The Mendel’s Theory of crossing

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Introduction
 
Mendel’s hybridization experiments in Pisum sativum (Pea plants) during the 1850’s and 1860’s set the foundation for what is now known as classical genetics  (PG3.1 Gregor Mendel Discovered the Basic Principles of Heredity).  Mendel derived laws of inheritance through observing how specific traits were passed on from parent to subsequent generations of offspring.  Mendel’s first law, the Law of Segregation, states that hereditary factors (now known as alleles) controlling a trait separate during gamete (sperm, egg) formation, and upon fertilization, each gamete contributes a single factor to the now developing offspring (3.2 Monohybrid Crosses Reveal the Principle of Segregation and the Concept of Dominance).  Mendel also developed a second law, the Law of Independent Assortment through his discovery that factors for a trait (eg, factors for pea color and factors for seed shape) assort independently of the factors of another trait during gamete formation (3.3 Dihybrid Crosses Reveal the Principle of Independent Assortment).
 
Mendelian genetics
 
You should remember the basics of Mendelian genetics from your biology course. This submodule provides a review. Make sure to be completely familiar and comfortable with it. The following video by professor Comai should help.
 
Video lecture on Mendel’s (Links to an external site.)Links to an external site. First and Second laws. 5:30 min. Luca Comai.
 
Important terms
 
You are expected to know each of these terms.
 

Character: a physical, physiological or biochemical characteristic of an individual. Examples: height, weight, color of hair, skin, leaves, flowers, eye, response to a pathogen, etc.

 

Trait: the type of a character, such as brown vs blue eye, red vs yellow flower, yellow vs green pea seed, tall vs short, etc.

 

Gene: unit of inheritance, such as the gene responsible for color of the pea. The names and symbols of genes vary and derive mostly from their history and the trait they affect (see below).

 

Locus: a site on a chromosome.

 

Alleles: different and alternative forms of a locus. When their locus correspond to a gene, the alleles typically confer different properties to the organism (see phenotype). One allele of the color gene results in the yellow pea trait, the other in the green pea trait. In diploids such as pea and human, individuals carry two alleles of every gene. Alleles are given names and symbols. For pea color, there is a yellow and a green allele.

 

Genotype: description of genes and alleles in an individual or a cell.

 

Phenotype: informative description of the physical appearance, physiology, and biochemistry of an individual. The phenotype can include one or more of the characters that can be observed.

 

Dominance: the quality of an allele that makes its activity prevail when another allele type of the same gene is present. The yellow allele of pea is dominant over the green and is represented by the letter Y for yellow.

 

Recessivity: the opposite quality of an allele. it makes its activity be hidden when a dominant allele of the same gene is present. The green allele of pea is recessive to yellow. To indicate its allelic state, it is called y.

 

Gametes: specialized cells dedicated to reproduction that typically carry a single allele for each gene

Sperm: one of two gamete types. It is produced by male individuals and it is typically small and motile. In plants, the sperm is carried by pollen.

Egg: the other gamete type, typically larger and non motile.

Male: an individual that produces sperm.

Female: an individual that produces eggs.

Hermaphrodite: an individual that produces both sperm and eggs.

Dioecious: a species whose individuals are either female or male. Humans, dogs, kiwifruits and marijuana plants are all dioecious.

 

Selfing: in a hermaphrodite, such as a plant or a worm, the sperm fertilizes the egg (both gametes have been produced by the same individual).

 

Crossing: the sperm of one individual fertilizes the egg of another individual.

 

Zygote: single cell produced by fertilization of egg by sperm.

 

Homozygous: an individual carrying two identical or equivalent alleles of the gene of interest.

 

Heterozygous: an individual carrying two different alleles of the gene of interest

 

Pure line: individuals that are nearly or perfectly identical and are homozygous at most or all loci. When selfed or intercrossed, they produce individuals of the same type.

 

Parental generation. The two parents of a cross. In mendelian genetics, these are often pure lines. Can be referred to as P1 and P2.

 

Filial generation. The first or successive generation from a cross.

 

F1. The first generation after crossing two pure lines. Often, this is heterozygous at the loci of interest.

 

F2. The second generation obtained by selfing or by sib-crossing (siblings=brothers and sisters).

 
Mendel First Law
 
The two alleles of a gene segregate into gametes. This is illustrated by the behavior of one factor cross. Yellow and green pure pea lines are crossed. The F1 progeny are pea seeds with Yy genotype and yellow phenotype. After germinating and flowering, each F1 makes two types of gametes: Y and y. The gamete types and corresponding F2 zygotes from such a parent are often visualized in a 2 x 2 square first proposed by Reginald Punnett, an early geneticist.
 

P1 and P2: Yellow and green, YY and yy, respectively

 

F1, Yellow and Yy

 

F2 phenotypes: Yellow and Green, 3:1

 

F2 genotypes: YY, Yy, and yy, 1:2:1

 

Yellow F2 are represented also with the following genotype: Y_, for either YY or Yy.

 

Green F2 are always yy.

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