Rr x Rr.

The Punnett square is provided, and various probabilities are calculated, including:

• Number of non-tongue rolling children out of 8
• Phenotypic ratio $3 roller : 1 non-roller$
• Probability of a tongue-rolling child (75%)

The page concludes by introducing Mendel's Laws:

1. Principle of Segregation: Alleles of a gene separate when gametes are formed.
2. Principle of Independent Assortment: The segregation of alleles of one gene is independent of the segregation of alleles of another gene during gamete formation.

Dihybrid Crosses and Mendel's Laws

This page focuses on dihybrid crosses and provides a detailed example to illustrate the concept.

Definition: Dihybrid cross: A mating involving two parents that differ in two genes (two independent traits).

The page presents an example of a dihybrid cross in summer squash, involving fruit color and shape:

Example: In summer squash, white fruit color is dominant over yellow, and disk-shaped fruit is dominant over sphere-shaped fruit. The cross involves a plant heterozygous for both traits $WwDd$ and a plant true-breeding for yellow, sphere-shaped fruit (wwdd).

A Punnett square for this cross is provided, along with a detailed analysis of the results:

1. Genotypic ratio: 1 WwDd : 1 Wwdd : 1 wwDd : 1 wwdd
2. Phenotypic ratio: 1 white disk-shaped : 1 white sphere-shaped : 1 yellow disk-shaped : 1 yellow sphere-shaped
3. Probability of yellow color and disk-shaped offspring: 1/4 or 25%

This example demonstrates the application of Mendel's Principle of Independent Assortment, showing how traits for color and shape are inherited independently.

Highlight: Understanding dihybrid crosses is crucial for predicting the inheritance of multiple traits and forms the basis for more complex genetic analyses.

Introduction to Genetics

This page introduces the fundamental concepts of genetics and heredity, highlighting the pioneering work of Gregor Mendel.

Genetics is defined as the study of heredity, a branch of biology that investigates how biological hereditary information is passed from one generation to the next. Heredity refers to the transmission of traits or characteristics, such as eye color, from parents to offspring.

Highlight: Gregor Mendel $1882-1884$, an Austrian monk, is considered the "Father of Genetics" due to his groundbreaking work with pea plants that laid the foundation for modern genetics.

The page explains why Mendel chose garden peas for his experiments, citing several advantages:

1. Contrasting expression of traits
2. Fast reproduction rate
3. Easy cultivation
4. Adaptation for self-pollination
5. Possibility of manual cross-pollination

Mendel's experimental design involved studying one character with two contrasting expressions at a time, allowing plants to self-pollinate to produce pure-breds, and then cross-pollinating two pure-breds with contrasting expressions.

Example: Mendel's pea plant experiment focused on seven distinct characteristics, including seed color, seed shape, pod color, pod shape, flower position, flower color, and plant height.