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AP®︎/College Biology
Course: AP®︎/College Biology > Unit 5
Lesson 2: Mendelian genetics- Introduction to heredity
- Fertilization terminology: gametes, zygotes, haploid, diploid
- Alleles and genes
- Worked example: Punnett squares
- Mendel and his peas
- The law of segregation
- The law of independent assortment
- Probabilities in genetics
- Pedigrees
- Mendelian genetics
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Alleles and genes
Genes and alleles, two fundamental concepts in genetics, differ in their roles. Genes are DNA sections that code for specific proteins or functional RNA, playing a crucial part in biological functions. Alleles, however, are variations of these genes, leading to diverse traits such as eye color. Understanding these differences is key to grasping genetic complexity. Created by Sal Khan.
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- How random can the patterns of ATCG be? Is there an infinite amount of randomness, or is there a way that they can be similar? Also can you take into account identical twins and how they are almost clones?(21 votes)
- The four "letters" A,T, C and G form three-letter "words" (called codons) - Sal's analogy. Continuing on with this analogy, the "words" form "sentences" depending on the way they are arranged, similar to how word order in an actual sentence is important. These "sentences" are proteins, that have a very specific 3D shape depending on the order of the codons used to make the protein (or words to make the sentence). The 3D shape of the protein determines its function. The collection of proteins in an organism determines the organism - different species have different characteristics, due to the different proteins they can produce.
Just to round the analogy off nicely, we can think of our entire genome as one big instruction manual. The genome is the manual, the proteins are the sentences within the manual, the codons/amino acids are the words within the sentences and the nucleotides (A, T, C and G) are the letters within the words.(14 votes)
- Can anyone explain me how do rRNA, tRNA and rRNA work? Thx before.(7 votes)
- messenger RNA (mRNA) is a large family of RNA molecules that convey genetic information from DNA to the ribosome, where they specify the amino acid sequence of the protein products of gene expression.
Transfer ribonucleic acid (tRNA) is a type of RNA molecule that helps decode a messenger RNA (mRNA) sequence into a protein. tRNAs function at specific sites in the ribosome during translation, which is a process that synthesizes a protein from an mRNA molecule.
Ribosomal ribonucleic acid (rRNA) is the RNA component of the ribosome, and is essential for protein synthesis in all living organisms. It constitutes the predominant material within the ribosome, which is approximately 60% rRNA and 40% protein by weight, or 3/5 of ribosome mass.(12 votes)
- If the genes have different alleles, can the chromosomes still be homologous?(7 votes)
- Yes. An "allele" of a gene can be as simple as a single nucleotide difference, or an insertion or deletion of hundred of bases. While the two genes are not necessarily perfect homologs of one another, there's still enough homology across the chromosome for synapsis (the pairing of homologous chromosomes) to occur.(12 votes)
- difference between gene and alelle?(5 votes)
- A gene is the factor that controls a trait, while an allege is a form of gene.(4 votes)
- If my mom has dark brown eyes and my dad has blue eyes, how did my brother end up with Blue Eyes and his older sister end up with a green eye and a hazel eye? lol(6 votes)
- Your mother must be heterozygous for eye color ("Ee") and your father recessive homozygous (recessive phenotypes must have recessive homozygous genotypes, "ee") The cross is Ee x ee
resulting in Ee, Ee, ee, ee where you had 50% prob of having blue eyes and 50% brown eyes when you were conceived, the same goes for your brother. Some traits have co-dominance (both recessive and domiant traits are present, such as with spotted fur on dogs or multi color eyes) or incomplete domiance (the recessive and domiant "blend" such as with a red flower and a white flower creating a pink flower). Your brother's sister must have be recessive green in one eye and have co-domiance with green and brown to have a hazel eye.My information may not be correct so I recommend doing some more studying/research on classical genetics!(9 votes)
- why do people not have black eyes?(4 votes)
- Black eyes do not exist actually. "Black" eyes are really just very dark brown eyes with lots of melanin in the three layers of the iris. The only other case of "black" eyes are those with aniridia (absence of an iris), as your pupil is just a black hole.(11 votes)
- Atsal was trying to say the plural or singular of hippo. What is the singular of hippo and what is the plural? ( I mean hippopotamus). 3:57(5 votes)
- One hippo, many hippopotami. Try look up the pronunciation it's a fun word.(6 votes)
- What does RNA stand for?(4 votes)
- how many amino acids may a single protein has(3 votes)
- Lactase has over 10,000, some have as low as 20(4 votes)
- If we can survive with only one set of chromosomes, why did we evolve to be diploid? Why do we have two sets when one seems like it's enough?(3 votes)
- For backup most likely. We all have redundant systems in the body (such as 2 kidneys) just in case one breaks down, so that's why I assume we have 2 sets of chromosomes(4 votes)
Video transcript
- What I hope to do in this video is hopefully give you some clarity on terms you might hear
used in a fairly related way and those are the terms Gene and Allele. Gene versus Allele. So let's do a little bit of review. Let's just reorient ourselves
in the world of DNA and RNA. Let's say that this,
this yellow squiggly line is a length of, I don't know, say my DNA, and let's say this little
light section right over here, that's if we were to zoom in, and we've represented
the various base pairs. And the sequence of base pairs is really the information content in DNA, and here I've just kind of
drawn it as a, as a ladder. We know that the real structure of DNA is a is kind of this twisted ladder, this double helix. Now if we talk about this
whole yellow squiggly line, and it could be even a section of a longer yellow squiggly line, this could code for multiple
for multiple things, especially multiple proteins. So different regions of this could code for different proteins. So for example, this
section right over here could be part of this region
that I'm highlighting in blue that codes for a specific protein, and so we would call this a Gene. We would call this a Gene. This might be a protein that is involved in, I don't know, I'll make something up. It's a protein that evolved, that's involved in the immune system. Maybe, maybe this stretch, let
me do it in different color. Maybe this stretch of DNA right over here, this stretch of DNA maybe
it's a longer stretch of DNA. Maybe it codes, it codes
for a protein that's used-- Maybe it's a protein that
helps regulate DNA replication. Maybe over here is another
we encode for another protein that maybe, maybe it in
some ways affects, affects the pigmentation of your skin, or the pigmentation of your eyes, and so you these stretches of DNA that code for specific things. And actually it doesn't have
to just be even for a protein. We are, we always talked about even if you do code for a protein you go from the DNA to messenger RNA, to messenger RNA and actually
go to pre-messenger RNA. That gets processed so you could actually lose some sections of it, but you go to messenger RNA
and then that messenger RNA, every three of these
base pairs is a Codon. Let me, so let's say that's one codon. One, two, three, that's another codon. One, two, three, each of those-- Maybe I'll draw them next to each other. Each of of them codes for an amino acid that is kind of connected
together to form, connected together to form a protein. So that's one amino acid right over there. This could be another amino
acid right over there. We can keep going on and on and on and on. You could have another
Amino Acid right over here, and then they all bond to
each other and they're brought actually to the mRNA from a, by a functional RNA group. And so there are functional things other than proteins that
this could code for. Like tRNA, tRNA which is really helping to transport the appropriate Amino Acids to the mRNA in the Ribosomes so that you can construct these proteins. So you can have tRNA and
we've seen this before in previous videos. It's this little squiggly line, matches up the the appropriate Codon, and then puts that Amino Acid in place. You also have things like Ribosomal RNA that make up the structure
of the actual Ribosomes. So RNA doesn't have to
only play this kind of in between messenger function. It actually can play a functional or a structural role. In fact there are theories
that the earliest life, the most primitive life was
nothing but self replicating RNA and then the systems became
more, and more, and more complicated and complex
until eventually you end up with things like redwood
trees and hippopotami. Hippopotamuses, hippopotami whatever. Elephants, but whatever else, but it all started with
potentially self replicating RNA. Some people say it might
be some type of proteins are able to replicate, who knows, but RNA is definitely, is definitely an interesting character in this. So you go from Gene to
RNA, that's transcription, and then RNA to protein, to protein that is translation but sometimes
you just stop at the RNA, and the RNA by itself plays a function. That's functional RNA. So each of these Genes they can code for a type of protein or
even a functional RNA. That's what a Gene is. Now what about an Allele? When the Allele is a specific
variation of the Gene. So for example, let's
say that you look at the at the same stretch of DNA. Let's say this is my DNA and if I were to take your DNA out and if were to look on the same chromosome at the same region. We're both human beings and we have for the most part very similar DNA. So this is-- Actually let me straighten it out. So, let's say this is my
DNA, a section of my DNA, and let's say this right over here, this in white is a section of your DNA, and so if we look at that
Gene, that blue Gene, that's that on my DNA. Now if we look at that
and this is the blue Gene, this is the blue Gene on your DNA. Now we're both human beings and most of our genetic material is fairly similar, but we might have variations
in how this Gene is coded. For example, you might
have or I might have a let's say, I have a
an Adenine right there, but right at that exact spot you might have a different base. You might have a, I don't know, you might have a, you might have-- Actually let me just-- You might have a Thymine right over there. So it's encoding for a
protein, or you know, functional RNA that's
playing the same role. Maybe it has a role in the immune system or role in your skin color or role in how your brain develops, but there's a variation. There's a variation in how it's coded. Now some of these
variations which could arise through mutations, it might
not have any impact in the function of the eventual
protein that gets constructed. You might just have a
different Amino Acid sometimes. In fact, you might not even
have a different Amino Acid because many times you
have two Codons coding for the same Amino Acid, but even in a case you might have one different Amino Acid in a protein that has 4,000 Amino
Acids it doesn't change how that protein acts or how it functions. Or sometimes it might. It might change how
that protein functions. It might change how that
protein regulates other things and whoever knows whatever else, and so you could imagine
that you have Genes. This Gene right over here. Maybe it has a role in eye color, and because of this variation
or because of other variations that show up in both cases they code for the protein that
say regulates eye color, or regulates the amount
of pigment you have, but because your variation right over here might lead or help lead-- And these things are very complex, it's very seldom do you
have a gene just for this, but this might make you-- especially if you have a Gene like this from both of your parents, maybe this one would go for blue eyes. Blue eyes, it somehow
helps produce blue eyes. While this, while mine somehow
helps produce brown eyes. And obviously I'd want to think about which variant of this Gene
that I get from my mother, and the variant of this Gene
that I get from my father. We all have two copies in
our regular somatic cells and our body cells. We have except for-- If we think about the, xx and the xy chromosomes, the sex determining chromosomes, on all the other chromosomes
we have two copies of the same Genes. We just have two-- It's just they're different variants. One variant from your mother and one variant from your father, or you could say that they
are different Alleles. So Alleles are just different variants. So these are two different Alleles. They code, they're the same Gene. They're the Gene that
somehow deals with eye color, but they're different
variations for that Gene. So the Gene you're speaking generally to that region of DNA. That region of the DNA strand that codes for some functional molecule, usually protein but it could be RNA. While the Allele is
that specific variation. That flavor of that Gene. Hopefully that helps.