How do I know which alleles the parents have?

I have the following assignment where I am to look at the "tree" (not sure the english word) and assign whether or not they can be autosomal dominant or recessive as well whether they can be X-linked recessive.

My way to try and complete this assignment is by drawing a punnet's square but I quickly run into some confusion. When I draw my square, I know that is I'm checking for autosomal dominant, then I would draw the mother as A'a, the A' denoting the sick dominant allele. What I'm not sure is this: Can't I also draw her as A'A'? That would also mean she would be autosomal dominant but with 2 sick alleles. What about the father? Do I draw him as Aa, aa or AA? I'm a bit confused here!

Let's go step my step here.

Going by your notation I'll call the square as Male and Circle as female. So from this information I can confidently say that you disease is not X linked dominant.


Because Females carry XX, that would mean to be X linked dominant she had to be X'X' for the disease allele to manifest it's phenotype. But if it were X'X, it would lead to a loss in fitness. And since the Male is normal and XY he does not carry the disease which tells us that the disease is definitely not X linked dominant.

Therefore in a cross of X'X' and XY you get individuals X'X and X'Y in the ratio 1:1

Coming to AD Autosomal Dominant may mean A'A' as the disease allele is present in homozygous condition. A heterozygous condition would lead to loss in fitness, not necessarily a disease.

A Father may be AA or AA'

so this cross would give you individuals

  1. AA' for A'A' x AA
  2. A'A A'A' for AA' x A'A'

so in this case you always get the disease in autosomal dominant, what remains is recessive.

AA or AA' Father x A'A' Mother

Products AA' for AA x A'A' or AA' and A'A' for AA' x A'A'

which tells us that the disease is actually Autosomal recessive on the mother.

The father's genotype for the allele is AA or homozygous dominant

and the disease is autosomal recessive for the disease in the mother.

How do I know which alleles the parents have? - Biology

For example, let's say that in the pea plant the height and flower-color genes are on the same chromosome. and that red (R) is dominant to white (r), and tall (T) is dominant to short (t). A tall red plant that is that is the F1 hybrid of a cross between a "pure" tall red line (plants RR, TT) and a short white line (plants rr, tt) is heterozygous for flower-color (Rr) and for height (Tt) and has has the following two chromosomal arrangements of these genes: If no recombination occurs between these two genetic loci, the plant will pass on either the combination RT or the combination rt to an offspring. These non-recombinant types are sometimes know as parental types, since the combination one of the ones received by the plant from its parents.
If recombination occurs, then in the gamete copied to the next generation, the arrangement on the transmitted chromosome would be: A recombination occurs if there are an odd number of crossover events (usually just one) between the two loci. An even number of crossovers (usually 0, maybe 2) would return these genes to their original chromosomal arrangement.

Recombination between two genes on a chromosome happens in meiosis with some probability that is known as the recombination frequency. If the genes are close together on the chromosome, the recombination frequency is very small. If the genes are far apart on a chromosome, or on different chromosomes, the recombination frequency is 50%. In this case, inheritance of alleles at the two loci are independent. If the recombination frequency is less than 50% we say the two loci are linked. Under most models of meiosis, recombination frequencies cannot be larger than 50%.

You can tell if the genes are linked by looking at the offspring. For example, let's say that we breed our above parent with genotype RT/rt to a parent who is rt/rt. If the offspring are white and short, you know the first parent contributed rt. If they are tall and red, you know the first parent contributed RT. If they are red and short, you know the first parent contributed Rt. If they are white and tall, you know the first parent contributed rT.

Suppose, these two plants have 100 offspring, and that 5 are red and short, 10 are white and tall, 40 are red and tall, and 45 are white and short. The red short and white tall plants exist because the first parent's chromosomes underwent recombination. Remember the original forms were RT and rt. 15 of the offspring have phenotypes that could have only resulted from recombination.

Next, you calculate the recombination frequency. This is the proportion of offspring that have genotypes, assuming you can determine them, which exist because of recombination. In this example, the recombination frequency in the sample of 100 offspring is 15/100 = 0.15.

What is a Pedigree?

While a pedigree may resemble a simple family tree, it contains more information. Specifically, a pedigree allows you to track how a particular genetic trait has been passed down through several family generations.

Scientists use pedigrees to study how certain genetic traits are inherited, and to predict how a trait may be passed on to future generations.

This pedigree traces the inheritance of hemophilia in the royal family of Queen Victoria

The really cool thing about a pedigree is that it is a tool that allows you to use an individual’s phenotype—the outward expression of a trait, to determine that individual’s genotype—what genes they possess.

What do I mean by that? Let me explain.

You already know that, for the most part, your observable traits are controlled by your genes. Often, more than one allele exists for a given gene. When more than one allele for a gene exists, one allele will be dominant over other alleles.

This is the case with eye color*.

What color eyes do you have? Brown? Blue? Green? Hazel? Your eye color is your phenotype—the outward expression of the genes you possess for eye color. The actual genes you possess is known as your genotype.

The allele for brown eyes (which we will designate B) is dominant over the allele for blue eyes (which we will designate b). Since we have two copies of every gene (one copy inherited from each parent during conception), we have two possible alleles for eye color.

A person with two copies of the brown eye allele (BB) will have brown eyes, while a person with two copies of the blue eye allele (bb) will have blue eyes. What color eyes will a person with one brown eye allele and one blue eye allele have (Bb)? They will have brown eyes. Why? Because having just a single copy of the dominant brown eye allele will mask expression from the recessive blue eye allele.

Our genotype (what gene alleles we carry) controls our phenotype (our outward expression of a trait)

In this example, there are two different eye color phenotypes: blue and brown. But there are three possible genotypes (BB, Bb, bb).

While it is possible with commercial DNA testing kits to determine your genotype, you may be able to determine what genotype you and your family members have based on your phenotypes using a pedigree.

What is hereditary alpha tryptasemia syndrome?

In addition to having higher blood tryptase levels, individuals with more alpha tryptase copies also report more shared symptoms. These symptoms can be associated with multiple organ systems and may be hard to explain. These symptoms may include allergic-like symptoms such as skin itching, flushing, hives, and even anaphylaxis gastrointestinal (GI) symptoms such as bloating, abdominal pain, diarrhea and/or constipation (frequently diagnosed as irritable bowel syndrome or IBS), heartburn, reflux, and difficulty swallowing connective tissue symptoms such as hypermobile joints and scoliosis cardiac symptoms such as a racing or pounding heartbeat or blood pressure swings sometimes with fainting as well as anxiety, depression, chronic pain, panic attacks, and others. These patients may find that others in their family have similar or related symptoms, as this is a genetic syndrome. Others may have few if any symptoms—and would be said only to have the trait and not the syndrome associated with the trait. In cases such as these, a person may only find out because a relative was more severely affected with the syndrome.


Zygosity refers to the grade of similarity between the alleles that determine one specific trait in an organism. In its simplest form, a pair of alleles can be either homozygous or heterozygous. Homozygosity, with homo relating to same while zygous pertains to a zygote, is seen when a combination of either two dominant or two recessive alleles code for the same trait. Recessive are always lowercase letters. For example, using 'A' as the representative character for each allele, a homozygous dominant pair's genotype would be depicted as 'AA', while homozygous recessive is shown as 'aa'. Heterozygosity, with hetero associated with different, can only be 'Aa' (the capital letter is always presented first by convention). The phenotype of a homozygous dominant pair is 'A', or dominant, while the opposite is true for homozygous recessive. Heterozygous pairs always have a dominant phenotype. [10] To a lesser degree, hemizygosity [11] and nullizygosity [12] can also be seen in gene pairs.

"Mono-" means "one" this cross indicates that the examination of a single trait. This could mean (for example) eye color. Each genetic locus is always represented by two letters. So in the case of eye color, say "B = Brown eyes" and "b = green eyes". In this example, both parents have the genotype Bb. For the example of eye color, this would mean they both have brown eyes. They can produce gametes that contain either the B or the b allele. (It is conventional in genetics to use capital letters to indicate dominant alleles and lower-case letters to indicate recessive alleles.) The probability of an individual offspring's having the genotype BB is 25%, Bb is 50%, and bb is 25%. The ratio of the phenotypes is 3:1, typical for a monohybrid cross. When assessing phenotype from this, "3" of the offspring have "Brown" eyes and only one offspring has "green" eyes. (3 are "B?" and 1 is "bb")

The way in which the B and b alleles interact with each other to affect the appearance of the offspring depends on how the gene products (proteins) interact (see Mendelian inheritance). This can include lethal effects and epistasis (where one allele masks another, regardless of dominant or recessive status).

More complicated crosses can be made by looking at two or more genes. The Punnett square works, however, only if the genes are independent of each other, which means that having a particular allele of gene "A" does not alter the probability of possessing an allele of gene "B". This is equivalent to stating that the genes are not linked, so that the two genes do not tend to sort together during meiosis.

The following example illustrates a dihybrid cross between two double-heterozygote pea plants. R represents the dominant allele for shape (round), while r represents the recessive allele (wrinkled). A represents the dominant allele for color (yellow), while a represents the recessive allele (green). If each plant has the genotype RrAa, and since the alleles for shape and color genes are independent, then they can produce four types of gametes with all possible combinations: RA, Ra, rA, and ra.

RA Ra rA ra
Ra RRAa RRaa RrAa Rraa
rA RrAA RrAa rrAA rrAa
ra RrAa Rraa rrAa rraa

Since dominant traits mask recessive traits (assuming no epistasis), there are nine combinations that have the phenotype round yellow, three that are round green, three that are wrinkled yellow, and one that is wrinkled green. The ratio 9:3:3:1 is the expected outcome when crossing two double-heterozygous parents with unlinked genes. Any other ratio indicates that something else has occurred (such as lethal alleles, epistasis, linked genes. etc.).

Forked-line method Edit

The forked-line method (also known as the tree method and the branching system) can also solve dihybrid and multi-hybrid crosses. A problem is converted to a series of monohybrid crosses, and the results are combined in a tree. However, a tree produces the same result as a Punnett square in less time and with more clarity. The example below assesses another double-heterozygote cross using RrYy x RrYy. As stated above, the phenotypic ratio is expected to be 9:3:3:1 if crossing unlinked genes from two double-heterozygotes. The genotypic ratio was obtained in the diagram below, this diagram will have more branches than if only analyzing for phenotypic ratio.

How is The Hitchhiker's Thumb Trait Determined

Briefly explain in complete sentences how the Hitchhiker's thumb trait is determined using the following words: allele, dominant, recessive, homozygous, and heterozygous.

You may use this website:

Hitchhiker's thumb trait is a autosomal recessive trait. This means that in order for a person to have this trait they must carry both allele's ( for example a) of the trait, which is in essence means that the person must be homozygous recessive (for example aa) in order to have the trait. If they are heterozygous (for example: Aa) for the trait then they are only a carrier of the trait and do not have it.

If you need to find out if someone has the trait or not given the allele's of the couple then use a punnett square to figure out the chances of the person having the trait. Although if one is homozygous dominant (don't have the trait so AA) and the other is homozygous recessive (do have the trait so aa) then the child will only be a carrier of the trait.

Family studies

Reiss (1999) summarized data from 18 studies of parents and offspring, with the following totals across all studies:

Reiss (1999) concluded that there may be some genetic basis for this character (because LxL matings produce more L offspring than do RxR matings), but it is not a simple one-gene, two-allele genetic character. If the myth were true, two R parents could not have an L child, but almost a third of the children of RxR matings are L. In the first study on this character, Lutz (1908) reached the same conclusion based on the same kind of data, and it is not clear what the purpose was of the 17 family studies done in the subsequent 90 years.

How do I know which alleles the parents have? - Biology


Determining the phenotype of an organism is fairly straight forward . you look at it (since phenotype means physical characteristics).
Determining genotype isn't as cut & dry because you can't see an organism's genes by just looking at it.

Before we dive into the TEST CROSS, let's review our three possible genotypes & the phenotypes they create. For our example let's use guinea pig fur color black being dominant (B) & white being recessive (b).

Homozygous Dominant
(Pure Dominant)
BB Black
Bb Black
Homozygous Recessive
(Pure Recessive)
bb White

So remember,

If I handed you a black guinea pig & asked,"What's its phenotype for fur color?" You would gently hold the guinea pig, look at it and reply, "Black, you dummy . all you gotta do is look at it". And I would say, "Correct, & please don't call me dummy".

If I handed you the same guinea pig & asked, "What's the genotype of this guinea pig with respect to its fur color?" You wouldn't be able to tell me, and I wouldn't be able to tell you either. The reason we don't know is because there are two genotypes that BOTH produce a dominant trait phenotype, homozygous dominant (BB) & heterozygous (Bb), & we can't see the actual alleles (letters) without serious scientific chromosomal-type analysis --- and that's assuming that a Guinea Pig Genome Project has been completed for us to refer to, & I don't think it has.

Now, I must tell you that in your life, as an exceptional biology student, you will most likely NEVER actually PERFORM a test cross. What you need to understand are the possible results of a test cross & what they mean. Our black guinea pig is either BB or Bb, which one is it?
To perform an actual test cross with this guinea pig, we would need a guinea pig (of the opposite sex) that is homozygous recessive ("bb"). In other words, we would need a white guinea pig to mate with our black guinea pig. We would give them a little privacy, hope that the female becomes pregnant, wait for however long the gestation period of a guinea pig is, & THEN we would look at the offspring.

In our scenario, if we see any white baby guinea piglets, our black parent pig is "Bb". If all the baby piglets are black, the black parent is "BB".

I should mention that the reliability of a test cross increases with the number of offspring produced. So ideally (in our example) we would want a large litter of guinea pigs to look at. If a small litter is produced (like only 4 or 5 or 6), we would probably have those same parent guinea pigs "do it" again & make more offspring so that our conclusion is more reliable.

That's the bottom-line on a test cross, that stuff in the table. But you don't want to settle for just the bottom line. You want to understand the concept in a little more detail, don't you? Yes, believe me, you do.

In our guinea pig example, our mystery black pig is either BB or Bb. Allow me to use "B?" for the mystery genotype. Thanks.
The white guinea pig is "bb" because white is the recessive trait & the only way a recessive trait appears is if the genotype is homozygous recessive. Let's put this info in a series of Punnett Squares.

For the offspring in that bottom row, their phenotype depends on what that second ("?") allele is in our black guinea pig parent.

As you can see, if our mystery genotype pig has a "B" in the "?" spot, all of the offspring from the test cross will be heterozygous (Bb) & have the dominant phenotype --- black fur.

There is NO WAY white guinea pigs can be produced because to be white the offspring need to inherit one little "b" from each parent, and in this scenario the black parent doesn't have ANY little "b's" to pass on.

On the other hand, if the mystery allele "?" = "b", then we can predict that half (2 of 4 boxes) of the offspring from the test cross are going to have the recessive phenotype i.e. have white fur.

The only way to get white guinea pig piglets is if both parent guinea pigs have at least one "b". We KNOW the white parent has got 'em (the only way to be white is to be "bb"), and we KNOW the black parent has one big "B" (has to in order to be black), so if we get white offspring, that second allele in the black parent is "b".

Our test cross = mystery black genotype x recessive white genotype = B__ x bb

Any white offspring must be "bb".
One "b" came from the white parent, the other must be in that "blank", making the mystery genotype "Bb".


1. In African-violet plants, purple flowers are dominant to white flowers. You purchase an African-violet plant with white flowers. It's genotype could be represented as :

Like I stated ealier, it is highly unlikely that you will be asked to actually perform a test cross --- like really mate two organisms & then analyze their offspring. If you do, consider yourself lucky & more power to ya. Perhaps you'll be fortunate enough to run some computer simulations or something in lab. Anyway, the concepts behind a TEST CROSS are important. So study-up, do your best.

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1. In African-violet plants, purple flowers are dominant to white flowers. You purchase an African-violet plant with white flowers. It's genotype could be represented as :

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Why do children from the same parents have different heights, different eye colors, body builds, personalities, etc.?

-A curious adult from California

Children from the same parents do not always look or act alike. In fact, it can sometimes be hard to tell which children are siblings just by looking at them.

This is what makes each one of us so unique. Could you imagine if every child from the same parents looked and acted identically? We would not have nearly as much diversity in our world. Also, you can imagine that families would not be nearly as interesting.

The answer to why this happens has to do with our genes and how they are passed on. Each gene has the instructions for one small part of you. You are who you are because of the particular set of 25,000 genes you got from your parents and the environment you developed and grew up in.

So part of the explanation is easy. you and your siblings grew up in different environments so you are bound to be different. But you also each inherited a completely different set of genes from each parent. This means you are a completely new, never before seen genetic combination.

The next question is how can you each get a completely different set of genes from the same set of parents? The answer has to do with the fact that each parent actually has two different sets of genes. And that each parent passes only half of their genes to their child. And that the half that gets passed down is random.

All of this together ensures that each child ends up with a different, unique set of genes. I am going to spend the rest of our time trying to explain how this works with one gene--MC1R.

MC1R is a good gene to look at because, as you'll see, it is genetically very simple. It determines whether or not someone will have red hair, pale skin, and/or freckles.

Then I'll expand the answer to include all of our other genes. In the end, you'll see that parents can make an almost infinite variety of possible kids. Which is why siblings can be so different.

Genes can Come in Different Versions

Everyone has two copies of most of their genes. We get one copy from our mom and one copy from our dad.

So everyone has two copies of the MC1R gene. Of course having two copies of a gene wouldn't matter if each copy was the same. But they aren't. Each gene can come in different versions (scientists call these different versions alleles).

MC1R comes in two different versions, red and not-red. Since we have two copies of MC1R, everyone has one of three possible combinations. They can have two reds, a red and a not-red or two not-reds.

People with two copies of the red version of MC1R have red hair, pale skin and freckles. People with one version of each often have pale skin and freckles and people with two not-red versions can have any shade of hair or skin color (depending on their other genes).

So people are not different because they have different genes. As humans, we all share the same genes. What makes us different is that we have different versions of the same genes.

Which Gene Copy you Get is Random

A crucial piece to the puzzle in figuring out how siblings can look so different when they come from the same parents has to do with how genes are passed down. Which of your parents' two gene copies you end up with is chosen at random.

If your parent has two different copies of a gene, you have an equal chance of getting either one. And the same is true for each of your siblings.

It's like flipping a coin. You might get heads but your brother or sister might get tails.

Let's put this all together to see how it works. First we'll use the MC1R gene. Then we'll expand to the other 25,000 or so genes.

Imagine two parents that have one red and one not-red copy of the MC1R gene. They can have any hair color and probably have pale skin and freckles. Let's use an example of a family tree to see what their kids might look like:

As you can see, because of this one gene, the parents can have three types of kids:

  1. If both parents pass a red version down, then that child will have red hair, pale skin and freckles
  2. If one parent passes a red and the other a not-red, then that child will not have red hair and, most likely, will have pale skin and freckles
  3. If both parents pass a not-red version, then that child will have a variety of other hair and skin colors

Because of the other thousands of genes with their different gene versions, there is a huge variety within each group too. So the red haired group can be tall, short, happy, grumpy, have blue, green, brown, hazel, etc., eyes, and so on. Same thing with the other groups as well.

It's like having two big bags of 50,000 different colored marbles. Imagine pulling 25,000 out of each bag, combining them and then recording the result. When you're done, you then return the marbles back to their original bags and then take another 25,000 out of each. Odds are you'll get two very different sets. Just like you'd get two very different kids.

Genes are Not the Whole Story

Genes are not the only reason siblings are different. If you know any identical twins, you'll know they are definitely different people even though they share the exact same genes.

So why do even identical twins, with the same DNA, have different personalities? Or body builds? The last piece of the puzzle comes from our environment.

For many of our characteristics, such as body build and personality, our genes and our environment work together to make us who we are. A person's environment can include things like exposures to chemicals, exercise habits and eating habits. For our personality, it is also important to remember that things like a child's social and family environment and relationships play an important role.

So if children from the same family live in the same household, does that mean that they are exposed to the same environment? No! Children will have different relationships, might be in different schools, and might have different habits for things like eating and exercise.

Given all of the possibilities for different genes and an individual's environment, it is no surprise that even children from the same parents are very unique. In fact, your unique genetic combination was never seen before you and will never be seen again.

How to Solve a Punnet Square

1. Determine the genotypes (letters) of the parents. Bb x Bb
2. Set up the punnet square with one parent on each side.
3. Fill out the Punnet square middle
4. Analyze the number of offspring of each type.

In pea plants, round seeds are dominant to wrinkled. The genotypes and phenotypes are:

If you get stuck make a "key". Sometimes the problems won't give you obvious information.

Example: In radishes, a bent root is a dominant trait, though some roots are straight (which is recessive). If a straight rooted plant is crossed with a heterozyous bent root plant, how many of the offspring will have straight roots?

Say what? First of all, this problem doesn't make it easy. Start by assigning genotypes and phenotypes. It doesn't matter what letter you pick, but it may be easiest to pick a letter that represents the dominant trait. In this case, use the letter B for bent.

BB = bent
Bb = bent
bb = straight

Now use the key to figure out your parents. In this case you have a straight root plant (bb) crossed with a heterozyous bent plant (Bb). Once you've figured that out, the cross is simple!

If a heteroyzous round seed is crossed with itself (Rr x Rr) a punnett square can help you figure out the ratios of the offspring.