Heredity and Traits

Genetics : 

Genetics Principles of Heredity

Heredity and Genetics: 

Heredity and Genetics Has anyone ever told you that you look just like one of your relatives? The shape of your face, color of your eyes, size of your nose, and other physical characteristics are traits that are inherited from your parents. The passing of traits from parent to offspring is called heredity .  Traits, such as eye color, are generally controlled by genes. Genes are segments of DNA that provide instructions for making proteins.  Genes are located on chromosomes (gene-carrying structures that consist of DNA and proteins). The different forms of a gene are called alleles .  For example, one gene that controls eye color has two forms, a blue allele and a brown allele.  One allele is inherited from each of your parents.  Depending upon the alleles present in your parents, you inherited either a blue or brown allele from your mom, and a blue or brown allele from your dad. The study of the patterns of inheritance of traits is called genetics .  The foundation of genetics is based on the work of a scientist named Gregor Mendel.

Gregor Mendel: The Father of Genetics : 

Gregor Mendel: The Father of Genetics Gregor Mendel is known as the father of genetics.  He was the first scientist to trace the inheritance of traits through several generations and keep detailed records of his experiments.  His work formed the basis for our understanding of heredity.

Gregor Mendel: The Father of Genetics : 

Gregor Mendel: The Father of Genetics Mendel studied heredity in plants; one particular plant he worked with was the pea plant.  He studied seven different characteristics of pea plants, which are shown in the diagram below.

Mendel's Experiment: Inheritance of a Single Trait : 

Mendel's Experiment: Inheritance of a Single Trait Mendel's experiments with pea plants involved crossing plants that had different traits and recording how those traits were passed on from one generation to the next. One trait he studied was seed shape.  To begin his experiment, he crossed plants that produced round seeds with plants that produced wrinkled seeds. When he observed the first generation of offspring, he noticed that one trait seemed to disappear.  All of the offspring had round seeds and none had wrinkled seeds. Next, he allowed the first generation plants to self-fertilize and produce a second generation.  Upon observing the second generation, Mendel discovered that the wrinkled seed trait had reappeared.  The second generation consisted of plants that produced round seeds and plants that produced wrinkled seeds. He repeated this experiment numerous times and received the same results.  This experiment and many other similar ones led to the formulation of two basic principles: the principle of dominance and the principle of segregation.

Mendel's Experiment: 

Mendel's Experiment

Principle of Dominance: 

Principle of Dominance The principle of dominance states that some factors (alleles) are dominant, whereas others are recessive.  The dominant allele masks the other allele when they are present together in an organism.  The recessive allele is the form that seems to disappear. Organisms carry two alleles for each gene.  They may have two dominant alleles, two recessive alleles, or one dominant and one recessive allele. In pea plants, round is the dominant allele and wrinkled is the recessive allele for seed shape.  Plants carrying two round alleles, or one round allele and one wrinkled allele will have round seeds.  The only way that pea plants can produce wrinkled seeds is if they carry two wrinkled seed alleles. These combinations of alleles are called an organism's genotype.

Genotype & Phenotype : 

Genotype & Phenotype You just learned that genotype is the combination of alleles present in an organism.  It is the genetic makeup of an organism. An organism that has two of the same alleles for a trait is called homozygous .  One parent in the seed crossing experiment was homozygous dominant (two round alleles) and the other parent was homozygous recessive (two wrinkled alleles). An organism that has two different alleles for a trait is called heterozygous .  The first generation of offspring were heterozygous, since they each carried one round allele and one wrinkled allele. The second generation was a mixture of homozygous and heterozygous plants. The observable traits that result from an organism's genotype is called the phenotype .  The phenotypes for seed shape are round and wrinkled.

Genotype & Phenotype: 

Genotype & Phenotype

Principle of Segregation : 

Principle of Segregation The second principle that developed from Mendel's experimental results is the principle of segregation.  This principle explains what happens to allele pairs when an organism produces reproductive cells, or gametes. The principle of segregation states that during gamete production, the alleles for a trait separate, or segregate, so that each gamete has only one of the alleles for that trait. The diagram below demonstrates the principle of segregation using a homozygous round parent and a homozygous wrinkled parent.  Remember that homozygous means the organism has two of the same alleles. When the homozygous round parent produces gametes, its two round alleles (R) separate so each of its gametes carries only one allele.  The same process occurs in the homozygous wrinkled parent, and each of its gametes carries only one wrinkled allele. During fertilization, a gamete from each parent unites to create a new organism.

Principle of Segregation: 

Principle of Segregation What will be the first generation's genotype?

Principle of Segregation Continued : 

Principle of Segregation Continued The genotype for the first generation of offspring is heterozygous , since a gamete carrying the dominant allele combines with a gamete carrying the recessive allele. Unlike the parents, which each could only produce one kind of gamete, the first generation produces two types of gametes.  The gametes contain either the dominant allele or the recessive allele. The gametes produced by the first generation may combine in the following ways during self-fertilization: a gamete carrying a dominant allele may combine with another gamete carrying a dominant allele a gamete carrying a dominant allele may combine with a gamete carrying a recessive allele a gamete carrying a recessive allele may combine with another gamete carrying a recessive allele

Principle of Segregation Continued : 

Principle of Segregation Continued

Mendel's Experiment: Inheritance of Two Traits : 

Mendel's Experiment: Inheritance of Two Traits Once Mendel established inheritance patterns for a single trait, he conducted experiments to study the inheritance of two or more traits. To study the inheritance of two traits, Mendel crossed pea plants that were homozygous for two dominant traits with pea plants that were homozygous for two recessive traits. For example, the yellow seed allele is dominant to the green seed allele, and the round seed allele is dominant to the wrinkled seed allele.  In an experiment, Mendel crossed yellow, round seed plants with green, wrinkled seed plants. The first generation all looked like the yellow, round parent.  This generation was allowed to self-fertilize, and then the second generation was observed. The second generation consisted of mostly yellow, round plants, about the same number of green, round and yellow, wrinkled plants, and very few green, wrinkled plants.

Mendel's Experiment: Inheritance of Two Traits : 

Mendel's Experiment: Inheritance of Two Traits What was significant about these results? It showed that color and shape were inherited separately from one another since any combination of color and shape was possible.

Principle of Independent Assortment : 

Principle of Independent Assortment The results from Mendel's experiment involving the inheritance of two traits led to the development of the principle of independent assortment.  The principle of independent assortment states that as gametes form, the alleles for different genes separate independently of one another (as long as the genes are located on separate chromosomes). Let's look at the experiment involving two traits again.  If the alleles for various genes were connected such that, for example, the allele for yellow was always attached to the allele for round, then it would not be possible to produce a yellow wrinkled plant or a green round plant.

Mendel's Principles and You : 

Mendel's Principles and You The principles of heredity that were discovered by Mendel do not apply just to plants; they also apply to you and many other organisms. Mendel's experiments make it easier to understand how you inherited your traits from your parents.  Just like pea plants pass on traits such as seed color and seed shape, humans pass on traits to their offspring too.  Your eye color, hair color, and blood type are some of the traits you inherited from your parents in the same way that pea plants inherited their traits from their parents.