<< 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

Is there any other type of a eukaryotic cell.
Grace Reply
what is bionomial nomenclature
Rachaelda Reply
state the role of mitochondria
Rachaelda
mitochondria ia power House of the cell. it provides energy and as ATP. Cells energy currency.
Haider
The scientific method of giving short names on the basis of genius and species.
Haider
it is introduce by carlous Lennieus
Haider
what is element
Kofi Reply
Structure of water molecule and it's biological significance. .....help guys
Ashly
what is the formula for chemical equetion
Justo Reply
Why mitochondria is called the power house of the congo the bahamas cell
Farrukh Reply
how can I learn this subject?
mascuud Reply
what's microscope?
Mathias Reply
A device used to study a very small specimen thst cannt br seen with the naked eyes for example cells, or microorganisms.
Danisha
a medical device used to study cells bacteria viruses and parasites e.g electron microscope for studying cells.
Grace
exactly microscope
Randa
what does multi seminar mean
Grace Reply
how many cells on the human
Amar Reply
how is genetic testing?
Nyabuoy
test
Nyuongatdet
which party of an internal leaf which represent organ and tissue
fernando
3 trilleons cells on the human
Jyoti
name the groups of bacteria, what they cause and explain the components of bacterial cell
Emmanuel
what are the three level of relationship that exist between organism?
Chinedu
trillions of cells
Grace
who many cell are in the human body
Ayasso Reply
trillions of cells
Grace
what causes coloring of skin variation
Prince Reply
what is your answer
Jonathan Reply
which qn
Randa
what is chemosynthesis
Irene
who many cell are in the human body
Ayasso
there are billion cells in human body
fazeela
what are three stages of mitosis
jerry
they're alot cells in our body
jerry
what are the stages of mitosis
jerry
they are prophez methaphez anaphez. thelophez
fazeela
anyone to explain each of the following,, prophase, metaphase, anaphase and telophase
jerry
what is a filial
Mbah Reply
what is the difference between chlorophyll and photosynthesis
Rahman Reply
Chloe is the green pigment found in green plants while photosynthesis is the process by which plant produce their own food
mary
photosynthesis is the production of food by plant while chlorophyll is the green pigment that is found in chloroplast..
jerry
chrolophyll (green colouring matter in leaves) while photosynthesis (process by which green plants make their own food)
Nakhombi
What isaac life
Farrukh
chlorophyll is the pigment that gives a plant its green color while photosynthesis is when a plant makes it's own food.
Grace
what are the functions of parts of microscope
Bami Reply
base to provide support
Ian
only base what about the other
Bami
has only one function
Mark
Mirror ... used to reflect light
Irene

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?

Ask