X-Linked Inheritance & Polygenic Inheritance

My AP Biology Thoughts Unit 5 Heredity

Welcome to My AP Biology Thoughts podcast, my name is Corrinna and I am your host for episode #100 called Unit 5 Heredity: X-Linked Inheritance & Polygenic Inheritance. Today we will be discussing how X-linked inheritance and polygenic inheritance work.

Segment 1: Introduction to X-Linked Inheritance & Polygenic Inheritance

• X-linked and polygenic are both different types of non-mendelian inheritance. Mendelian inheritance follows the laws of segregation and independent assortment, so X-linked and polygenic traits by definition do not follow these laws.
• X-linked traits are traits whose alleles are carried on the sex chromosomes instead of the normal chromosomes. Males have XY sex chromosomes, and females have XX sex chromosomes. In x-linked inheritance, the allele for a trait is only carried on the x chromosome. This means that any male who inherits the allele will express the trait since he only has one copy of the x chromosome. X-linked traits can be dominant or recessive, just like in mendelian inheritance. However, since the alleles are located on the x chromosome, any male who gets the linked allele will express the trait. In females, x-linked inheritance works like mendelian inheritance - if both x chromosomes have the recessive allele, it is expressed, otherwise the dominant allele will always be expressed.
• Polygenic inheritance is when a single phenotypic trait is controlled by multiple genes. This is different from multiple alleles because in polygenic inheritance, all of the polygenic genes can be found in an individual, while in multiple alleles, only two of the alleles are present in an individual. Polygenic inheritance also produces a range of phenotypes, rather than specific distinct phenotypes. Polygenic inheritance also doesn’t exhibit complete dominance, which is why there is a range of phenotypes.

Segment 2: More About X-Linked Inheritance & Polygenic Inheritance

• One example of an x-linked trait is hemophilia. In males who possess the altered copy of the gene, they will always express the phenotype for hemophilia because they only have one copy of the X chromosome. However women need two copies of the altered gene to express the phenotype. This is because hemophilia is an x-linked RECESSIVE trait. If it was x-linked dominant, the woman would only need one copy of the altered gene to express the phenotype.
• Polygenic inheritance includes traits like hair color, height, and skin color, as well as non visible traits like blood pressure and intelligence. Skin color is one example of this. Skin color is controlled by the pigment melanin, and darker skin results from more melanin. Hypothetically, the production of melanin is controlled by contributing alleles which will be called A, B, and C, which results in dark skin color. The non contributing alleles lowercase a, b, and c, produce a light skin color. Since polygenic alleles don’t display dominance, each contributing allele gives an additive effect in which the different alleles create a spectrum of possible phenotypes. In this example, individuals with all uppercase alleles will have the darkest skin, and individuals with all lowercase alleles will have the lightest skin color.