Lesson 8.2: Codominance, Multiple Alleles and Sex Linkage

Introduction

Welcome to Lesson 8.2 of Foundation Biology! In this lesson, we'll be diving into the fascinating world of genetics, focusing on codominance, multiple alleles, and sex-linked inheritance. By the end of this lesson, you should be able to:

Understand the concepts of codominance and incomplete dominance.
Explain how multiple alleles influence traits, using ABO blood groups as a prime example.
Understand the principles of sex determination and sex-linked inheritance, including conditions like color blindness and hemophilia.
Interpret pedigree diagrams to track inheritance patterns across generations.

Hook: Real-World Genetics

Think about blood types. Have you ever wondered why some people have type A blood, others have type B, and some have AB or O? This variation is a result of genetic principles we'll discuss today! 🧬

Understanding Codominance and Incomplete Dominance

Codominance

In codominance, both alleles in a gene pair are fully expressed in the phenotype of the organism. This means that neither allele is dominant or recessive; they work together to create a unique trait.

Example:

Consider the case of flower color in certain plants, such as roan cattle. If one parent has red flowers (denoted as $R$) and the other has white flowers (denoted as $W$), the offspring will have a mix of both red and white flowers. We can represent this using the notation:

Parental Generation: $RR$ (red) x $WW$ (white)
F1 Generation: All $RW$ (roan, showing both colors)

Incomplete Dominance

Unlike codominance, incomplete dominance occurs when the dominant allele does not completely mask the recessive allele, resulting in a blend of the two phenotypes.

Example:

Using the same flower analogy, if red flowers ($R$) and white flowers ($W$) combine under incomplete dominance, the offspring would be pink ($RW$):

Parental Generation: $RR$ (red) x $WW$ (white)
F1 Generation: All $RW$ (pink)

Summary of Differences

Codominance: Both traits visible (e.g., roan cattle)
Incomplete Dominance: Blended trait (e.g., pink flowers)

Exploring Multiple Alleles

ABO Blood Groups

The ABO blood group system is a classic example of multiple alleles. In this system, there are three alleles: $I^A$, $I^B$, and $i$.

Individuals can have either two of the same alleles (homozygous) or one of each (heterozygous).
The possible blood types are:
Type A: $I^A I^A$ or $I^A i$
Type B: $I^B I^B$ or $I^B i$
Type AB: $I^A I^B$
Type O: $ii$

Example of Inheritance:

If a parent has type $A$ blood ($I^A i$) and the other has type $B$ ($I^B i$), the potential offspring blood types include:

$I^A I^B$ (type AB)
$I^A i$ (type A)
$I^B i$ (type B)
$ii$ (type O)

Impact of Multiple Alleles

This diversity in blood types can affect transfusions, paternity tests, and provides a great example of how genetic variation arises in populations!

Sex Determination and Sex-Linked Inheritance

Sex Determination

Sex in humans is determined by the chromosome pair XX (female) or XY (male). The Y chromosome carries the SRY gene, leading to male development.

Sex-Linked Traits

Some traits are linked to the sex chromosomes. For instance:

Color Blindness: A recessive trait carried on the X chromosome. Males (XY) are more frequently affected because they have only one X chromosome.
Hemophilia: Another recessive trait on the X chromosome, leading to issues with blood clotting, primarily affecting males.

Example of Color Blindness

Let’s say $ X^C $ is the normal vision allele and $ X^c $ is the color blindness allele. A color-blind male would have the genotype $ X^cY $ and a carrier female $ X^C X^c $. The potential offspring could have the following genotypes:

Sons: $ X^cY $ (color blind) or $ X^CY $ (normal vision)
Daughters: $ X^C X^c $ (carrier) or $ X^C X^C $ (normal vision)

Interpreting Pedigree Diagrams

A pedigree diagram is a family tree that shows the inheritance of traits over generations. It uses symbols to depict males (squares) and females (circles), with shaded shapes indicating individuals exhibiting the trait.

Example: In a pedigree for color blindness, if we see that the trait skips generations, it indicates that the trait is likely recessive.

Conclusion

In this lesson, we explored:

The key concepts of codominance and incomplete dominance.
How multiple alleles, such as in the ABO blood group system, create genetic diversity.
The principles of sex-linked inheritance, including how certain traits can affect one sex more than the other.
How to interpret pedigree diagrams to track genetic traits across generations.

Study Notes

Codominance: Both alleles expressed (e.g., roan cattle).
Incomplete Dominance: Blended traits (e.g., pink flowers).
Multiple Alleles: More than two alleles for a gene (e.g., ABO blood types).
Sex Linked Traits: Traits associated with sex chromosomes (e.g., color blindness, hemophilia).
Pedigree Diagrams: Charts showing inheritance patterns within families.