Question

what alleles does a person have to have to receive a recessive trait?

Answer 1

In order to have a recessive trait, a person would have to inherit the recessive alleles for that trait, from both parents.

I can show you, by using an example.

Let's talk about "cystic fibrosis."

(It's a disease where mucus builds up in the lungs and digestive tracts. It is life threatening.)

Cystic fibrosis is a recessive disease.
A recessive trait.

In order to have cystic fibrosis,
a child has to receive the gene for cystic fibrosis, from both parents.

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Here, let's make a Punnett Square.

By doing this, I can show you how recessive traits work.

DOMINANT alleles are CAPTIALIZED.

RECESSIVE alleles are LOWERCASE.

Since cystic fibrosis is RECESSIVE,
we can represent it with a LOWERCASE allele.

Let's use a lowercase c for a cystic fibrosis gene.

Let's use a Captial C, for a normal gene.

Now, there are two parents.
and each parent has to have a genotype, made up of two letters.

Take a look at this Punnett Square:



At the TOP of the Punnett Square, we have one of the parents,
with the genotype (C c).

At the LEFT SIDE of the Punnett Square, we have the other parent,
with the genotype (C c).

Take a look at that genotype: (C c)

If you have the genotype (C c),
then that means that you don't have cystic fibrosis.

But you CARRY THE GENE for cystic fibrosis,
because there is still a lowercase c in there.

In order to ACTUALLY HAVE cystic fibrosis,
your genotype would have to be both lowercase c's. Like (c c).

It works the same way, for all recessive traits.

In order to have a recessive trait,
BOTH letters in the genotype must be LOWERCASE.

Remember,
LOWERCASE alleles are RECESSIVE.
CAPITALIZED alleles are DOMINANT.

Take another look at the table.

There are 4 squares in all, you see?

Each of those 4 squares is the possible genotype of the child.

FOUR squares in all.
And ONE of those four squares has the genotype (c c).

That means that if both parents have the genotype (C c),
then there's a 25% chance that the child will have cystic fibrosis,
with the genotype (c c).