<< Chapter < Page Chapter >> Page >

Finally, epistasis can be reciprocal such that either gene, when present in the dominant (or recessive) form, expresses the same phenotype. In the shepherd’s purse plant ( Capsella bursa-pastoris ), the characteristic of seed shape is controlled by two genes in a dominant epistatic relationship. When the genes A and B are both homozygous recessive ( aabb ), the seeds are ovoid. If the dominant allele for either of these genes is present, the result is triangular seeds. That is, every possible genotype other than aabb results in triangular seeds, and a cross between heterozygotes for both genes ( AaBb x AaBb ) would yield offspring with a phenotypic ratio of 15 triangular:1 ovoid.

As you work through genetics problems, keep in mind that any single characteristic that results in a phenotypic ratio that totals 16 is typical of a two-gene interaction. Recall the phenotypic inheritance pattern for Mendel’s dihybrid cross, which considered two non-interacting genes—9:3:3:1. Similarly, we would expect interacting gene pairs to also exhibit ratios expressed as 16 parts. Note that we are assuming the interacting genes are not linked; they are still assorting independently into gametes.

For an excellent review of Mendel’s experiments and to perform your own crosses and identify patterns of inheritance, visit the Mendel’s Peas web lab.

Section summary

Mendel postulated that genes (characteristics) are inherited as pairs of alleles (traits) that behave in a dominant and recessive pattern. Alleles segregate into gametes such that each gamete is equally likely to receive either one of the two alleles present in a diploid individual. In addition, genes are assorted into gametes independently of one another. That is, alleles are generally not more likely to segregate into a gamete with a particular allele of another gene. A dihybrid cross demonstrates independent assortment when the genes in question are on different chromosomes or distant from each other on the same chromosome. For crosses involving more than two genes, use the forked line or probability methods to predict offspring genotypes and phenotypes rather than a Punnett square.

Although chromosomes sort independently into gametes during meiosis, Mendel’s law of independent assortment refers to genes, not chromosomes, and a single chromosome may carry more than 1,000 genes. When genes are located in close proximity on the same chromosome, their alleles tend to be inherited together. This results in offspring ratios that violate Mendel's law of independent assortment. However, recombination serves to exchange genetic material on homologous chromosomes such that maternal and paternal alleles may be recombined on the same chromosome. This is why alleles on a given chromosome are not always inherited together. Recombination is a random event occurring anywhere on a chromosome. Therefore, genes that are far apart on the same chromosome are likely to still assort independently because of recombination events that occurred in the intervening chromosomal space.

Whether or not they are sorting independently, genes may interact at the level of gene products such that the expression of an allele for one gene masks or modifies the expression of an allele for a different gene. This is called epistasis.

Art connections

[link] In pea plants, purple flowers (P) are dominant to white flowers (p) and yellow peas (Y) are dominant to green peas (y). What are the possible genotypes and phenotypes for a cross between PpYY and ppYy pea plants? How many squares do you need to do a Punnett square analysis of this cross?

[link] The possible genotypes are PpYY, PpYy, ppYY, and ppYy. The former two genotypes would result in plants with purple flowers and yellow peas, while the latter two genotypes would result in plants with white flowers with yellow peas, for a 1:1 ratio of each phenotype. You only need a 2 × 2 Punnett square (four squares total) to do this analysis because two of the alleles are homozygous.

Got questions? Get instant answers now!

Questions & Answers

how can a human being die by drinking too much of water
Olaleke Reply
Scientific name of a dog in canis familiarise?
Jaymez Reply
what's biology?
Jaymez
Biology is the study of living organism and their interaction with one another and their environment
Sharon
an other questions please
Elsabet
what is biosphere.
Chinyi
where are pollen grains produced pls.
Chinyi
And what are emphatic factors.
Chinyi
@Chinyi Bahiyah -pollen grains are produced in the anther of plants. the anther is one part of the male part of the plant the other being filament and they together form the stamen. hoped that helped :) !!
xjuicy_editzz
what is science?
Benedict Reply
Science is the method of acquiring skills and knowledge through experimentation and observation in the World Series
Richmond
what is ecology
Bitwell
is the study that deals with the interaction between living things and their environment
Alexander
what is the main fuction of root hair cells?
Liteboho
the main function is to uptake of water and nutrients from the rhizosphere.
Benedict
State the genius to where the dog belong.
Jaymez
where does meiosis occur in male plant reproductive organ
Gimai Reply
plant cell is plasmolysed as a result of the
Favour Reply
of the plant losing water
Esnart
"Head of a pin" means what?
Theresah Reply
types of genotype and types of blood group
Adebusuyi Reply
what is biology
Musa Reply
is the study of cell and structure of an organism
Atem
is the study of characteristics,life processes and phenomana of living organism
Atem
What is Monomer of protein?
Dimpho Reply
What's nutrition and types of nutrition
Jessica Reply
Why is cheetah d fastest animal
Jessica Reply
because they are capable of reaching speeds of up to 70mph
Abu
Thanks
Jessica
what is a cell
Reuben Reply
what is an enzymes
Reuben
a cell is a biological catalyst that speeds up the rate of a chemical reaction without itself been changed
Israel
a cell is smallest unit of life
Francis
an Enzyme helps speed up a chemical reaction
Francis
is the basic structural and functional unit of all organism
Atem
what is deamination
Mutebi Reply
Is the removal of amino radical from amino acid or any other amino compound.
Esnart
what is metabolism
Allen Reply
metabolism is the the combination of all the reaction that occur within your body
xjuicy_editzz
These reaction can be either anabolic (which is the building up of molecules) or can be catabolism (breaking of molecules) but these processes occur simultaneously to maintain homeostasis(internal body environment) within the body.
xjuicy_editzz
hope that helps :D !!
xjuicy_editzz
list 20 element in their order
Dor Reply
hydrogen helium lithium beryllium boron carbon nitrogen oxygen fluorine neon sodium Magnesium aluminium silicon phosphorus sulfur chlorine Aaron potassium calcium -Also if anyone is having trouble remembering the elements I recommend watching "The Periodic Table of elements song" :)
xjuicy_editzz

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Biology' conversation and receive update notifications?

Ask