Why do more men than women have colorblindness?

Name: Problem 12, Chapter 9: Campbell Biology Concepts & Connections
Uploaded: 2020-04-16T03:50:28-08:00
Duration: 6 min
Channel: Dennis Howard
Description: Why do more men than women have colorblindness?

Key Concepts

X-linked Inheritance

Colorblindness is a prime example of an X-linked trait, meaning that the gene responsible for the condition is located on the X chromosome. Men, having one X and one Y chromosome, manifest the condition with a single mutated copy, whereas women, with two X chromosomes, would require mutations in both copies to become colorblind.

Recessive Alleles

The genetic mutation that leads to colorblindness is recessive, which means that an individual needs two copies of the defective gene to express the condition fully. Since men have only one X chromosome, they do not have a second, potentially normal gene that could compensate, making them more likely to be colorblind.

Hemizygosity in Males

Men are hemizygous for genes on the X chromosome, meaning they have only one copy. This lack of a second allele for these genes results in a higher incidence of X-linked conditions like colorblindness in males, as there is no backup copy to mask the effect of a defective gene.

Sexual Dimorphism in Genetic Inheritance

The differences between male and female chromosome composition contribute to the differing prevalence of X-linked disorders. Females typically have a lower incidence of colorblindness because they possess two X chromosomes, which provides a buffer against a single defective allele, whereas males only have one X chromosome.

Transcript

00:01 Problem number 12 poses a great question.

00:02 Why do more men than women have colorblindness? well, the answer lies in the genetic makeup behind colorblindness, right? so we usually depict the colorblind allele as an x with a superscript of a lowercase c.

00:19 And that is for color blindness.

00:25 And then for the normal or just normal vision, we would have an x with a superscript of a capital see and that is normal vision.

00:37 So having this x right here, what does that tell us? well, that tells us that it is a sex -linked, sex -linked trait, and it's carried on that x -chrome zone.

00:51 Okay? so using that knowledge, also we would want to know just to help us out here that color blindness is recessive, whereas the normal vision, that will be better right there, normal vision is going to be dominant.

01:14 Okay, so tell me if we have this combination of alleles, what is that? is that a normal vision or is that a person with colorblindness? well, we would know that since this is dominant, right, the normal vision is dominant, and its x superscript capital c is dominant over this recessive colorblindness allele, we'd know that this x right here would dominate over the second recessive allele.

01:47 So they would simply have normal vision, right? so now let's try again.

01:57 Let's make it x superscript, lowercase c, x superscript lowercase c.

02:02 Would this person have normal vision? or would they be colorblind? well, once again looking over here, we see that having recessive traits to recessive alleles would mean that the recessive phenotype is expressed.

02:18 So we see that this person would be colorblind.

02:21 Now is this person a male or a female? well, we see that there are two x chromosomes.

02:25 There are no y chromosomes.

02:27 So that would tell us that this is a female.

02:29 So this is a colorblind female.

02:30 Okay? now let's look at a colorblind male.

02:33 What would that entail? so so we know that a male is xy.

02:37 Okay, so for them to be colorblind, which of these two alleles would they have? well, that's right.

02:45 It's going to be this allel right here.

02:47 So we're going to draw in a little lowercase c.

02:50 So this would be a colorblind male...