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By the end of this section, you will be able to:
  • Identify non-Mendelian inheritance patterns such as incomplete dominance, codominance, multiple alleles, and sex linkage from the results of crosses
  • Explain the effect of linkage and recombination on gamete genotypes
  • Explain the phenotypic outcomes of epistatic effects among genes

Mendel studied traits with only one mode of inheritance in pea plants. The inheritance of the traits he studied all followed the relatively simple pattern of dominant and recessive alleles for a single characteristic. There are several important modes of inheritance, discovered after Mendel’s work, that do not follow the dominant and recessive, single-gene model.

Alternatives to dominance and recessiveness

Mendel’s experiments with pea plants suggested that: 1) two types of “units” or alleles exist for every gene; 2) alleles maintain their integrity in each generation (no blending); and 3) in the presence of the dominant allele, the recessive allele is hidden, with no contribution to the phenotype. Therefore, recessive alleles can be “carried” and not expressed by individuals. Such heterozygous individuals are sometimes referred to as “carriers.” Since then, genetic studies in other organisms have shown that much more complexity exists, but that the fundamental principles of Mendelian genetics still hold true. In the sections to follow, we consider some of the extensions of Mendelism.

Incomplete dominance

Mendel’s results, demonstrating that traits are inherited as dominant and recessive pairs, contradicted the view at that time that offspring exhibited a blend of their parents’ traits. However, the heterozygote phenotype occasionally does appear to be intermediate between the two parents. For example, in the snapdragon, Antirrhinum majus ( [link] ), a cross between a homozygous parent with white flowers ( C W C W ) and a homozygous parent with red flowers ( C R C R ) will produce offspring with pink flowers ( C R C W ). (Note that different genotypic abbreviations are used for Mendelian extensions to distinguish these patterns from simple dominance and recessiveness.) This pattern of inheritance is described as incomplete dominance    , meaning that one of the alleles appears in the phenotype in the heterozygote, but not to the exclusion of the other, which can also be seen. The allele for red flowers is incompletely dominant over the allele for white flowers. However, the results of a heterozygote self-cross can still be predicted, just as with Mendelian dominant and recessive crosses. In this case, the genotypic ratio would be 1 C R C R :2 C R C W :1 C W C W , and the phenotypic ratio would be 1:2:1 for red:pink:white. The basis for the intermediate color in the heterozygote is simply that the pigment produced by the red allele (anthocyanin) is diluted in the heterozygote and therefore appears pink because of the white background of the flower petals.

Photo is of a snapdragon with a pink flower.
These pink flowers of a heterozygote snapdragon result from incomplete dominance. (credit: "storebukkebruse"/Flickr)

Codominance

A variation on incomplete dominance is codominance    , in which both alleles for the same characteristic are simultaneously expressed in the heterozygote. An example of codominance occurs in the ABO blood groups of humans. The A and B alleles are expressed in the form of A or B molecules present on the surface of red blood cells. Homozygotes ( I A I A and I B I B ) express either the A or the B phenotype, and heterozygotes ( I A I B ) express both phenotypes equally. The I A I B individual has blood type AB. In a self-cross between heterozygotes expressing a codominant trait, the three possible offspring genotypes are phenotypically distinct. However, the 1:2:1 genotypic ratio characteristic of a Mendelian monohybrid cross still applies ( [link] ).

Questions & Answers

learning is facilitated by
Quitta Reply
What
Angel
What is Repression?
Sharmin Reply
Repression, In psychoanalytic theory, the exclusion of distressing memories, thoughts, or feelings from the conscious mind. Often involving sexual or aggressive urges or painful childhood memories, these unwanted mental contents are pushed into the unconscious mind.
Forqan
in psychoanalytic terms a basic defense mechanism that shuts out anxiety arousing thoghts feelings and memories from one consciousness
Berrel
Repression is holding something painful inside because you're afraid to let it out for fear of what trouble it might csuse
Connie
It helps students to understand the behavior of people they encounter.
Anjonette Reply
what are the types of learning?
Ogbeyemi Reply
auditory, visual, kinesthetic, reading/writing
frederick
thanks @Frederick. But I still want to understand something. Classical conditioning, Operants conditioning and Observational learning are not types of learning?
Ogbeyemi
I believe there are several types of learning besides just the four I mentioned.
frederick
so, I can also say those I mentioned are part of learning?
Ogbeyemi
i would say so
frederick
Operant conditioning is a type of associative learning process through which the strength of a behavior is modified by reinforcement or punishment.
Forqan
what is psychology
Mukoone Reply
in simple explanation, it is the study and understanding of the human behavior
Edmar
the scientific study of the human mind and its functions, especially those affecting behaviour in a given context.
naina
the scientific study of human mind and behaviour
NishaOfficial
but I thought psychology is not limited to humans. I think it is the scientific study of behavior and mental process of organisms
Ogbeyemi
processes*
Ogbeyemi
what happened to my reply?
Trish
the study of human behavior, the reasons for the behavior, and ways to change the undesirable behaviors into desirable behaviors
RD
psychology is the study of the soul or mind of man through scientific studies relating to environmental factors and human behaviours.
Audley
not only man but animals as well
Audley
psychology is the study of the mind and behaviors*
Emely
the definition you gave Audley is incorrect
Emely
Hey Audley, also humans are animals, so the correct way to say it is.. Not only man but other animals as well. 👍🏽
Emely
Audley, you are both correct in the definition of psychology. Yours was textually from the book, while the other was simple and to the heart of it where a novice would understand.
Bosie
Emely, you stated that man is a animal as well. Explain?
Bosie
_____are the memory images that are related to particular place ,date and time .
Raghav Reply
what is psychology
Ayado Reply
can join your discussions
Ayado
he or she can
Edward
I have been wondering of minor Autism I have estrange daughter that has I have been divorced and things like the ex wife sister told bad things about Me, Which I was not there to defend my self was said. one part) the other is the ex wife mom took care of her and never gave her kind of up bringing
Edward
Much and now she is 28 has a pension cause of her Autism she has everyone like her 2brothers her mom and her father had said to go on andfor-get the past she had not wanting to only seen her once I tried to make contact online she does not my contact from G circle
Edward
too bad
Ibrahim
God bless you Edward 🙏 I pray that God heals you and your family he will make things right
Joseph
Ibrahim it's too bad you couldn't take the time to give encouragement to your brother with a few more positive words at the end were all brothers and sisters in Christ 🙏
Joseph
what is rootology
Glory Reply
Rootology is a blend of 13 concentrated herbal extracts that support against sneezing, congestion, sinus pressure, runny nose, itchy watery eyes, and headache.
Labdhee
a child playing with the friend without complaining is called
Valerie Reply
free child
Nisha
psycosocial development
Anita
playful & sporty child
Jaga
The scope of Experimental Psychology is widening with the invention of new tools and instruments for experiments.
Den Reply
like what kind?
MissRoyal
hello, what kind of people talk throught here
Marcela Reply
the kind looking for answers
Juan
?
John
thought is autonomous. it is up to us compartmentalizing these random various thoughts that we too often incorrectly give them the title of (ideas)
John
what is the scope of experimental psychology?
Fareha
who is pavlo
ROGATH Reply
he believed in classic conditioning
just
he created classical conditioning
naina
Ivan Petrovich Pavlov was a Russian physiologist known primarily for his work in classical conditioning.
lavanya
he developed the theory is classical conditioning on a dog
NishaOfficial
Hello... Gyatri, angel and thinker.. I am MJ.
Mujtaba Reply
hi
naina
Hello
Jeanica
How are you...?
Mujtaba
Fine
Jeanica
Where are you from?
Jeanica
hi everyone 😊
Luis
I'm not trying to be a killjoy but I don't think this is a conversation thread for meeting new people.
Aspen
there are plenty of apps for that
Aspen
hai
kumpulan
@Aspen German well you're right there are other apps that offers ways into meeting new people. however I don't see anything wrong in here? just saying hi and hello.
Luis
but if they turn it into something like a "dating app" then for me that's another story. just saying. 😊.
Luis
hello Everyone
artgalang
Hello Luis, this app is for friends to everyone. 😊🙏
Jeanica
Anyone having problem can leave
Mujtaba
Hi
Red
Hi
Esther
Hello
Jeanica
hello^^
Girlie
is this a Psychology Course GC?
artgalang
Hi I'm from Philippines 🇵🇭
Esther
hellow
Erick
Can anyone.... What is basic criteria for abnormalities...?
Mujtaba
Is here any psychologist or psychiatrist.
Mujtaba
hi Jeanica yes I know that 😊
Luis
hello
Fermin
yes psychologist I am
kirti
me too
artgalang
Can you please explain me Diathesis stress model and its application
Mujtaba
And how can it applied..
Mujtaba
What is randomization
Joseph Reply
definition of stimulus
Thomas Reply
an idea, object or effect via the help of a mediational process which elicits a response
Wayne
Which event leads to a diploid cell in a life cycle
Nicole Reply

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Source:  OpenStax, University of georgia biology. OpenStax CNX. Dec 09, 2013 Download for free at https://legacy.cnx.org/content/col11585/1.6
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