<< Chapter < Page Chapter >> Page >

For the same tetrahybrid cross, what is the expected proportion of offspring that have the dominant phenotype at all four loci? We can answer this question using phenotypic proportions, but let’s do it the hard way—using genotypic proportions. The question asks for the proportion of offspring that are 1) homozygous dominant at A or heterozygous at A, and 2) homozygous at B or heterozygous at B , and so on. Noting the “or” and “and” in each circumstance makes clear where to apply the sum and product rules. The probability of a homozygous dominant at A is 1/4 and the probability of a heterozygote at A is 1/2. The probability of the homozygote or the heterozygote is 1/4 + 1/2 = 3/4 using the sum rule. The same probability can be obtained in the same way for each of the other genes, so that the probability of a dominant phenotype at A and B and C and D is, using the product rule, equal to 3/4 × 3/4 × 3/4 × 3/4, or 27/64. If you are ever unsure about how to combine probabilities, returning to the forked-line method should make it clear.

Rules for multihybrid fertilization

Predicting the genotypes and phenotypes of offspring from given crosses is the best way to test your knowledge of Mendelian genetics. Given a multihybrid cross that obeys independent assortment and follows a dominant and recessive pattern, several generalized rules exist; you can use these rules to check your results as you work through genetics calculations ( [link] ). To apply these rules, first you must determine n , the number of heterozygous gene pairs (the number of genes segregating two alleles each). For example, a cross between AaBb and AaBb heterozygotes has an n of 2. In contrast, a cross between AABb and AABb has an n of 1 because A is not heterozygous.

General Rules for Multihybrid Crosses
General Rule Number of Heterozygous Gene Pairs
Number of different F 1 gametes 2 n
Number of different F 2 genotypes 3 n
Given dominant and recessive inheritance, the number of different F 2 phenotypes 2 n

Linked genes violate the law of independent assortment

Although all of Mendel’s pea characteristics behaved according to the law of independent assortment, we now know that some allele combinations are not inherited independently of each other. Genes that are located on separate non-homologous chromosomes will always sort independently. However, each chromosome contains hundreds or thousands of genes, organized linearly on chromosomes like beads on a string. The segregation of alleles into gametes can be influenced by linkage    , in which genes that are located physically close to each other on the same chromosome are more likely to be inherited as a pair. However, because of the process of recombination, or “crossover,” it is possible for two genes on the same chromosome to behave independently, or as if they are not linked. To understand this, let’s consider the biological basis of gene linkage and recombination.

Homologous chromosomes possess the same genes in the same linear order. The alleles may differ on homologous chromosome pairs, but the genes to which they correspond do not. In preparation for the first division of meiosis, homologous chromosomes replicate and synapse. Like genes on the homologs align with each other. At this stage, segments of homologous chromosomes exchange linear segments of genetic material ( [link] ). This process is called recombination, or crossover, and it is a common genetic process. Because the genes are aligned during recombination, the gene order is not altered. Instead, the result of recombination is that maternal and paternal alleles are combined onto the same chromosome. Across a given chromosome, several recombination events may occur, causing extensive shuffling of alleles.

Questions & Answers

what do we mean by transgenic organisms?
what is or are the functions of the Islets of Langarhaans
They are the regions of the pancreas that contains the endocrine cell
is the studly of life
Aisha Reply
what is biology
Asunta Reply
what is soil
Mukisa Reply
the top layer of the earth in which plant's, tree's
type of soil
function of cell wall
Nthati Reply
function of cell wall
To protect the cell from bursting
to protect the cell from bursting
What is escherichia coli
Tumise Reply
in what type of cells is meiosis taking place?
Rhyeann Reply
sex cells
reproductive system of earthworm plzz describes
applications of biology
Namawejje Reply
what is dormancy?
Aliyu Reply
hello guys what's the difference between prokaryotes and eukaryotes
Nwachukwu Reply
hlo what are the applications of biology?
eukaryotic cells have DNA in their nucleus while prokaryotic cells have their DNA present freely in their cytoplasm.
deviation from mendelian
Ogali Reply
what is lethal allele
Explain how chemical , bioligical and physical interaction between themselves and the non living components ?
Beyan Reply
what is Tissues
Faith Reply
Group of similar cells performing some related functions We have some type of tissues Connective Muscle Epithelial Nervous tissues
group of cells that is called tissue.
tissue is the arrangement or combination of similar cells that perform the same function .
what is abiotic?
Williams Reply
state 2 abiotic factors that affect the rate of transpiration in plants?
non living things
1) temperature:the temperature of the atmosphere affects the opening and closing of the stomata in the leaves of plants .high temperature encourages water loss . 2)wind : air flow encourages transpiration in plants . example,plants that are found on high areas like mountains transpire easily
I agree with Fadilat Muhammad In addition, water plants tend to reserve water during dry seasons so if they are not given plenty of water, they keep the water to them selves
what is a cell
Esther Reply
structural and functional group is called cells
the structural and functional unit of life is called cell. the cell was discovered by Robert hook in 1665Ad.
The structural functional of unit is called cell
The structural and functional unit of cell
Halim I am. A biological cell is the basic unit of life made up of protoplasm, organelles, a mitochondria where ATP, adenosinetriphosphate produces our energy, and a membrane barrier that envelopes the entire cell separating the internal and the external like our epidermis skin.
is isn't the cell
cell is the functional unit or basic unit of life
hello.the cell is defined as the basic structural and functional unit of life.it is responsible for the information and activities of an organism.
Robert Hooke discovered cells while observing the bark of a tree under a microscope.the name CELL was given to the tiny structures or compartments because they looked like the cells of monks

Get the best Biology course in your pocket!

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?