AssessmentsIndependent assortment of genes and linkage
Independent assortment of genes and linkage
USMLE® Step 1 style questions USMLE
A group of geneticists are studying characteristics of a novel frog species. They find that within this species, there is a tendency for progeny to inherit two specific traits together which are red eyes and yellow-spotted backs. Which of the following best describes this phenomenon?
Content Reviewers:Rishi Desai, MD, MPH
Inheritance is possible because of chromosomes.
These chromosomes come in pairs - one from mom and one from dad - so they’re called homologous chromosomes.
Each chromosome has genes, which are segments of DNA that carry genetic information for a specific trait.
And different versions of the same gene are called alleles.
As an example, brown eye color and blue eye color are both alleles for the eye color gene.
And each parent offers one allele of a gene.
Now, these alleles can be either dominant often represented with a capital letter, or recessive, represented with the corresponding lowercase letter, the difference being that it only takes one dominant allele for its traits to be expressed, whereas it takes two recessive alleles for its traits to be expressed.
Human somatic cells - that is, all of the cells aside from the sperm and eggs, which are called gametes - have 23 pairs of chromosomes; 22 somatic pairs and one sexual pair - adding up to 46 chromosomes in total.
These chromosomes, along with the alleles they carry, segregate during meiosis - which is the process of making new gametes.
Gametes only carry half the genetic information of the parent - so 23 chromosomes.
Once the male and female gametes merge during fertilization, their alleles combine to make the genotype —or genetic information— of the new organism.
For every gene, alleles can combine to give rise to three possible genotypes, homozygous dominant - or AA, heterozygous - or Aa - and homozygous recessive - or aa.
Now, independent assortment means that no matter which alleles an organism inherits for one gene that codes for a trait like eye color, it won’t affect the alleles it inherits for another gene that codes for a different trait, like hair color.
Let’s start with a simple example. Let’s represent the eye color gene with the letter “a” and the hair color gene with the letter “d”.
Now, the dominant allele for eye color - A - stands for brown eyes and the recessive allele - a - stands for blue eyes.
On the other side, the dominant allele for hair color - D - stands for dark hair while the recessive allele - d - stands for blond hair.
So let’s say we have a person with a heterozygous genotype —Aa— for eye color and heterozygous genotype —Dd— for hair color.
This person would have the dominant allele features - so brown eyes and dark hair, even though they still carry a recessive allele for blond hair and blue eyes.
Now, at the molecular level we know that the eye color gene is physically located on a pair of homologous chromosomes, and in this case, let’s say that the chromosome from mom carries the dominant allele —A—, and the chromosome from dad carries the recessive allele —a— .
Similarly, let’s say that the hair color gene is actually physically located on another pair of homologous chromosomes.
And let’s say that the chromosome from mom carries the dominant allele —D— , and the chromosome from dad carries the recessive allele —d—.
Now in meiosis, different pairs of homologous chromosomes independently separate into different gametes.
In other words, how one pair of homologous chromosomes splits into daughter cells does not affect how another pair of homologous chromosomes decides to split into those same daughter cells.
As a result, a person that has a heterozygous genotype for both hair color and eye color can produce four different types of gametes: One that carries the two chromosomes from the mother, and thus the dominant alleles A and D.
One that carries the two chromosomes from the father, and thus the recessive alleles a and d.
One that carries the first chromosome from the mother and the second from the father, and thus the alleles dominant A and recessive d.
And one that carries the first chromosome from the father and the second from the mother, and thus the recessive alleles a and dominant D.
So far so good, but let’s bring two more genes along! One that determines the skin color, let’s represent it with the letter “b”.
So the dominant allele for skin color - B - stands for dark skin and the recessive allele - b - stands for white skin.
And another one that determines the type of earwax someone has let’s represent it with the letter “c”.
So the dominant allele - C - stands for wet earwax, while the recessive allele - c - stands for dry earwax.
Genes are randomly assorted during the production of sperm and eggs. This is called independent assortment. This process is responsible for the different combination of genes that children receive from their parents.
If two genes are located close together on a chromosome, they are said to be linked. If these genes are passed on to a child, they will tend to be passed on together.