<< Chapter < Page Chapter >> Page >

Dominant and recessive alleles

Our discussion of homozygous and heterozygous organisms brings us to why the F 1 heterozygous offspring were identical to one of the parents, rather than expressing both alleles. In all seven pea-plant characteristics, one of the two contrasting alleles was dominant, and the other was recessive. Mendel called the dominant allele the expressed unit factor; the recessive allele was referred to as the latent unit factor. We now know that these so-called unit factors are actually genes on homologous chromosome pairs. For a gene that is expressed in a dominant and recessive pattern, homozygous dominant and heterozygous organisms will look identical (that is, they will have different genotypes but the same phenotype). The recessive allele will only be observed in homozygous recessive individuals ( [link] ).

Human Inheritance in Dominant and Recessive Patterns
Dominant Traits Recessive Traits
Achondroplasia Albinism
Brachydactyly Cystic fibrosis
Huntington’s disease Duchenne muscular dystrophy
Marfan syndrome Galactosemia
Neurofibromatosis Phenylketonuria
Widow’s peak Sickle-cell anemia
Wooly hair Tay-Sachs disease

Several conventions exist for referring to genes and alleles. For the purposes of this chapter, we will abbreviate genes using the first letter of the gene’s corresponding dominant trait. For example, violet is the dominant trait for a pea plant’s flower color, so the flower-color gene would be abbreviated as V (note that it is customary to italicize gene designations). Furthermore, we will use uppercase and lowercase letters to represent dominant and recessive alleles, respectively. Therefore, we would refer to the genotype of a homozygous dominant pea plant with violet flowers as VV , a homozygous recessive pea plant with white flowers as vv , and a heterozygous pea plant with violet flowers as Vv .

The punnett square approach for a monohybrid cross

When fertilization occurs between two true-breeding parents that differ in only one characteristic, the process is called a monohybrid    cross, and the resulting offspring are monohybrids. Mendel performed seven monohybrid crosses involving contrasting traits for each characteristic. On the basis of his results in F 1 and F 2 generations, Mendel postulated that each parent in the monohybrid cross contributed one of two paired unit factors to each offspring, and every possible combination of unit factors was equally likely.

To demonstrate a monohybrid cross, consider the case of true-breeding pea plants with yellow versus green pea seeds. The dominant seed color is yellow; therefore, the parental genotypes were YY for the plants with yellow seeds and yy for the plants with green seeds, respectively. A Punnett square    , devised by the British geneticist Reginald Punnett, can be drawn that applies the rules of probability to predict the possible outcomes of a genetic cross or mating and their expected frequencies. To prepare a Punnett square, all possible combinations of the parental alleles are listed along the top (for one parent) and side (for the other parent) of a grid, representing their meiotic segregation into haploid gametes. Then the combinations of egg and sperm are made in the boxes in the table to show which alleles are combining. Each box then represents the diploid genotype of a zygote, or fertilized egg, that could result from this mating. Because each possibility is equally likely, genotypic ratios can be determined from a Punnett square. If the pattern of inheritance (dominant or recessive) is known, the phenotypic ratios can be inferred as well. For a monohybrid cross of two true-breeding parents, each parent contributes one type of allele. In this case, only one genotype is possible. All offspring are Yy and have yellow seeds ( [link] ).

Questions & Answers

what structure help root hair cells to take up water.
Jackson Reply
which two part of a plant cell are affected when the is immersed in sucrose solution?
Jackson
which two part of a plant cell are affected when the is immersed in sucrose solution?
Jackson
the xylem cells absorbs water and mineral salt from the soil to all part of the plant.
Nana
And the phloem cell take up food prepared from the leaves to all part of the plant
Nana
what is genetic engineering
Mavis Reply
what are the three main type of ecosystem
Mavis
biosphere,
Bigenis
biosphere
Adeniyi
what is mitochondria
Might Reply
please what's genetics erngee
Abanke Reply
what is genetic engineering
Eveline Reply
what is the meaning of term mitosis
Lwitiko Reply
outline the significance of mitosis to organisms
Lwitiko
significant of meiosis are to increase in variation and leads to the formation of haploid gamete
Eveline
thanks
Abanke
name the resources to be conserved
Oreva Reply
define natural resources
Oreva
name the agencies responsible for the conservation of natural resources
Oreva
land,water,forests,
Kiiza
nema,uwa
Kiiza
wassup guyz
Peace
land ,water
Erika
Good Land and Water
garnhial
food, fish, livelihoods,forest, land and water
garnhial
explain the importance of carbon dioxide in the body
Kiiza Reply
how does that works
SAMUEL
diagram of Prokaryotic cells
Magreth Reply
where is it?
Yazi
ʜɪ
Malikie
ʜɪ
Malikie
where is the diagram?
Yazi
waiting.....
Yazi
what
Malikie
good morning guyz
Joelia
gd morning
Hannalyn
morning how are u doing
Paul
doing all fyn en u
Joelia
👍👍👍
Gruxz
morning everyone.. by is Grace we have be saved... Ephesians 2:8....
Cosmo
hi
Kisa
hey guys am new
Ellie
u a most wlcm
Joelia
🍑🐕
Ken
Hello guys im new
Sulaiman
u are welcome
Peace
thanks
Ellie
..amitabatha..
Ken
He guys happy New month
Peace
hello everyone,I'm new here!!
Reine
welcome
James
thanks so much!!
Reine
how re u Reine Balla
James
Happy New mnth to guys
Peace
HAPPY new month to u
James
Thanks and same to u
Peace
welcome one more peace
James
hi guys
Tafadzwa
Define the term Biology element atom
James
my name is rons am asking the question what is blood compatibility
Rons
Do you really no the one you text with.
Israel
I'm fine James, just boredom want to finish me!!
Reine
what is a cell of a bacteria called
Sahfe Reply
what is genotype
James
genotype: this is the combination of alleles an organism has for a given characteristic..
Cosmo
what is maiotic
Gabriel Reply
what is sexual reproduction
Gabriel
is system which usually happens to sexual things
GABRIEL
what are the effects of concentration gradient in the uptake of water by plant's
Harleen Reply
ls it ok if you have sex during pregnancy
Kags Reply
no
Bernard
yes!!!
Yazi
Yes
Babie
hello
Cabdi
Babie Maseuse .. Hello .. how are you ?
kf
yes it make the baby's strength stay longer
REAL
Who are you?
Babie
why not it's nicer and more enjoyable more than ok for the woman
Lamin
danso from the Gambia and you why do you ask
Lamin
as long as the woman is comfortable and in any possible position good for her
Lamin
unless medically advised to stop or hault and those conditions are also applicable to certain problem in pregancies. Pls any further comment pls never hesitate to ask
Lamin
how tell us
Tony
an assignment for you pls
Lamin
it's recommended
Nyakato
Hello
george
hi how are you?
Lamin
Fine
george
hello everyone
Cosmo
Good night to everyone am from Zambia may the Grace of the Lord be with you all
Cosmo
u too hv gd dreams
Joelia
thanks colleagues and wish you all the best insha Allah
Lamin
InshaAllah
Joelia
Marsha Allah
Lamin
yes
James
What is the meaning of organ
Ronald Reply
tissue combined to form organ
zameer
join
Elishs
orga- these are tissues join together to perform a specific functions.
Elishs
then what are tissues
Kags
group of cells working together to perform a particular function
Harleen
tissue are group of cells put together to perform a certain goal
Bernard
Organs are group of tissues working together to perform a specific function
Peter
What will happen when read blood cell placed in 0.5 % Nacl solution
zameer Reply
it will shrink and collapse eventually since the concentration of the solution is higher than the cell content
Peter

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