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By the end of this section, you will be able to:
  • Explain Mendel’s law of segregation and independent assortment in terms of genetics and the events of meiosis
  • Use the forked-line method and the probability rules to calculate the probability of genotypes and phenotypes from multiple gene crosses
  • Explain the effect of linkage and recombination on gamete genotypes
  • Explain the phenotypic outcomes of epistatic effects between genes

Mendel generalized the results of his pea-plant experiments into four postulates, some of which are sometimes called “laws,” that describe the basis of dominant and recessive inheritance in diploid organisms. As you have learned, more complex extensions of Mendelism exist that do not exhibit the same F 2 phenotypic ratios (3:1). Nevertheless, these laws summarize the basics of classical genetics.

Pairs of unit factors, or genes

Mendel proposed first that paired unit factors of heredity were transmitted faithfully from generation to generation by the dissociation and reassociation of paired factors during gametogenesis and fertilization, respectively. After he crossed peas with contrasting traits and found that the recessive trait resurfaced in the F 2 generation, Mendel deduced that hereditary factors must be inherited as discrete units. This finding contradicted the belief at that time that parental traits were blended in the offspring.

Alleles can be dominant or recessive

Mendel’s law of dominance    states that in a heterozygote, one trait will conceal the presence of another trait for the same characteristic. Rather than both alleles contributing to a phenotype, the dominant allele will be expressed exclusively. The recessive allele will remain “latent” but will be transmitted to offspring by the same manner in which the dominant allele is transmitted. The recessive trait will only be expressed by offspring that have two copies of this allele ( [link] ), and these offspring will breed true when self-crossed.

Since Mendel’s experiments with pea plants, other researchers have found that the law of dominance does not always hold true. Instead, several different patterns of inheritance have been found to exist.

Photo shows an albino child with his black mother.
The child in the photo expresses albinism, a recessive trait.

Equal segregation of alleles

Observing that true-breeding pea plants with contrasting traits gave rise to F 1 generations that all expressed the dominant trait and F 2 generations that expressed the dominant and recessive traits in a 3:1 ratio, Mendel proposed the law of segregation    . This law states that paired unit factors (genes) must segregate equally into gametes such that offspring have an equal likelihood of inheriting either factor. For the F 2 generation of a monohybrid cross, the following three possible combinations of genotypes could result: homozygous dominant, heterozygous, or homozygous recessive. Because heterozygotes could arise from two different pathways (receiving one dominant and one recessive allele from either parent), and because heterozygotes and homozygous dominant individuals are phenotypically identical, the law supports Mendel’s observed 3:1 phenotypic ratio. The equal segregation of alleles is the reason we can apply the Punnett square to accurately predict the offspring of parents with known genotypes. The physical basis of Mendel’s law of segregation is the first division of meiosis, in which the homologous chromosomes with their different versions of each gene are segregated into daughter nuclei. The role of the meiotic segregation of chromosomes in sexual reproduction was not understood by the scientific community during Mendel’s lifetime.

Questions & Answers

complete the table below based on the levels of biological organization
Lovely Reply
Give me Examples of living thing which have 2 or more flagella?
Mahesh Reply
insect and plants
bacteria and chlamydompnas
reproduction it's full meaning
Gift Reply
full meaning of ATP
A life process in which living things increase their population through sexual or non sexual intercouse
please explaination
Gifty ATP means Adenosine tri phosphate
the process by which organisms produce their own kind.
reproduction is the process where living organisms producess their offspring
jerry Reply
what is reproduction
Nmesoma Reply
why some kinds of students are failed
Ahmadi Reply
lack of concentration
lack of guidance and counseling
what's the divination of openstax
don't mind about reading
lack of focus
What is the meaning of optic
Kisaky Reply
Giving a specific section of the alimentary canal,describe 3 ways in which physical digestion occurs.
mouth when chewing
what is population
Ivy Reply
total number of people living in an area
a number of people lives in one catigorize area or named area
what is a cell
Chiko Reply
basic and functional unit of life
cell is tissues that makes up functional life in human or un animal.
is the smallest basic unit of life.
Is the smallest baic unit. o
why cell is very important to human body
what is diffusion
diffusion is a process of mix of particles from higher concentration to the lower one,to make the body functional normal
what is effusion
what is soil
Is the finely divided material covering the earth crust.
is the upper moist of layer of the earth surface
what is reducing sugar
Erica Reply
in genetics which disease is also termed as the queen disease
Phinihas Reply
what are the types of cell
Teye Reply
prokaryote ,eukaryote, akaryotes
what is akaryotes ?
multicellular and unicellular
akaryotes are organisms that function as eukaryotes and prokaryotes
akaryotes are cell with no nucleus
biology is the study of living organisms
bonney Reply
what's the divination of open stax?
biology is designed for multi- semester biology course for science Major
what are the important of cells in the body
Nharnhar Reply

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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