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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

Is the "growth and maintenance phase" in a cell's life cycle when cell division is about to occur
Somto Reply
what is the common name of Basidiomycetes
Ogechukwu Reply
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yes
tariq
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tariq
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which are present In Body And such elements Have Great role in our Body there are 16 bio elements that maintains human Body but on The basis of amount There are 6 bio elements present in Concen. of 99% and More Valuable And Highly Concen. element is Oxygent with 65 %
Haider
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Lamina
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Ivanovic Reply
Complex traits such as height result from 
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Nkalubo Reply
chloroplast in plants and bacterial cell ; mitochondria in animal cells
aung
Diagram of a living cell
Eliza Reply
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Sule
A cell is the smallest basic unit of life.
John
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Ogochukwu Reply
this is da study of living and non-living thing in an eco-system
Nutty
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Akufia
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Winner
why do plants store carbohydrates in form of starch and not glucose?
Nutty Reply
Describe the structure of starch?
Nutty
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Winner
water is life!.. Discuss?
Nutty Reply
why do plants store carbohydrates in form if starch not glucose!
Nutty
study of living thing
Dennis Reply
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The surgical removal of d spermduct
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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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