Understanding Codominance & Sex-Linked Characteristics (Extended)

 

What Are Codominance and Sex-Linked Characteristics?

Codominance

Codominance occurs when both alleles of a gene are expressed equally in the phenotype of a heterozygous organism.

• Example: In cows, if a white-coated cow $(W)$ and a red-coated cow $(R)$ breed, their offspring may have both red and white patches (roan).

 

Codominance in Depth

ABO Blood Groups

• There are three alleles of the blood group gene that control it.
• The $I^A$ and $I^B$ alleles are codominant, while $I^O$ is recessive.
• In case you were confused the I represents the gene and the A, B and O represent the alleles
• Blood type phenotypes depend on allele combinations:
  • $I^A I^A$ or $I^A I^O$ = Blood type A.
  • $I^B I^B$ or $I^B I^O$ = Blood type B.
  • $I^A I^B$ = Blood type AB (both A and B are equally expressed).
  • $I^O I^O$ = Blood type O.

Punnett Square Example:

Cross between $I^A I^O$ (Blood Type A) and  $I^B I^O$ (Blood type B)

 

	$I^A$	$I^O$
$I^B$	$I^A I^B$ (AB)	$I^B I^O$ (B)
$I^O$	$I^A I^O$ (A)	$I^O I^O$ (O)

• Phenotypes:
  • $25\%$ AB, $25\%$ A, $25\%$ B, $25\%$ O.

 

Sex-Linked Characteristics

Some traits are determined by genes on the sex chromosomes (X or Y).

• Linked alleles are alleles that are on the same chromosome
• Due to the structure of the male and female chromsome we can say some characteristics (alleles) are sex linked.
• Key Idea: Males (XY) have only one X chromosome, so any gene on it shows in the phenotype. 
• Due to this sex linked conditions are more likely so show in males compared to females
• Example: Red-green color blindness is more common in males because the gene responsible is on the X chromosome.
• Females (XX) having both X chromosomes are more likely to have a dominate allele which can "hide" the recessive allele

 

Key Terms

Term	Definition
Codominance	A situation where both alleles contribute equally to the phenotype.
Sex-Linked Characteristic	A trait controlled by a gene located on the sex chromosomes.
X-Linked Gene	A gene located on the X chromosome.
Y-Linked Gene	A gene located on the Y chromosome (rare).
Carrier	A female who has one normal allele and one affected allele for an X-linked gene (does not show the trait).

 

 

Sex-Linked Inheritance in Depth

Red-Green Color Blindness

• Caused by a recessive allele on the X chromosome.
• Represented as:
  • $\text{X}^N$: Normal vision
  • $\text{X}^n$: Color blindness
• Male Genotypes: $\text{X}^N\text{Y}$ (Normal), $\text{X}^n\text{Y}$ (Color Blind)
• Female Genotypes:
  • $\text{X}^N\text{X}^N$: Normal
  • $\text{X}^N\text{X}^n$: Carrier
  • $\text{X}^n\text{X}^n$: Color Blind

 

Punnett Square Example:

Carrier female $(\text{X}^N\text{X}^n)$ and normal male $(\text{X}^N\text{Y})$.

	$\text{X}^N$	$\text{X}^n$
$\text{X}^N$	$\text{X}^N\text{X}^N$ (Normal Female)	$\text{X}^N\text{X}^n$ (Carrier Female)
$\text{Y}$	$\text{X}^N\text{Y}$ (Normal Male)	$\text{X}^n\text{Y}$ (Color Blind Male)

• Phenotypes:
  • $50\%$ females (1 normal, 1 carrier).
  • $50\%$ males (1 normal, 1 color blind).

 

 

Steps to Solve Codominance and Sex-Linked Problems

1. Understand the alleles:

   • Identify dominant, recessive, and codominant alleles.
   • Determine if the gene is on an autosome or a sex chromosome.

2. Write parent genotypes:

   • Include $\text{X}$ and $\text{Y}$ for sex-linked traits.

3. Set up a Punnett square:

   • Label rows and columns with alleles from each parent.

4. Fill in the Punnett square:

   • Combine alleles to determine offspring genotypes.

5. Analyze the results:

   • Count the phenotypes and genotypes.