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By the end of this section, you will be able to:
  • Explain the relationship between genotypes and phenotypes in dominant and recessive gene systems
  • Use a Punnett square to calculate the expected proportions of genotypes and phenotypes in a monohybrid cross
  • Explain Mendel’s law of segregation and independent assortment in terms of genetics and the events of meiosis
  • Explain the purpose and methods of a test cross

The seven characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits. Mendel deduced from his results that each individual had two discrete copies of the characteristic that are passed individually to offspring. We now call those two copies genes, which are carried on chromosomes. The reason we have two copies of each gene is that we inherit one from each parent. In fact, it is the chromosomes we inherit and the two copies of each gene are located on paired chromosomes. Recall that in meiosis these chromosomes are separated out into haploid gametes. This separation, or segregation, of the homologous chromosomes means also that only one of the copies of the gene gets moved into a gamete. The offspring are formed when that gamete unites with one from another parent and the two copies of each gene (and chromosome) are restored.

For cases in which a single gene controls a single characteristic, a diploid organism has two genetic copies that may or may not encode the same version of that characteristic. For example, one individual may carry a gene that determines white flower color and a gene that determines violet flower color. Gene variants that arise by mutation and exist at the same relative locations on homologous chromosomes are called alleles . Mendel examined the inheritance of genes with just two allele forms, but it is common to encounter more than two alleles for any given gene in a natural population.

Phenotypes and genotypes

Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics. The observable traits expressed by an organism are referred to as its phenotype    . An organism’s underlying genetic makeup, consisting of both the physically visible and the non-expressed alleles, is called its genotype    . Mendel’s hybridization experiments demonstrate the difference between phenotype and genotype. For example, the phenotypes that Mendel observed in his crosses between pea plants with differing traits are connected to the diploid genotypes of the plants in the P, F 1 , and F 2 generations. We will use a second trait that Mendel investigated, seed color, as an example. Seed color is governed by a single gene with two alleles. The yellow-seed allele is dominant and the green-seed allele is recessive. When true-breeding plants were cross-fertilized, in which one parent had yellow seeds and one had green seeds, all of the F 1 hybrid offspring had yellow seeds. That is, the hybrid offspring were phenotypically identical to the true-breeding parent with yellow seeds. However, we know that the allele donated by the parent with green seeds was not simply lost because it reappeared in some of the F 2 offspring ( [link] ). Therefore, the F 1 plants must have been genotypically different from the parent with yellow seeds.

Questions & Answers

Cellular respiration
Lucy Reply
what are the characteristics of living things
Ruth Reply
Movement Respiration Nutrition/Feeding Irritability/Sensitivity Growth Excretion Reproduction Deat/Life span
Hashim
What makes children from the same father and mother sometimes don't look alike?
Hashim
identification of problems
Nana Reply
what happens in the process of raising the human arms
Nana
what is biology
Brandi Reply
first step in scientific method
Brandi
In an investigation the pancreatic duct of a mammal was blocked.It was found that the blood sugar regulation remained normal while food digestion was impaired.Explain
Mac Reply
To begin with, obstruction of pancreatic duct will alter the blood sugar level as the juices responsible for glucose regulation will be rendered inconsequential. This will in turn affect the rate of digestion and absorbtion of digested food substances by the Villus .
Muktar
characteristics of algae
OMIME Reply
Algae are eukaryotic organisms. Algae do not have roots and stems. Algae have chlorophyll and helps in carrying out photosynthesis.
Aditi
Cell wall is the rigid layer enclosed by membranes of plants and prokayortic cell, it maintains the shape of the cell and serve as a protective barrier.
chizoba Reply
ECOLOGY: is a branch of biology that studies the interactions among organisms and their biophysical environment, which includes both biotic and abiotic components. 
chizoba
via nutrient cycles and energy flows. For instance, the energy from the sun is captured by plants through photosynthesis. Photosynthesis is a biological process through which plants manufacture their own food with the aid of light from the sun and frc sources (e.g. cabon dioxide and water)
chizoba
What is cell wall
Taiwo Reply
cell wall is the outemost rigid covering of the plants ,that provides protection to the plants.
Aditi
what is ecology, ecosystem?
Nkeng Reply
what is digestive system
Lucky Reply
digestive system is the human syman system that icludes esopuges stomach o braking down of food in to useful substance to our body
samrawit
definition of biology basics
Ritu Reply
the potential energy of a molecule can be inquired by their number of?
Jesus Reply
what is the full meaning of RNA
Ayo Reply
ribose nucleic acid
Nikita
Ribonucleic acid
Jesus
Ribo Nucleic Acid
Aditi
ribonucleic acid
Nana
discuss, describe at least three (3) methods that could be used to improve photosynthesis..
Marvel Reply
Improve the efficiency with which plants capture light Improve the efficiency by which plants turn light into energy The smart canopy concept develop crop planting schemes that increase the penetration of sunlight into lower-level leaves.
Jesus
what is osmosis
Aon Reply
movement of water molecule from higher to lower concentration through a semipereable membrene.
Dr
what of in the case of solute
Aon
osmosis is the movement of molecules from higher concentration region to lower concentration region through semi-permeable membrane.
Broad
in case of solute means that water moves from the region with lower solutes to the region with higher solute. so it is vice versa to water.
Broad
osmosis is the movement of water molecule from a region of lower concentration to a region of higher concentration through a semi permeable membrane
Nana

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Source:  OpenStax, Concepts of biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11487/1.9
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